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    SubjectRe: [patch 5/5]thp: split huge page if head page is isolated
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    On Fri, 2011-10-28 at 17:50 +0800, Minchan Kim wrote:
    > On Fri, Oct 28, 2011 at 04:25:56PM +0800, Shaohua Li wrote:
    > > On Fri, 2011-10-28 at 15:30 +0800, Minchan Kim wrote:
    > > > 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).
    > > yes, but a lot of other pages already isolated. we will reclaim those
    > > pages first. for example, reclaim huge page A, B. current reclaim order
    > > is A, B, A+1, ... B+1, because we will isolated A and B first, all tail
    > > pages are not isolated yet. While with my patch, the order is A, A
    > > +1, ... B, B+1,.... with my patch, we can avoid unnecessary page split
    > > or page isolation. This is exactly why my patch reduces the thp_split
    > > count.
    >
    > It's possbile but I doubt how it is effective becuase add_to_swap has a unlikely as follows
    >
    > if (unlikely(PageTransHuge(page)))
    >
    > I don't mean unlikely assumption is absolutely right.
    > But at least, you have to convince us of it's wrong.
    > Personally, I don't want to add more logic and handling THP pages
    > different with normal page unless it's real concern.
    if you actually use THP, you will find it's a problem. The data I posted
    already clearly showed it.



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