Re: [PATCH mmotm/next] mm: memcontrol: rewrite charge API: fix shmem_unuse

[Johannes Weiner]

On Mon, Jun 30, 2014 at 03:48:39PM -0700, Hugh Dickins wrote:
> Under shmem swapping and swapoff load, I sometimes hit the
> VM_BUG_ON_PAGE(!page->mapping) in mem_cgroup_commit_charge() at
> mm/memcontrol.c:6502!  Each time it has been a call from shmem_unuse().
> 
> Yes, there are some cases (most commonly when the page being unswapped
> is in a file being unlinked and evicted at that time) when the charge
> should not be committed.  In the old scheme, the page got uncharged
> again on release; but in the new scheme, it hits that BUG beforehand.
> 
> It's a useful BUG, so adapt shmem_unuse() to allow for it.  Which needs
> more info from shmem_unuse_inode(): so abuse -EAGAIN internally to
> replace the previous !found state (-ENOENT would be a more natural
> code, but that's exactly what you get when the swap has been evicted).
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Thanks, Hugh!
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