Message-Id: <20081015153641.afcc94e5.akpm@linux-foundation.org>
Date:	Wed, 15 Oct 2008 15:36:41 -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, cmm@...ibm.com
Subject: Re: [PATCH] vmscan: set try_to_release_page's gfp_mask to 0

On Wed, 13 Aug 2008 15:24:40 +0900
Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp> wrote:

> At 12:21 08/08/13, Andrew Morton wrote:
> >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!
> 
> Sorry, I do not have performance number yet. I'll try this.
> 

This patch remains in a stalled state...

And then there's this:

: 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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