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

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Date:   Fri, 4 Dec 2020 13:06:31 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
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 Fri, Dec 04, 2020 at 11:24:56AM -0500, Pavel Tatashin wrote:
> 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.

It looks like the intent is that it will call gup again, then goto
check_again, and once again try to isolate the LRU. ie it loops.

If it gets to a point where all the CMA pages fail to isolate then it
simply exits with success as the cma_page_list will be empty.

Is this a bug? It seems like a bug, the invariant here is to not
return with a CMA page, so why do we have a path that does return with
a CMA page?

Jason