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    SubjectRe: [RFC] mm: a question about high-order check in __zone_watermark_ok()
    On Mon, Sep 26, 2016 at 01:02:31PM +0200, Michal Hocko wrote:
    > On Mon 26-09-16 18:17:50, Xishi Qiu wrote:
    > > On 2016/9/26 17:43, Michal Hocko wrote:
    > >
    > > > On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
    > > >> On 2016/9/26 16:58, Michal Hocko wrote:
    > > >>
    > > >>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
    > > >>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
    > > >>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
    > > >>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
    > > >>>> quietly fix a bug. Please see the following.
    > > >>>>
    > > >>>> Before this patch, the high-order check is this:
    > > >>>> __zone_watermark_ok()
    > > >>>> ...
    > > >>>> for (o = 0; o < order; o++) {
    > > >>>> /* At the next order, this order's pages become unavailable */
    > > >>>> free_pages -= z->free_area[o].nr_free << o;
    > > >>>>
    > > >>>> /* Require fewer higher order pages to be free */
    > > >>>> min >>= 1;
    > > >>>>
    > > >>>> if (free_pages <= min)
    > > >>>> return false;
    > > >>>> }
    > > >>>> ...
    > > >>>>
    > > >>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
    > > >>>>
    > > >>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
    > > >>>> and there are a lot of high-order cma pages, but little high-order unmovable
    > > >>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
    > > >>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
    > > >>>
    > > >>> AFAIR CMA wmark check was always tricky and the above commit has made
    > > >>> the situation at least a bit more clear. Anyway IIRC
    > > >>>
    > > >>> #ifdef CONFIG_CMA
    > > >>> /* If allocation can't use CMA areas don't use free CMA pages */
    > > >>> if (!(alloc_flags & ALLOC_CMA))
    > > >>> free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
    > > >>> #endif
    > > >>>
    > > >>> if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
    > > >>> return false;
    > > >>>
    > > >>> should reduce the prioblem because a lot of CMA pages should just get us
    > > >>> below the wmark + reserve boundary.
    > > >>
    > > >> Hi Michal,
    > > >>
    > > >> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
    > > >> are also enough, but they are fragment, then it will triger the problem.
    > > >> If we alloc a high-order unmovable page, water mark check return *true*, but we
    > > >> will alloc *failed*, right?
    > > >
    > > > As Vlastimil has written. There were known issues with the wmark checks
    > > > and high order requests.
    > >
    > > Shall we backport to stable?
    >
    > I dunno, it was a part of a larger series with high atomic reserves and
    > changes which sound a bit intrusive for the stable kernel. Considering
    > that CMA was known to be problematic and there are still some issues
    > left I do not think this is worth the trouble/risk.

    CMA problem is known one. I mentioned it on my ZONE_CMA series v1 but
    removed due to Mel's high atomic reserve series.

    That series is rather large and has some problems so I think that it
    is not suitable for stable tree.

    Thanks.

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