.. _gfp_mask_from_fs_io:

================================= GFP masks used from FS/IO context

:Date: May, 2018 :Author: Michal Hocko [email protected]

Introduction

Code paths in the filesystem and IO stacks must be careful when allocating memory to prevent recursion deadlocks caused by direct memory reclaim calling back into the FS or IO paths and blocking on already held resources (e.g. locks - most commonly those used for the transaction context).

The traditional way to avoid this deadlock problem is to clear __GFP_FS respectively __GFP_IO (note the latter implies clearing the first as well) in the gfp mask when calling an allocator. GFP_NOFS respectively GFP_NOIO can be used as shortcut. It turned out though that above approach has led to abuses when the restricted gfp mask is used "just in case" without a deeper consideration which leads to problems because an excessive use of GFP_NOFS/GFP_NOIO can lead to memory over-reclaim or other memory reclaim issues.

New API

Since 4.12 we do have a generic scope API for both NOFS and NOIO context memalloc_nofs_save, memalloc_nofs_restore respectively memalloc_noio_save, memalloc_noio_restore which allow to mark a scope to be a critical section from a filesystem or I/O point of view. Any allocation from that scope will inherently drop __GFP_FS respectively __GFP_IO from the given mask so no memory allocation can recurse back in the FS/IO.

.. kernel-doc:: include/linux/sched/mm.h :functions: memalloc_nofs_save memalloc_nofs_restore .. kernel-doc:: include/linux/sched/mm.h :functions: memalloc_noio_save memalloc_noio_restore

FS/IO code then simply calls the appropriate save function before any critical section with respect to the reclaim is started - e.g. lock shared with the reclaim context or when a transaction context nesting would be possible via reclaim. The restore function should be called when the critical section ends. All that ideally along with an explanation what is the reclaim context for easier maintenance.

Please note that the proper pairing of save/restore functions allows nesting so it is safe to call memalloc_noio_save or memalloc_noio_restore respectively from an existing NOIO or NOFS scope.

What about __vmalloc(GFP_NOFS)

Since v5.17, and specifically after the commit 451769ebb7e79 ("mm/vmalloc: alloc GFP_NO{FS,IO} for vmalloc"), GFP_NOFS/GFP_NOIO are now supported in [k]vmalloc by implicitly using scope API.

In earlier kernels vmalloc didn't support GFP_NOFS semantic because there were hardcoded GFP_KERNEL allocations deep inside the allocator. That means that calling vmalloc with GFP_NOFS/GFP_NOIO was almost always a bug.

In the ideal world, upper layers should already mark dangerous contexts and so no special care is required and vmalloc should be called without any problems. Sometimes if the context is not really clear or there are layering violations then the recommended way around that (on pre-v5.17 kernels) is to wrap vmalloc by the scope API with a comment explaining the problem.