linux-kernel - Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone

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Date:   Fri, 4 Dec 2020 11:24:56 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone

On Thu, Dec 3, 2020 at 2:36 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Thu, Dec 03, 2020 at 02:15:36PM -0500, Pavel Tatashin wrote:
>
> > I studied some more, and I think this is not a race:
> > list_add_tail(&head->lru, &cma_page_list) is called only when
> > isolate_lru_page(head) succeeds.
> > isolate_lru_page(head) succeeds only when PageLRU(head) is true.
> > However, in this function we also clear LRU flag before returning
> > success.
> > This means, that if we race with another thread, the other thread
> > won't get to unprotected list_add_tail(&head->lru, &cma_page_list)
> > until head is is back on LRU list.
>
> Oh interesting, I totally didn't see how that LRU stuff is
> working. So.. this creates a ridiculously expensive spin lock? Not
> broken, but yikes :|

Not really a spin lock, the second thread won't be able to isolate
this page, and will skip migration of this page.

>
> Jason