1 .. _gfp_mask_from_fs_io: 2 3 ================================= 4 GFP masks used from FS/IO context 5 ================================= 6 7 :Date: May, 2018 8 :Author: Michal Hocko <mhocko@kernel.org> 9 10 Introduction 11 ============ 12 13 Code paths in the filesystem and IO stacks must be careful when 14 allocating memory to prevent recursion deadlocks caused by direct 15 memory reclaim calling back into the FS or IO paths and blocking on 16 already held resources (e.g. locks - most commonly those used for the 17 transaction context). 18 19 The traditional way to avoid this deadlock problem is to clear __GFP_FS 20 respectively __GFP_IO (note the latter implies clearing the first as well) in 21 the gfp mask when calling an allocator. GFP_NOFS respectively GFP_NOIO can be 22 used as shortcut. It turned out though that above approach has led to 23 abuses when the restricted gfp mask is used "just in case" without a 24 deeper consideration which leads to problems because an excessive use 25 of GFP_NOFS/GFP_NOIO can lead to memory over-reclaim or other memory 26 reclaim issues. 27 28 New API 29 ======== 30 31 Since 4.12 we do have a generic scope API for both NOFS and NOIO context 32 ``memalloc_nofs_save``, ``memalloc_nofs_restore`` respectively ``memalloc_noio_save``, 33 ``memalloc_noio_restore`` which allow to mark a scope to be a critical 34 section from a filesystem or I/O point of view. Any allocation from that 35 scope will inherently drop __GFP_FS respectively __GFP_IO from the given 36 mask so no memory allocation can recurse back in the FS/IO. 37 38 .. kernel-doc:: include/linux/sched/mm.h 39 :functions: memalloc_nofs_save memalloc_nofs_restore 40 .. kernel-doc:: include/linux/sched/mm.h 41 :functions: memalloc_noio_save memalloc_noio_restore 42 43 FS/IO code then simply calls the appropriate save function before 44 any critical section with respect to the reclaim is started - e.g. 45 lock shared with the reclaim context or when a transaction context 46 nesting would be possible via reclaim. The restore function should be 47 called when the critical section ends. All that ideally along with an 48 explanation what is the reclaim context for easier maintenance. 49 50 Please note that the proper pairing of save/restore functions 51 allows nesting so it is safe to call ``memalloc_noio_save`` or 52 ``memalloc_noio_restore`` respectively from an existing NOIO or NOFS 53 scope. 54 55 What about __vmalloc(GFP_NOFS) 56 ============================== 57 58 vmalloc doesn't support GFP_NOFS semantic because there are hardcoded 59 GFP_KERNEL allocations deep inside the allocator which are quite non-trivial 60 to fix up. That means that calling ``vmalloc`` with GFP_NOFS/GFP_NOIO is 61 almost always a bug. The good news is that the NOFS/NOIO semantic can be 62 achieved by the scope API. 63 64 In the ideal world, upper layers should already mark dangerous contexts 65 and so no special care is required and vmalloc should be called without 66 any problems. Sometimes if the context is not really clear or there are 67 layering violations then the recommended way around that is to wrap ``vmalloc`` 68 by the scope API with a comment explaining the problem.
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