Re: [PATCH 7/7] mm: compaction: Introduce sync-light migration foruse by compaction

[Nai Xia]

On Wed, Nov 23, 2011 at 7:00 PM, Mel Gorman <mgorman@xxxxxxx> wrote:
> On Wed, Nov 23, 2011 at 10:01:53AM +0800, Nai Xia wrote:
>> On Tue, Nov 22, 2011 at 2:56 PM, Shaohua Li <shaohua.li@xxxxxxxxx> 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 ?
>>
>
> Where are you adding this check?
>
> If you mean in __unmap_and_move(), the check is unnecessary unless
> another subsystem starts using sync-light compaction. With this series,
> only direct compaction cares about MIGRATE_SYNC_LIGHT. If the page is

But I am still a little bit confused that if MIGRATE_SYNC_LIGHT is only
used by direct compaction and  another mode can be used by it:
MIGRATE_ASYNC also does not write dirty pages, then why not also
do an (current->flags & PF_MEMALLOC) test before writing out pages,
like we already did for the page lock condition, but adding a new mode
instead?


> not up to date, it is also locked during the IO and unlocked after
> setting Uptodate in the IO completion handler.
>
> As the page is locked, compaction will fail trylock_page, do the
> PF_MEMALLOC check and bail as it is not safe for direct compaction
> to call lock_page as the comment in __unmap_and_move explains. This
> should avoid the stall.
>
> Did I misunderstand your suggestion?
>
> --
> Mel Gorman
> SUSE Labs
>
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