kernel_samsung_a34x-permissive/drivers/md/dm-user.c

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// SPDX-License-Identifier: GPL-2.0+
/*
* Copyright (C) 2020 Google, Inc
* Copyright (C) 2020 Palmer Dabbelt <palmerdabbelt@google.com>
*/
#include <linux/device-mapper.h>
#include <uapi/linux/dm-user.h>
#include <linux/bio.h>
#include <linux/init.h>
#include <linux/mempool.h>
#include <linux/miscdevice.h>
#include <linux/module.h>
#include <linux/poll.h>
#include <linux/uio.h>
#include <linux/wait.h>
#define DM_MSG_PREFIX "user"
#define MAX_OUTSTANDING_MESSAGES 128
/*
* dm-user uses four structures:
*
* - "struct target", the outermost structure, corresponds to a single device
* mapper target. This contains the set of outstanding BIOs that have been
* provided by DM and are not actively being processed by the user, along
* with a misc device that userspace can open to communicate with the
* kernel. Each time userspaces opens the misc device a new channel is
* created.
* - "struct channel", which represents a single active communication channel
* with userspace. Userspace may choose arbitrary read/write sizes to use
* when processing messages, channels form these into logical accesses.
* When userspace responds to a full message the channel completes the BIO
* and obtains a new message to process from the target.
* - "struct message", which wraps a BIO with the additional information
* required by the kernel to sort out what to do with BIOs when they return
* from userspace.
* - "struct dm_user_message", which is the exact message format that
* userspace sees.
*
* The hot path contains three distinct operations:
*
* - user_map(), which is provided a BIO from device mapper that is queued
* into the target. This allocates and enqueues a new message.
* - dev_read(), which dequeues a message, copies it to userspace.
* - dev_write(), which looks up a message (keyed by sequence number) and
* completes the corresponding BIO.
*
* Lock ordering (outer to inner)
*
* 1) miscdevice's global lock. This is held around dev_open, so it has to be
* the outermost lock.
* 2) target->lock
* 3) channel->lock
*/
struct message {
/*
* Messages themselves do not need a lock, they're protected by either
* the target or channel's lock, depending on which can reference them
* directly.
*/
struct dm_user_message msg;
struct bio *bio;
size_t posn_to_user;
size_t total_to_user;
size_t posn_from_user;
size_t total_from_user;
struct list_head from_user;
struct list_head to_user;
/*
* These are written back from the user. They live in the same spot in
* the message, but we need to either keep the old values around or
* call a bunch more BIO helpers. These are only valid after write has
* adopted the message.
*/
u64 return_type;
u64 return_flags;
};
struct target {
/*
* A target has a single lock, which protects everything in the target
* (but does not protect the channels associated with a target).
*/
struct mutex lock;
/*
* There is only one point at which anything blocks: userspace blocks
* reading a new message, which is woken up by device mapper providing
* a new BIO to process (or tearing down the target). The
* corresponding write side doesn't block, instead we treat userspace's
* response containing a message that has yet to be mapped as an
* invalid operation.
*/
struct wait_queue_head wq;
/*
* Messages are delivered to userspace in order, but may be returned
* out of order. This allows userspace to schedule IO if it wants to.
*/
mempool_t message_pool;
u64 next_seq_to_map;
u64 next_seq_to_user;
struct list_head to_user;
/*
* There is a misc device per target. The name is selected by
* userspace (via a DM create ioctl argument), and each ends up in
* /dev/dm-user/. It looks like a better way to do this may be to have
* a filesystem to manage these, but this was more expedient. The
* current mechanism is functional, but does result in an arbitrary
* number of dynamically created misc devices.
*/
struct miscdevice miscdev;
/*
* Device mapper's target destructor triggers tearing this all down,
* but we can't actually free until every channel associated with this
* target has been destroyed. Channels each have a reference to their
* target, and there is an additional single reference that corresponds
* to both DM and the misc device (both of which are destroyed by DM).
*
* In the common case userspace will be asleep waiting for a new
* message when device mapper decides to destroy the target, which
* means no new messages will appear. The destroyed flag triggers a
* wakeup, which will end up removing the reference.
*/
struct kref references;
int dm_destroyed;
};
struct channel {
struct target *target;
/*
* A channel has a single lock, which prevents multiple reads (or
* multiple writes) from conflicting with each other.
*/
struct mutex lock;
struct message *cur_to_user;
struct message *cur_from_user;
ssize_t to_user_error;
ssize_t from_user_error;
/*
* Once a message has been forwarded to userspace on a channel it must
* be responded to on the same channel. This allows us to error out
* the messages that have not yet been responded to by a channel when
* that channel closes, which makes handling errors more reasonable for
* fault-tolerant userspace daemons. It also happens to make avoiding
* shared locks between user_map() and dev_read() a lot easier.
*
* This does preclude a multi-threaded work stealing userspace
* implementation (or at least, force a degree of head-of-line blocking
* on the response path).
*/
struct list_head from_user;
/*
* Responses from userspace can arrive in arbitrarily small chunks.
* We need some place to buffer one up until we can find the
* corresponding kernel-side message to continue processing, so instead
* of allocating them we just keep one off to the side here. This can
* only ever be pointer to by from_user_cur, and will never have a BIO.
*/
struct message scratch_message_from_user;
};
static inline struct target *target_from_target(struct dm_target *target)
{
WARN_ON(target->private == NULL);
return target->private;
}
static inline struct target *target_from_miscdev(struct miscdevice *miscdev)
{
return container_of(miscdev, struct target, miscdev);
}
static inline struct channel *channel_from_file(struct file *file)
{
WARN_ON(file->private_data == NULL);
return file->private_data;
}
static inline struct target *target_from_channel(struct channel *c)
{
WARN_ON(c->target == NULL);
return c->target;
}
static inline size_t bio_size(struct bio *bio)
{
struct bio_vec bvec;
struct bvec_iter iter;
size_t out = 0;
bio_for_each_segment (bvec, bio, iter)
out += bio_iter_len(bio, iter);
return out;
}
static inline size_t bio_bytes_needed_to_user(struct bio *bio)
{
switch (bio_op(bio)) {
case REQ_OP_WRITE:
return sizeof(struct dm_user_message) + bio_size(bio);
case REQ_OP_READ:
case REQ_OP_FLUSH:
case REQ_OP_DISCARD:
case REQ_OP_SECURE_ERASE:
case REQ_OP_WRITE_SAME:
case REQ_OP_WRITE_ZEROES:
return sizeof(struct dm_user_message);
/*
* These ops are not passed to userspace under the assumption that
* they're not going to be particularly useful in that context.
*/
default:
return -EOPNOTSUPP;
}
}
static inline size_t bio_bytes_needed_from_user(struct bio *bio)
{
switch (bio_op(bio)) {
case REQ_OP_READ:
return sizeof(struct dm_user_message) + bio_size(bio);
case REQ_OP_WRITE:
case REQ_OP_FLUSH:
case REQ_OP_DISCARD:
case REQ_OP_SECURE_ERASE:
case REQ_OP_WRITE_SAME:
case REQ_OP_WRITE_ZEROES:
return sizeof(struct dm_user_message);
/*
* These ops are not passed to userspace under the assumption that
* they're not going to be particularly useful in that context.
*/
default:
return -EOPNOTSUPP;
}
}
static inline long bio_type_to_user_type(struct bio *bio)
{
switch (bio_op(bio)) {
case REQ_OP_READ:
return DM_USER_REQ_MAP_READ;
case REQ_OP_WRITE:
return DM_USER_REQ_MAP_WRITE;
case REQ_OP_FLUSH:
return DM_USER_REQ_MAP_FLUSH;
case REQ_OP_DISCARD:
return DM_USER_REQ_MAP_DISCARD;
case REQ_OP_SECURE_ERASE:
return DM_USER_REQ_MAP_SECURE_ERASE;
case REQ_OP_WRITE_SAME:
return DM_USER_REQ_MAP_WRITE_SAME;
case REQ_OP_WRITE_ZEROES:
return DM_USER_REQ_MAP_WRITE_ZEROES;
/*
* These ops are not passed to userspace under the assumption that
* they're not going to be particularly useful in that context.
*/
default:
return -EOPNOTSUPP;
}
}
static inline long bio_flags_to_user_flags(struct bio *bio)
{
u64 out = 0;
typeof(bio->bi_opf) opf = bio->bi_opf & ~REQ_OP_MASK;
if (opf & REQ_FAILFAST_DEV) {
opf &= ~REQ_FAILFAST_DEV;
out |= DM_USER_REQ_MAP_FLAG_FAILFAST_DEV;
}
if (opf & REQ_FAILFAST_TRANSPORT) {
opf &= ~REQ_FAILFAST_TRANSPORT;
out |= DM_USER_REQ_MAP_FLAG_FAILFAST_TRANSPORT;
}
if (opf & REQ_FAILFAST_DRIVER) {
opf &= ~REQ_FAILFAST_DRIVER;
out |= DM_USER_REQ_MAP_FLAG_FAILFAST_DRIVER;
}
if (opf & REQ_SYNC) {
opf &= ~REQ_SYNC;
out |= DM_USER_REQ_MAP_FLAG_SYNC;
}
if (opf & REQ_META) {
opf &= ~REQ_META;
out |= DM_USER_REQ_MAP_FLAG_META;
}
if (opf & REQ_PRIO) {
opf &= ~REQ_PRIO;
out |= DM_USER_REQ_MAP_FLAG_PRIO;
}
if (opf & REQ_NOMERGE) {
opf &= ~REQ_NOMERGE;
out |= DM_USER_REQ_MAP_FLAG_NOMERGE;
}
if (opf & REQ_IDLE) {
opf &= ~REQ_IDLE;
out |= DM_USER_REQ_MAP_FLAG_IDLE;
}
if (opf & REQ_INTEGRITY) {
opf &= ~REQ_INTEGRITY;
out |= DM_USER_REQ_MAP_FLAG_INTEGRITY;
}
if (opf & REQ_FUA) {
opf &= ~REQ_FUA;
out |= DM_USER_REQ_MAP_FLAG_FUA;
}
if (opf & REQ_PREFLUSH) {
opf &= ~REQ_PREFLUSH;
out |= DM_USER_REQ_MAP_FLAG_PREFLUSH;
}
if (opf & REQ_RAHEAD) {
opf &= ~REQ_RAHEAD;
out |= DM_USER_REQ_MAP_FLAG_RAHEAD;
}
if (opf & REQ_BACKGROUND) {
opf &= ~REQ_BACKGROUND;
out |= DM_USER_REQ_MAP_FLAG_BACKGROUND;
}
if (opf & REQ_NOWAIT) {
opf &= ~REQ_NOWAIT;
out |= DM_USER_REQ_MAP_FLAG_NOWAIT;
}
if (opf & REQ_NOUNMAP) {
opf &= ~REQ_NOUNMAP;
out |= DM_USER_REQ_MAP_FLAG_NOUNMAP;
}
if (unlikely(opf)) {
pr_warn("unsupported BIO type %x\n", opf);
return -EOPNOTSUPP;
}
WARN_ON(out < 0);
return out;
}
/*
* Not quite what's in blk-map.c, but instead what I thought the functions in
* blk-map did. This one seems more generally useful and I think we could
* write the blk-map version in terms of this one. The differences are that
* this has a return value that counts, and blk-map uses the BIO _all iters.
* Neither advance the BIO iter but don't advance the IOV iter, which is a bit
* odd here.
*/
static ssize_t bio_copy_from_iter(struct bio *bio, struct iov_iter *iter)
{
struct bio_vec bvec;
struct bvec_iter biter;
ssize_t out = 0;
bio_for_each_segment (bvec, bio, biter) {
ssize_t ret;
ret = copy_page_from_iter(bvec.bv_page, bvec.bv_offset,
bvec.bv_len, iter);
/*
* FIXME: I thought that IOV copies had a mechanism for
* terminating early, if for example a signal came in while
* sleeping waiting for a page to be mapped, but I don't see
* where that would happen.
*/
WARN_ON(ret < 0);
out += ret;
if (!iov_iter_count(iter))
break;
if (ret < bvec.bv_len)
return ret;
}
return out;
}
static ssize_t bio_copy_to_iter(struct bio *bio, struct iov_iter *iter)
{
struct bio_vec bvec;
struct bvec_iter biter;
ssize_t out = 0;
bio_for_each_segment (bvec, bio, biter) {
ssize_t ret;
ret = copy_page_to_iter(bvec.bv_page, bvec.bv_offset,
bvec.bv_len, iter);
/* as above */
WARN_ON(ret < 0);
out += ret;
if (!iov_iter_count(iter))
break;
if (ret < bvec.bv_len)
return ret;
}
return out;
}
static ssize_t msg_copy_to_iov(struct message *msg, struct iov_iter *to)
{
ssize_t copied = 0;
if (!iov_iter_count(to))
return 0;
if (msg->posn_to_user < sizeof(msg->msg)) {
copied = copy_to_iter((char *)(&msg->msg) + msg->posn_to_user,
sizeof(msg->msg) - msg->posn_to_user, to);
} else {
copied = bio_copy_to_iter(msg->bio, to);
if (copied > 0)
bio_advance(msg->bio, copied);
}
if (copied < 0)
return copied;
msg->posn_to_user += copied;
return copied;
}
static ssize_t msg_copy_from_iov(struct message *msg, struct iov_iter *from)
{
ssize_t copied = 0;
if (!iov_iter_count(from))
return 0;
if (msg->posn_from_user < sizeof(msg->msg)) {
copied = copy_from_iter(
(char *)(&msg->msg) + msg->posn_from_user,
sizeof(msg->msg) - msg->posn_from_user, from);
} else {
copied = bio_copy_from_iter(msg->bio, from);
if (copied > 0)
bio_advance(msg->bio, copied);
}
if (copied < 0)
return copied;
msg->posn_from_user += copied;
return copied;
}
static struct message *msg_get_map(struct target *t)
{
struct message *m;
lockdep_assert_held(&t->lock);
m = mempool_alloc(&t->message_pool, GFP_NOIO);
m->msg.seq = t->next_seq_to_map++;
INIT_LIST_HEAD(&m->to_user);
INIT_LIST_HEAD(&m->from_user);
return m;
}
static struct message *msg_get_to_user(struct target *t)
{
struct message *m;
lockdep_assert_held(&t->lock);
if (list_empty(&t->to_user))
return NULL;
m = list_first_entry(&t->to_user, struct message, to_user);
list_del(&m->to_user);
return m;
}
static struct message *msg_get_from_user(struct channel *c, u64 seq)
{
struct message *m;
struct list_head *cur, *tmp;
lockdep_assert_held(&c->lock);
list_for_each_safe (cur, tmp, &c->from_user) {
m = list_entry(cur, struct message, from_user);
if (m->msg.seq == seq) {
list_del(&m->from_user);
return m;
}
}
return NULL;
}
static void message_kill(struct message *m, mempool_t *pool)
{
m->bio->bi_status = BLK_STS_IOERR;
bio_endio(m->bio);
bio_put(m->bio);
mempool_free(m, pool);
}
/*
* Returns 0 when there is no work left to do. This must be callable without
* holding the target lock, as it is part of the waitqueue's check expression.
* When called without the lock it may spuriously indicate there is remaining
* work, but when called with the lock it must be accurate.
*/
static int target_poll(struct target *t)
{
return !list_empty(&t->to_user) || t->dm_destroyed;
}
static void target_release(struct kref *ref)
{
struct target *t = container_of(ref, struct target, references);
struct list_head *cur, *tmp;
/*
* There may be outstanding BIOs that have not yet been given to
* userspace. At this point there's nothing we can do about them, as
* there are and will never be any channels.
*/
list_for_each_safe (cur, tmp, &t->to_user) {
message_kill(list_entry(cur, struct message, to_user),
&t->message_pool);
}
mempool_exit(&t->message_pool);
mutex_unlock(&t->lock);
mutex_destroy(&t->lock);
kfree(t);
}
static void target_put(struct target *t)
{
/*
* This both releases a reference to the target and the lock. We leave
* it up to the caller to hold the lock, as they probably needed it for
* something else.
*/
lockdep_assert_held(&t->lock);
if (!kref_put(&t->references, target_release))
mutex_unlock(&t->lock);
}
static struct channel *channel_alloc(struct target *t)
{
struct channel *c;
lockdep_assert_held(&t->lock);
c = kzalloc(sizeof(*c), GFP_KERNEL);
if (c == NULL)
return NULL;
kref_get(&t->references);
c->target = t;
c->cur_from_user = &c->scratch_message_from_user;
mutex_init(&c->lock);
INIT_LIST_HEAD(&c->from_user);
return c;
}
static void channel_free(struct channel *c)
{
struct list_head *cur, *tmp;
lockdep_assert_held(&c->lock);
/*
* There may be outstanding BIOs that have been given to userspace but
* have not yet been completed. The channel has been shut down so
* there's no way to process the rest of those messages, so we just go
* ahead and error out the BIOs. Hopefully whatever's on the other end
* can handle the errors. One could imagine splitting the BIOs and
* completing as much as we got, but that seems like overkill here.
*
* Our only other options would be to let the BIO hang around (which
* seems way worse) or to resubmit it to userspace in the hope there's
* another channel. I don't really like the idea of submitting a
* message twice.
*/
if (c->cur_to_user != NULL)
message_kill(c->cur_to_user, &c->target->message_pool);
if (c->cur_from_user != &c->scratch_message_from_user)
message_kill(c->cur_from_user, &c->target->message_pool);
list_for_each_safe (cur, tmp, &c->from_user)
message_kill(list_entry(cur, struct message, from_user),
&c->target->message_pool);
mutex_lock(&c->target->lock);
target_put(c->target);
mutex_unlock(&c->lock);
mutex_destroy(&c->lock);
kfree(c);
}
static int dev_open(struct inode *inode, struct file *file)
{
struct channel *c;
struct target *t;
/*
* This is called by miscdev, which sets private_data to point to the
* struct miscdevice that was opened. The rest of our file operations
* want to refer to the channel that's been opened, so we swap that
* pointer out with a fresh channel.
*
* This is called with the miscdev lock held, which is also held while
* registering/unregistering the miscdev. The miscdev must be
* registered for this to get called, which means there must be an
* outstanding reference to the target, which means it cannot be freed
* out from under us despite us not holding a reference yet.
*/
t = container_of(file->private_data, struct target, miscdev);
mutex_lock(&t->lock);
file->private_data = c = channel_alloc(t);
if (c == NULL) {
mutex_unlock(&t->lock);
return -ENOMEM;
}
mutex_unlock(&t->lock);
return 0;
}
static ssize_t dev_read(struct kiocb *iocb, struct iov_iter *to)
{
struct channel *c = channel_from_file(iocb->ki_filp);
ssize_t total_processed = 0;
ssize_t processed;
mutex_lock(&c->lock);
if (unlikely(c->to_user_error)) {
total_processed = c->to_user_error;
goto cleanup_unlock;
}
if (c->cur_to_user == NULL) {
struct target *t = target_from_channel(c);
mutex_lock(&t->lock);
while (!target_poll(t)) {
int e;
mutex_unlock(&t->lock);
mutex_unlock(&c->lock);
e = wait_event_interruptible(t->wq, target_poll(t));
mutex_lock(&c->lock);
mutex_lock(&t->lock);
if (unlikely(e != 0)) {
/*
* We haven't processed any bytes in either the
* BIO or the IOV, so we can just terminate
* right now. Elsewhere in the kernel handles
* restarting the syscall when appropriate.
*/
total_processed = e;
mutex_unlock(&t->lock);
goto cleanup_unlock;
}
}
if (unlikely(t->dm_destroyed)) {
/*
* DM has destroyed this target, so just lock
* the user out. There's really nothing else
* we can do here. Note that we don't actually
* tear any thing down until userspace has
* closed the FD, as there may still be
* outstanding BIOs.
*
* This is kind of a wacky error code to
* return. My goal was really just to try and
* find something that wasn't likely to be
* returned by anything else in the miscdev
* path. The message "block device required"
* seems like a somewhat reasonable thing to
* say when the target has disappeared out from
* under us, but "not block" isn't sensible.
*/
c->to_user_error = total_processed = -ENOTBLK;
mutex_unlock(&t->lock);
goto cleanup_unlock;
}
/*
* Ensures that accesses to the message data are not ordered
* before the remote accesses that produce that message data.
*
* This pairs with the barrier in user_map(), via the
* conditional within the while loop above. Also see the lack
* of barrier in user_dtr(), which is why this can be after the
* destroyed check.
*/
smp_rmb();
c->cur_to_user = msg_get_to_user(t);
WARN_ON(c->cur_to_user == NULL);
mutex_unlock(&t->lock);
}
processed = msg_copy_to_iov(c->cur_to_user, to);
total_processed += processed;
WARN_ON(c->cur_to_user->posn_to_user > c->cur_to_user->total_to_user);
if (c->cur_to_user->posn_to_user == c->cur_to_user->total_to_user) {
struct message *m = c->cur_to_user;
c->cur_to_user = NULL;
list_add_tail(&m->from_user, &c->from_user);
}
cleanup_unlock:
mutex_unlock(&c->lock);
return total_processed;
}
static ssize_t dev_write(struct kiocb *iocb, struct iov_iter *from)
{
struct channel *c = channel_from_file(iocb->ki_filp);
ssize_t total_processed = 0;
ssize_t processed;
mutex_lock(&c->lock);
if (unlikely(c->from_user_error)) {
total_processed = c->from_user_error;
goto cleanup_unlock;
}
/*
* cur_from_user can never be NULL. If there's no real message it must
* point to the scratch space.
*/
WARN_ON(c->cur_from_user == NULL);
if (c->cur_from_user->posn_from_user < sizeof(struct dm_user_message)) {
struct message *msg, *old;
processed = msg_copy_from_iov(c->cur_from_user, from);
if (processed <= 0) {
pr_warn("msg_copy_from_iov() returned %zu\n",
processed);
c->from_user_error = -EINVAL;
goto cleanup_unlock;
}
total_processed += processed;
/*
* In the unlikely event the user has provided us a very short
* write, not even big enough to fill a message, just succeed.
* We'll eventually build up enough bytes to do something.
*/
if (unlikely(c->cur_from_user->posn_from_user <
sizeof(struct dm_user_message)))
goto cleanup_unlock;
old = c->cur_from_user;
mutex_lock(&c->target->lock);
msg = msg_get_from_user(c, c->cur_from_user->msg.seq);
if (msg == NULL) {
pr_info("user provided an invalid messag seq of %llx\n",
old->msg.seq);
mutex_unlock(&c->target->lock);
c->from_user_error = -EINVAL;
goto cleanup_unlock;
}
mutex_unlock(&c->target->lock);
WARN_ON(old->posn_from_user != sizeof(struct dm_user_message));
msg->posn_from_user = sizeof(struct dm_user_message);
msg->return_type = old->msg.type;
msg->return_flags = old->msg.flags;
WARN_ON(msg->posn_from_user > msg->total_from_user);
c->cur_from_user = msg;
WARN_ON(old != &c->scratch_message_from_user);
}
/*
* Userspace can signal an error for single requests by overwriting the
* seq field.
*/
switch (c->cur_from_user->return_type) {
case DM_USER_RESP_SUCCESS:
c->cur_from_user->bio->bi_status = BLK_STS_OK;
break;
case DM_USER_RESP_ERROR:
case DM_USER_RESP_UNSUPPORTED:
default:
c->cur_from_user->bio->bi_status = BLK_STS_IOERR;
goto finish_bio;
}
/*
* The op was a success as far as userspace is concerned, so process
* whatever data may come along with it. The user may provide the BIO
* data in multiple chunks, in which case we don't need to finish the
* BIO.
*/
processed = msg_copy_from_iov(c->cur_from_user, from);
total_processed += processed;
if (c->cur_from_user->posn_from_user <
c->cur_from_user->total_from_user)
goto cleanup_unlock;
finish_bio:
/*
* When we set up this message the BIO's size matched the
* message size, if that's not still the case then something
* has gone off the rails.
*/
WARN_ON(bio_size(c->cur_from_user->bio) != 0);
bio_endio(c->cur_from_user->bio);
bio_put(c->cur_from_user->bio);
/*
* We don't actually need to take the target lock here, as all
* we're doing is freeing the message and mempools have their
* own lock. Each channel has its ows scratch message.
*/
WARN_ON(c->cur_from_user == &c->scratch_message_from_user);
mempool_free(c->cur_from_user, &c->target->message_pool);
c->scratch_message_from_user.posn_from_user = 0;
c->cur_from_user = &c->scratch_message_from_user;
cleanup_unlock:
mutex_unlock(&c->lock);
return total_processed;
}
static int dev_release(struct inode *inode, struct file *file)
{
struct channel *c;
c = channel_from_file(file);
mutex_lock(&c->lock);
channel_free(c);
return 0;
}
static const struct file_operations file_operations = {
.owner = THIS_MODULE,
.open = dev_open,
.llseek = no_llseek,
.read_iter = dev_read,
.write_iter = dev_write,
.release = dev_release,
};
static int user_ctr(struct dm_target *ti, unsigned int argc, char **argv)
{
struct target *t;
int r;
if (argc != 3) {
ti->error = "Invalid argument count";
r = -EINVAL;
goto cleanup_none;
}
t = kzalloc(sizeof(*t), GFP_KERNEL);
if (t == NULL) {
r = -ENOMEM;
goto cleanup_none;
}
ti->private = t;
/* Enable more BIO types. */
ti->num_discard_bios = 1;
ti->discards_supported = true;
ti->num_flush_bios = 1;
ti->flush_supported = true;
/*
* We begin with a single reference to the target, which is miscdev's
* reference. This ensures that the target won't be freed
* until after the miscdev has been unregistered and all extant
* channels have been closed.
*/
kref_init(&t->references);
mutex_init(&t->lock);
init_waitqueue_head(&t->wq);
INIT_LIST_HEAD(&t->to_user);
mempool_init_kmalloc_pool(&t->message_pool, MAX_OUTSTANDING_MESSAGES,
sizeof(struct message));
t->miscdev.minor = MISC_DYNAMIC_MINOR;
t->miscdev.fops = &file_operations;
t->miscdev.name = kasprintf(GFP_KERNEL, "dm-user/%s", argv[2]);
if (t->miscdev.name == NULL) {
r = -ENOMEM;
goto cleanup_message_pool;
}
/*
* Once the miscdev is registered it can be opened and therefor
* concurrent references to the channel can happen. Holding the target
* lock during misc_register() could deadlock. If registration
* succeeds then we will not access the target again so we just stick a
* barrier here, which pairs with taking the target lock everywhere
* else the target is accessed.
*
* I forgot where we ended up on the RCpc/RCsc locks. IIU RCsc locks
* would mean that we could take the target lock earlier and release it
* here instead of the memory barrier. I'm not sure that's any better,
* though, and this isn't on a hot path so it probably doesn't matter
* either way.
*/
smp_mb();
r = misc_register(&t->miscdev);
if (r) {
DMERR("Unable to register miscdev %s for dm-user",
t->miscdev.name);
r = -ENOMEM;
goto cleanup_misc_name;
}
return 0;
cleanup_misc_name:
kfree(t->miscdev.name);
cleanup_message_pool:
mempool_exit(&t->message_pool);
kfree(t);
cleanup_none:
return r;
}
static void user_dtr(struct dm_target *ti)
{
struct target *t = target_from_target(ti);
/*
* Removes the miscdev. This must be called without the target lock
* held to avoid a possible deadlock because our open implementation is
* called holding the miscdev lock and must later take the target lock.
*
* There is no race here because only DM can register/unregister the
* miscdev, and DM ensures that doesn't happen twice. The internal
* miscdev lock is sufficient to ensure there are no races between
* deregistering the miscdev and open.
*/
misc_deregister(&t->miscdev);
/*
* We are now free to take the target's lock and drop our reference to
* the target. There are almost certainly tasks sleeping in read on at
* least one of the channels associated with this target, this
* explicitly wakes them up and terminates the read.
*/
mutex_lock(&t->lock);
/*
* No barrier here, as wait/wake ensures that the flag visibility is
* correct WRT the wake/sleep state of the target tasks.
*/
t->dm_destroyed = true;
wake_up_all(&t->wq);
target_put(t);
}
/*
* Consumes a BIO from device mapper, queueing it up for userspace.
*/
static int user_map(struct dm_target *ti, struct bio *bio)
{
struct target *t;
struct message *entry;
t = target_from_target(ti);
/*
* FIXME
*
* This seems like a bad idea. Specifically, here we're
* directly on the IO path when we take the target lock, which may also
* be taken from a user context. The user context doesn't actively
* trigger anything that may sleep while holding the lock, but this
* still seems like a bad idea.
*
* The obvious way to fix this would be to use a proper queue, which
* would result in no shared locks between the direct IO path and user
* tasks. I had a version that did this, but the head-of-line blocking
* from the circular buffer resulted in us needing a fairly large
* allocation in order to avoid situations in which the queue fills up
* and everything goes off the rails.
*
* I could jump through a some hoops to avoid a shared lock while still
* allowing for a large queue, but I'm not actually sure that allowing
* for very large queues is the right thing to do here. Intuitively it
* seems better to keep the queues small in here (essentially sized to
* the user latency for performance reasons only) and rely on returning
* DM_MAPIO_REQUEUE regularly, as that would give the rest of the
* kernel more information.
*
* I'll spend some time trying to figure out what's going on with
* DM_MAPIO_REQUEUE, but if someone has a better idea of how to fix
* this I'm all ears.
*/
mutex_lock(&t->lock);
/*
* FIXME
*
* The assumption here is that there's no benefit to returning
* DM_MAPIO_KILL as opposed to just erroring out the BIO, but I'm not
* sure that's actually true -- for example, I could imagine users
* expecting that submitted BIOs are unlikely to fail and therefor
* relying on submission failure to indicate an unsupported type.
*
* There's two ways I can think of to fix this:
* - Add DM arguments that are parsed during the constructor that
* allow various dm_target flags to be set that indicate the op
* types supported by this target. This may make sense for things
* like discard, where DM can already transform the BIOs to a form
* that's likely to be supported.
* - Some sort of pre-filter that allows userspace to hook in here
* and kill BIOs before marking them as submitted. My guess would
* be that a userspace round trip is a bad idea here, but a BPF
* call seems resonable.
*
* My guess is that we'd likely want to do both. The first one is easy
* and gives DM the proper info, so it seems better. The BPF call
* seems overly complex for just this, but one could imagine wanting to
* sometimes return _MAPPED and a BPF filter would be the way to do
* that.
*
* For example, in Android we have an in-kernel DM device called
* "dm-bow" that takes advange of some portion of the space that has
* been discarded on a device to provide opportunistic block-level
* backups. While one could imagine just implementing this entirely in
* userspace, that would come with an appreciable performance penalty.
* Instead one could keep a BPF program that forwards most accesses
* directly to the backing block device while informing a userspace
* daemon of any discarded space and on writes to blocks that are to be
* backed up.
*/
if (unlikely((bio_type_to_user_type(bio) < 0) ||
(bio_flags_to_user_flags(bio) < 0))) {
mutex_unlock(&t->lock);
return DM_MAPIO_KILL;
}
entry = msg_get_map(t);
if (unlikely(entry == NULL)) {
mutex_unlock(&t->lock);
return DM_MAPIO_REQUEUE;
}
bio_get(bio);
entry->msg.type = bio_type_to_user_type(bio);
entry->msg.flags = bio_flags_to_user_flags(bio);
entry->msg.sector = bio->bi_iter.bi_sector;
entry->msg.len = bio_size(bio);
entry->bio = bio;
entry->posn_to_user = 0;
entry->total_to_user = bio_bytes_needed_to_user(bio);
entry->posn_from_user = 0;
entry->total_from_user = bio_bytes_needed_from_user(bio);
/* Pairs with the barrier in dev_read() */
smp_wmb();
list_add_tail(&entry->to_user, &t->to_user);
wake_up_interruptible(&t->wq);
mutex_unlock(&t->lock);
return DM_MAPIO_SUBMITTED;
}
static struct target_type user_target = {
.name = "user",
.version = { 1, 0, 0 },
.module = THIS_MODULE,
.ctr = user_ctr,
.dtr = user_dtr,
.map = user_map,
};
static int __init dm_user_init(void)
{
int r;
r = dm_register_target(&user_target);
if (r) {
DMERR("register failed %d", r);
goto error;
}
return 0;
error:
return r;
}
static void __exit dm_user_exit(void)
{
dm_unregister_target(&user_target);
}
module_init(dm_user_init);
module_exit(dm_user_exit);
MODULE_AUTHOR("Palmer Dabbelt <palmerdabbelt@google.com>");
MODULE_DESCRIPTION(DM_NAME " target returning blocks from userspace");
MODULE_LICENSE("GPL");