[CC Joonsoo, Mel] On 09/09/2015 08:31 PM, Vitaly Wool wrote: > Hi Laura, > > On Wed, Sep 9, 2015 at 7:56 PM, Laura Abbott <labbott@xxxxxxxxxx> wrote: > >> (cc-ing linux-mm) >> On 09/09/2015 07:44 AM, Vitaly Wool wrote: >> >>> __zone_watermark_ok() does not corrrectly take high-order >>> CMA pageblocks into account: high-order CMA blocks are not >>> removed from the watermark check. Moreover, CMA pageblocks >>> may suddenly vanish through CMA allocation, so let's not >>> regard these pages as free in __zone_watermark_ok(). >>> >>> This patch also adds some primitive testing for the method >>> implemented which has proven that it works as it should. >>> >>> >> The choice to include CMA as part of watermarks was pretty deliberate. >> Do you have a description of the problem you are facing with >> the watermark code as is? Any performance numbers? >> >> > let's start with facing the fact that the calculation in > __zone_watermark_ok() is done incorrectly for the case when ALLOC_CMA is > not set. While going through pages by order it is implicitly considered You're not the first who tried to fix it, I think Joonsoo tried as well? I think the main objection was against further polluting fastpaths due to CMA. Note that Mel has a patchset removing high-order watermark checks (in the last patch of https://lwn.net/Articles/655406/ ) so this will be moot afterwards. > that CMA pages can be used and this impacts the result of the function. > > This can be solved in a slightly different way compared to what I proposed > but it needs per-order CMA pages accounting anyway. Then it would have > looked like: > > 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; > #ifdef CONFIG_CMA > if (!(alloc_flags & ALLOC_CMA)) > free_pages -= z->free_area[o].nr_free_cma << o; > /* Require fewer higher order pages to be free */ > min >>= 1; > ... > > But what we have also seen is that CMA pages may suddenly disappear due to > CMA allocator work so the whole watermark checking was still unreliable, > causing compaction to not run when it ought to and thus leading to Well, watermark checking is inherently racy. CMA pages disappearing is no exception, non-CMA pages may disappear as well. > (otherwise redundant) low memory killer operations, so I decided to propose > a safer method instead. > > Best regards, > Vitaly > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>