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> Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
