Messages in this thread | | | Subject | Re: [RFC] mm: a question about high-order check in __zone_watermark_ok() | From | Vlastimil Babka <> | Date | Mon, 26 Sep 2016 10:52:42 +0200 |
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[+CC Joonsoo Kim]
On 09/26/2016 10:47 AM, 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?
Yeah I think this limitation was known to CMA people.
> Also if we doing __alloc_pages_slowpath(), the compact will not work, because > __zone_watermark_ok() always return true, and it lead to alloc a high-order > unmovable page failed, then do direct reclaim.
I guess that can happen as well.
> Thanks, > Xishi Qiu >
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