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    SubjectRe: [PATCH] mm: skip the page buddy block instead of one page
    Hello,

    On Thu, Aug 15, 2013 at 11:46:07AM +0800, Xishi Qiu wrote:
    > On 2013/8/15 10:44, Minchan Kim wrote:
    >
    > > Hi Xishi,
    > >
    > > On Thu, Aug 15, 2013 at 10:32:50AM +0800, Xishi Qiu wrote:
    > >> On 2013/8/15 2:00, Mel Gorman wrote:
    > >>
    > >>>>> Even if the page is still page buddy, there is no guarantee that it's
    > >>>>> the same page order as the first read. It could have be currently
    > >>>>> merging with adjacent buddies for example. There is also a really
    > >>>>> small race that a page was freed, allocated with some number stuffed
    > >>>>> into page->private and freed again before the second PageBuddy check.
    > >>>>> It's a bit of a hand grenade. How much of a performance benefit is there
    > >>>>
    > >>>> 1. Just worst case is skipping pageblock_nr_pages
    > >>>
    > >>> No, the worst case is that page_order returns a number that is
    > >>> completely garbage and low_pfn goes off the end of the zone
    > >>>
    > >>>> 2. Race is really small
    > >>>> 3. Higher order page allocation customer always have graceful fallback.
    > >>>>
    > >>
    > >> Hi Minchan,
    > >> I think in this case, we may get the wrong value from page_order(page).
    > >>
    > >> 1. page is in page buddy
    > >>
    > >>> if (PageBuddy(page)) {
    > >>
    > >> 2. someone allocated the page, and set page->private to another value
    > >>
    > >>> int nr_pages = (1 << page_order(page)) - 1;
    > >>
    > >> 3. someone freed the page
    > >>
    > >>> if (PageBuddy(page)) {
    > >>
    > >> 4. we will skip wrong pages
    > >
    > > So, what's the result by that?
    > > As I said, it's just skipping (pageblock_nr_pages -1) at worst case
    >
    > Hi Minchan,
    > I mean if the private is set to a large number, it will skip 2^private
    > pages, not (pageblock_nr_pages -1). I find somewhere will use page->private,
    > such as fs. Here is the comment about parivate.
    > /* Mapping-private opaque data:
    > * usually used for buffer_heads
    > * if PagePrivate set; used for
    > * swp_entry_t if PageSwapCache;
    > * indicates order in the buddy
    > * system if PG_buddy is set.
    > */

    Please read full thread in detail.

    Mel suggested following as

    if (PageBuddy(page)) {
    int nr_pages = (1 << page_order(page)) - 1;
    if (PageBuddy(page)) {
    nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1);
    low_pfn += nr_pages;
    continue;
    }
    }

    min(nr_pages, xxx) removes your concern but I think Mel's version
    isn't right. It should be aligned with pageblock boundary so I
    suggested following.

    if (PageBuddy(page)) {
    #ifdef CONFIG_MEMORY_ISOLATION
    unsigned long order = page_order(page);
    if (PageBuddy(page)) {
    low_pfn += (1 << order) - 1;
    low_pfn = min(low_pfn, end_pfn);
    }
    #endif
    continue;
    }

    so worst case is (pageblock_nr_pages - 1).
    but we don't need to add CONFIG_MEMORY_ISOLATION so my suggestion
    is following as.

    if (PageBuddy(page)) {
    unsigned long order = page_order(page);
    if (PageBuddy(page)) {
    low_pfn += (1 << order) - 1;
    low_pfn = min(low_pfn, end_pfn);
    }
    continue;
    }


    > Thanks,
    > Xishi Qiu
    >
    > > and the case you mentioned is right academically and I and Mel
    > > already pointed out that. But how often could that happen in real
    > > practice? I believe such is REALLY REALLY rare.
    > > So, as Mel said, if you have some workloads to see the benefit
    > > from this patch, I think we could accept the patch.
    > > Could you try and respin with the number?
    > > I guess big contigous memory range or memory-hotplug which are
    > > full of free pages in embedded CPU which is rather slower than server
    > > or desktop side could have benefit.
    > >
    > > Thanks.
    > >
    > >>
    > >>> nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1);
    > >>> low_pfn += nr_pages;
    > >>> continue;
    > >>> }
    > >>> }
    > >>>
    > >>> It's still race-prone meaning that it really should be backed by some
    > >>> performance data justifying it.
    > >>>
    > >>
    > >>
    > >>
    > >
    >
    >
    >

    --
    Kind regards,
    Minchan Kim


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