Re: [PATCH] mm: Allow GFP_IOFS for page_cache_read page cache allocation

[Michal Hocko]

On Thu 12-11-15 10:53:01, Jan Kara wrote:
> On Wed 11-11-15 15:13:53, mhocko@xxxxxxxxxx wrote:
> > From: Michal Hocko <mhocko@xxxxxxxx>
> > 
> > page_cache_read has been historically using page_cache_alloc_cold to
> > allocate a new page. This means that mapping_gfp_mask is used as the
> > base for the gfp_mask. Many filesystems are setting this mask to
> > GFP_NOFS to prevent from fs recursion issues. page_cache_read is
> > called from the vm_operations_struct::fault() context during the page
> > fault. This context doesn't need the reclaim protection normally.
> > 
> > ceph and ocfs2 which call filemap_fault from their fault handlers
> > seem to be OK because they are not taking any fs lock before invoking
> > generic implementation. xfs which takes XFS_MMAPLOCK_SHARED is safe
> > from the reclaim recursion POV because this lock serializes truncate
> > and punch hole with the page faults and it doesn't get involved in the
> > reclaim.
> > 
> > There is simply no reason to deliberately use a weaker allocation
> > context when a __GFP_FS | __GFP_IO can be used. The GFP_NOFS
> > protection might be even harmful. There is a push to fail GFP_NOFS
> > allocations rather than loop within allocator indefinitely with a
> > very limited reclaim ability. Once we start failing those requests
> > the OOM killer might be triggered prematurely because the page cache
> > allocation failure is propagated up the page fault path and end up in
> > pagefault_out_of_memory.
> > 
> > We cannot play with mapping_gfp_mask directly because that would be racy
> > wrt. parallel page faults and it might interfere with other users who
> > really rely on NOFS semantic from the stored gfp_mask. The mask is also
> > inode proper so it would even be a layering violation. What we can do
> > instead is to push the gfp_mask into struct vm_fault and allow fs layer
> > to overwrite it should the callback need to be called with a different
> > allocation context.
> > 
> > Initialize the default to (mapping_gfp_mask | __GFP_FS | __GFP_IO)
> > because this should be safe from the page fault path normally. Why do we
> > care about mapping_gfp_mask at all then? Because this doesn't hold only
> > reclaim protection flags but it also might contain zone and movability
> > restrictions (GFP_DMA32, __GFP_MOVABLE and others) so we have to respect
> > those.
> > 
> > Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> > ---
> > 
> > Hi,
> > this has been posted previously as a part of larger GFP_NOFS related
> > patch set (http://lkml.kernel.org/r/1438768284-30927-1-git-send-email-mhocko%40kernel.org)
> > but I think it makes sense to discuss it even out of that scope.
> > 
> > I would like to hear FS and other MM people about the proposed interface.
> > Using mapping_gfp_mask blindly doesn't sound good to me and vm_fault
> > looks like a proper channel to communicate between MM and FS layers.
> > 
> > Comments? Are there any better ideas?
> 
> Makes sense to me and the filesystems I know should be fine with this
> (famous last words ;). Feel free to add:
> 
> Acked-by: Jan Kara <jack@xxxxxxxx>

Thanks a lot! Are there any objections from other fs/mm people?
-- 
Michal Hocko
SUSE Labs
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