linux-kernel - Re: [PATCH] vmscan: set try_to_release_page's gfp

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Date:	Tue, 12 Aug 2008 20:21:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, Mingming Cao <cmm@...ibm.com>
Subject: Re: [PATCH] vmscan: set try_to_release_page's gfp_mask to 0

On Wed, 13 Aug 2008 11:21:16 +0900 Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp> wrote:

> Hi.
> 
> shrink_page_list passes gfp_mask to try_to_release_page.
> When shrink_page_list is called from kswapd or buddy system, gfp_mask is set
> and (gfp_mask & __GFP_WAIT) and (gfp_mask & __GFP_FS) check is positive.
> releasepage of jbd/jbd2(ext3/4, ocfs2) and XFS use this parameter. 
> If try_to_free_page fails due to bh busy in jbd/jbd2, jbd/jbd2 lets a thread wait for 
> committing transaction. I think this has big performance impacts for vmscan.
> So I modified shrink_page_list not to pass gfp_mask to try_to_release_page
> in ordered to improve vmscan performance.
> 
> Thanks.
> 
> Signed-off-by: Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
> 
> diff -Nrup linux-2.6.27-rc2.org/mm/vmscan.c linux-2.6.27-rc2.vmscan/mm/vmscan.c
> --- linux-2.6.27-rc2.org/mm/vmscan.c	2008-08-11 14:33:24.000000000 +0900
> +++ linux-2.6.27-rc2.vmscan/mm/vmscan.c	2008-08-12 18:57:05.000000000 +0900
> @@ -614,7 +614,7 @@ static unsigned long shrink_page_list(st
>  		* Otherwise, leave the page on the LRU so it is swappable.
>  		*/
>  		if (PagePrivate(page)) {
> -			if (!try_to_release_page(page, sc->gfp_mask))
> +			if (!try_to_release_page(page, 0))
>  				goto activate_locked;
>  			if (!mapping && page_count(page) == 1) {
>  				unlock_page(page);

I think the change makes sense.

Has this change been shown to improve any workloads?  If so, please
provide full information for the changelog.  If not, please mention
this and explain why benefits were not demonstrable.  This information
should _always_ be present in a "performance" patch's changelog!

Probably a better fix would be to explicitly tell
journal_try_to_free_buffers() when it need to block on journal commit,
rather than (mis)interpreting the gfp_t in this fashion.  I assume the
only caller who really cares is direct-io.  That would be quite a bit
of churn, and the asynchronous behaviour perhaps makes sense _anyway_
when called from page reclaim.

otoh, there is a risk that this change will cause page reclaim to sit
there burning huge amounts of CPU time and not achieving anything,
because all it is doing is scanning over busy pages.  In that case,
blocking behind a commit which would make those pages reclaimable is
correct behaviour.  But given that the offending code in
journal_try_to_free_buffers() has only been there for a few weeks, I
guess this isn't a concern.

Really, I think what this patch tells us is that 3f31fddf ("jbd: fix
race between free buffer and commit transaction") was an unpleasant
hack which had undesirable and unexpected side-effects.  I think - that
depends upon your as-yet-undisclosed testing results?

Perhaps we should revert 3f31fddf and have another think about how to
fix the direct-io -EIO problem.  One option would be to hold our noses
and add a new gfp_t flag for this specific purpose?

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