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Subject: Re: [PATCH 7/7] mm: compaction: Introduce sync-light migration for
 use by compaction
From: Shaohua Li <shaohua.li@intel.com>
To: Nai Xia <nai.xia@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>, Linux-MM <linux-mm@kvack.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Minchan Kim <minchan.kim@gmail.com>, Jan Kara <jack@suse.cz>,
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        LKML <linux-kernel@vger.kernel.org>
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Date: Wed, 23 Nov 2011 10:25:49 +0800
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On Wed, 2011-11-23 at 10:01 +0800, Nai Xia wrote:
> On Tue, Nov 22, 2011 at 2:56 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> > On Tue, 2011-11-22 at 02:36 +0800, Mel Gorman wrote:
> >> This patch adds a lightweight sync migrate operation MIGRATE_SYNC_LIGHT
> >> mode that avoids writing back pages to backing storage. Async
> >> compaction maps to MIGRATE_ASYNC while sync compaction maps to
> >> MIGRATE_SYNC_LIGHT. For other migrate_pages users such as memory
> >> hotplug, MIGRATE_SYNC is used.
> >>
> >> This avoids sync compaction stalling for an excessive length of time,
> >> particularly when copying files to a USB stick where there might be
> >> a large number of dirty pages backed by a filesystem that does not
> >> support ->writepages.
> > Hi,
> > from my understanding, with this, even writes
> > to /proc/sys/vm/compact_memory doesn't wait for pageout, is this
> > intended?
> > on the other hand, MIGRATE_SYNC_LIGHT now waits for pagelock and buffer
> > lock, so could wait on page read. page read and page out have the same
> > latency, why takes them different?
> 
> So for the problem you raised, I think my suggestion to Mel is to adopt the
> following logic:
> 
>            if (!trylock_page(page) && !PageUptodate(page))
>                       we are quite likely to block on read, so we
>                       depend on yet another MIGRATE_SYNC_MODE to decide
>                       if we really want to lock_page() and wait for this IO.
> 
> How do you think ?
assume the PageUptodate() is at the check for 'goto out'. yes, looks
reasonable to me. And we need similar check for buffer_head.

Thanks,
Shaohua