67 lines
2.9 KiB
ReStructuredText
67 lines
2.9 KiB
ReStructuredText
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=================================
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GFP masks used from FS/IO context
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=================================
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:Date: May, 2018
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:Author: Michal Hocko <mhocko@kernel.org>
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Introduction
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============
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Code paths in the filesystem and IO stacks must be careful when
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allocating memory to prevent recursion deadlocks caused by direct
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memory reclaim calling back into the FS or IO paths and blocking on
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already held resources (e.g. locks - most commonly those used for the
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transaction context).
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The traditional way to avoid this deadlock problem is to clear __GFP_FS
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respectively __GFP_IO (note the latter implies clearing the first as well) in
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the gfp mask when calling an allocator. GFP_NOFS respectively GFP_NOIO can be
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used as shortcut. It turned out though that above approach has led to
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abuses when the restricted gfp mask is used "just in case" without a
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deeper consideration which leads to problems because an excessive use
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of GFP_NOFS/GFP_NOIO can lead to memory over-reclaim or other memory
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reclaim issues.
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New API
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========
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Since 4.12 we do have a generic scope API for both NOFS and NOIO context
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``memalloc_nofs_save``, ``memalloc_nofs_restore`` respectively ``memalloc_noio_save``,
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``memalloc_noio_restore`` which allow to mark a scope to be a critical
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section from a filesystem or I/O point of view. Any allocation from that
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scope will inherently drop __GFP_FS respectively __GFP_IO from the given
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mask so no memory allocation can recurse back in the FS/IO.
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.. kernel-doc:: include/linux/sched/mm.h
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:functions: memalloc_nofs_save memalloc_nofs_restore
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.. kernel-doc:: include/linux/sched/mm.h
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:functions: memalloc_noio_save memalloc_noio_restore
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FS/IO code then simply calls the appropriate save function before
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any critical section with respect to the reclaim is started - e.g.
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lock shared with the reclaim context or when a transaction context
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nesting would be possible via reclaim. The restore function should be
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called when the critical section ends. All that ideally along with an
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explanation what is the reclaim context for easier maintenance.
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Please note that the proper pairing of save/restore functions
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allows nesting so it is safe to call ``memalloc_noio_save`` or
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``memalloc_noio_restore`` respectively from an existing NOIO or NOFS
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scope.
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What about __vmalloc(GFP_NOFS)
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==============================
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vmalloc doesn't support GFP_NOFS semantic because there are hardcoded
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GFP_KERNEL allocations deep inside the allocator which are quite non-trivial
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to fix up. That means that calling ``vmalloc`` with GFP_NOFS/GFP_NOIO is
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almost always a bug. The good news is that the NOFS/NOIO semantic can be
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achieved by the scope API.
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In the ideal world, upper layers should already mark dangerous contexts
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and so no special care is required and vmalloc should be called without
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any problems. Sometimes if the context is not really clear or there are
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layering violations then the recommended way around that is to wrap ``vmalloc``
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by the scope API with a comment explaining the problem.
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