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

Theodore Ts'o
2015-07-02T14:26:04Z

On Wed, Jul 01, 2015 at 03:37:15PM +0200, Michal Hocko wrote:
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 37e90db1520b..6c44d424968e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -995,7 +995,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> goto keep_locked;
>
> /* Case 3 above */
> - } else {
> + } else if (sc->gfp_mask & __GFP_FS) {
> wait_on_page_writeback(page);
> }
> }

Um, I've just taken a closer look at this code now that I'm back from
vacation, and I'm not sure this is right. This Case 3 code occurs
inside an

if (PageWriteback(page)) {
...
}

conditional, and if I'm not mistaken, if the flow of control exits
this conditional, it is assumed that the page is *not* under writeback.
This patch will assume the page has been cleaned if __GFP_FS is set,
which could lead to a dirty page getting dropped, so I believe this is
a bug. No?

It would seem to me that a better fix would be to change the Case 2
handling:

/* Case 2 above */
} else if (global_reclaim(sc) ||
- !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
+ !PageReclaim(page) || !(sc->gfp_mask & __GFP_FS)) {
/*
* This is slightly racy - end_page_writeback()
* might have just cleared PageReclaim, then
* setting PageReclaim here end up interpreted
* as PageReadahead - but that does not matter
* enough to care. What we do want is for this
* page to have PageReclaim set next time memcg
* reclaim reaches the tests above, so it will
* then wait_on_page_writeback() to avoid OOM;
* and it's also appropriate in global reclaim.
*/
SetPageReclaim(page);
nr_writeback++;

goto keep_locked;

Am I missing something?

- Ted