forked from Minki/linux
d43f2c98f6
Now that we have kerneldoc comments for
memalloc_no{fs,io}_{save_restore}(), go ahead and pull them into the docs.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
67 lines
2.9 KiB
ReStructuredText
67 lines
2.9 KiB
ReStructuredText
=================================
|
|
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.
|
|
|
|
.. 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)
|
|
==============================
|
|
|
|
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.
|