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

[Michal Hocko]

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.
-- 
Michal Hocko
SUSE Labs
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