Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS allocations

[Michal Hocko <mhocko@suse.cz>]

[CCing Hugh as well]

On Wed 01-07-15 08:17:31, Michal Hocko wrote:
> On Tue 30-06-15 17:17:17, Michal Hocko wrote:
> [...]
> > Hi,
> > the issue has been reported http://marc.info/?l=linux-kernel&m=143522730927480.
> > This obviously requires a patch ot make ext4_ext_grow_indepth call
> > sb_getblk with the GFP_NOFS mask but that one makes sense on its own
> > and Ted has mentioned he will push it. I haven't marked the patch for
> > stable yet. This is the first time the issue has been reported and
> > ext4 writeout code has changed considerably in 3.11 and I am not sure
> > the issue was present before. e62e384e9da8 which has introduced the
> > wait_on_page_writeback has been merged in 3.6 which is quite some time
> > ago. If we go with stable I would suggest marking it for 3.11+ and it
> > should obviously go with the ext4_ext_grow_indepth fix.
> 
> After Dave's additional explanation
> (http://marc.info/?l=linux-ext4&m=143570521212215) it is clear that the
> lack of __GFP_FS check was wrong from the very beginning. XFS is doing
> the similar thing from before the e62e384e9da8 was merged. I guess we
> were just lucky not to hit this problem sooner.
> 
> That being said I think the patch should be marked for stable and the
> changelog updated:
> 
> As per David Chinner the xfs is doing similar thing since 2.6.15 already
> so ext4 is not the only affected filesystem. Moreover he notes:
> : For example: IO completion might require unwritten extent conversion
> : which executes filesystem transactions and GFP_NOFS allocations. The
> : writeback flag on the pages can not be cleared until unwritten
> : extent conversion completes. Hence memory reclaim cannot wait on
> : page writeback to complete in GFP_NOFS context because it is not
> : safe to do so, memcg reclaim or otherwise.
> 
> Cc: stable # 3.6+
> Fixes: e62e384e9da8 ("memcg: prevent OOM with too many dirty pages")
> 
> Andrew let me know whether I should repost the patch with the updated
> changelog or you can take it from here.

Hmm, I have double checked the original commit and it turned out my
memory was failing me. e62e384e9da8 had may_enter_fs check. This has
been changed later in the same merge window by c3b94f44fcb0 ("memcg:
further prevent OOM with too many dirty pages") and the code has been
refactored later some more. So the changelog needs some more rewording:
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