Re: [patch 5/5]thp: split huge page if head page is isolated

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On Fri, Oct 28, 2011 at 01:11:55PM +0800, Shaohua Li wrote:
> On Fri, 2011-10-28 at 07:34 +0800, Minchan Kim wrote:
> > On Tue, Oct 25, 2011 at 10:59:40AM +0800, Shaohua Li wrote:
> > > With current logic, if page reclaim finds a huge page, it will just reclaim
> > > the head page and leave tail pages reclaimed later. Let's take an example,
> > > lru list has page A and B, page A is huge page:
> > > 1. page A is isolated
> > > 2. page B is isolated
> > > 3. shrink_page_list() adds page A to swap page cache. so page A is split.
> > > page A+1, page A+2, ... are added to lru list.
> > > 4. shrink_page_list() adds page B to swap page cache.
> > > 5. page A and B is written out and reclaimed.
> > > 6. page A+1, A+2 ... is isolated and reclaimed later.
> > > So the reclaim order is A, B, ...(maybe other pages), A+1, A+2 ...
> > 
> > I don't see your code yet but have a question.
> > You mitigate this problem by 4/5 which could add subpages into lru tail
> > so subpages would reclaim next interation of reclaim.
> > 
> > What do we need 5/5?
> > Do I miss something?
> Both patches are required. without this patch, current page reclaim will
> only reclaim the first page of a huge page, because the hugepage isn't
> split yet. The hugepage is split when the first page is being written to
> swap, which is too later and page reclaim might already isolated a lot
> of pages.

When split happens, subpages would be located in tail of LRU by your 4/5.
(Assume tail of LRU is old age).
In addtion, isolation happens 32 page chunk so the subpages would be isolated
and reclaimed in next iteration. I think 32 pages are not too many.

What do you think about it?
-- 
Kind regards,
Minchan Kim

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