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Date: Tue, 25 Aug 2020 17:42:57 -0700 From: Andrew Morton <akpm@...ux-foundation.org> To: Joonsoo Kim <js1304@...il.com> Cc: Linux Memory Management List <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, Michal Hocko <mhocko@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>, kernel-team@....com, Joonsoo Kim <iamjoonsoo.kim@....com> Subject: Re: [PATCH for v5.9] mm/page_alloc: handle a missing case for memalloc_nocma_{save/restore} APIs On Tue, 25 Aug 2020 14:34:32 +0900 Joonsoo Kim <js1304@...il.com> wrote: > > > > That's a bunch more code on a very hot path to serve an obscure feature > > which has a single obscure callsite. > > > > Can we instead put the burden on that callsite rather than upon > > everyone? For (dumb) example, teach __gup_longterm_locked() to put the > > page back if it's CMA and go get another one? > > Hmm... Unfortunately, it cannot ensure that we eventually get the non-CMA page. > I think that the only way to ensure it is to implement the > functionality here. We can > use 'unlikely' or 'static branch' to reduce the overhead for a really > rare case but > for now I have no idea how to completely remove the overhead. Gee, there must be something? Provide the gup code with a special entry point which takes the page straight from __rmqueue() and bypasses the pcp lists?
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