linux-ext4 - Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP

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Date:	Thu, 2 Jul 2015 10:25:51 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Nikolay Borisov <kernel@...p.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>, Marian Marinov <mm@...com>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS
 allocations

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
							
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