GFP masks used from FS/IO context

Date:May, 2018 Author:Michal Hocko <mhocko@kernel.org>

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.

unsigned int memalloc_nofs_save(void)

Marks implicit GFP_NOFS allocation scope.

Parameters

void

no arguments

Description

This functions marks the beginning of the GFP_NOFS allocation scope. All further allocations will implicitly drop __GFP_FS flag and so they are safe for the FS critical section from the allocation recursion point of view. Use memalloc_nofs_restore to end the scope with flags returned by this function.

This function is safe to be used from any context.

void memalloc_nofs_restore(unsigned int flags)

Ends the implicit GFP_NOFS scope.

Parameters

unsigned int flags

Flags to restore.

Description

Ends the implicit GFP_NOFS scope started by memalloc_nofs_save function. Always make sure that that the given flags is the return value from the pairing memalloc_nofs_save call.

unsigned int memalloc_noio_save(void)

Marks implicit GFP_NOIO allocation scope.

Parameters

void

no arguments

Description

This functions marks the beginning of the GFP_NOIO allocation scope. All further allocations will implicitly drop __GFP_IO flag and so they are safe for the IO critical section from the allocation recursion point of view. Use memalloc_noio_restore to end the scope with flags returned by this function.

This function is safe to be used from any context.

void memalloc_noio_restore(unsigned int flags)

Ends the implicit GFP_NOIO scope.

Parameters

unsigned int flags

Flags to restore.

Description

Ends the implicit GFP_NOIO scope started by memalloc_noio_save function. Always make sure that that the given flags is the return value from the pairing memalloc_noio_save call.

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)

vmalloc doesn’t support GFP_NOFS semantic because there are hardcoded GFP_KERNEL allocations deep inside the allocator which are quite non-trivial to fix up. That means that calling vmalloc with GFP_NOFS/GFP_NOIO is almost always a bug. The good news is that the NOFS/NOIO semantic can be achieved by the scope API.

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 is to wrap vmalloc by the scope API with a comment explaining the problem.