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

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Message-ID: <CA+CK2bBRqK6_ZOkTbz8qe4Gz6T5pwt-KjJQ0+NzYOC1qO1sPMg@mail.gmail.com>
Date:   Thu, 3 Dec 2020 12:14:11 -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 11:59 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Thu, Dec 03, 2020 at 11:40:15AM -0500, Pavel Tatashin wrote:
> > > Looking at this code some more.. How is it even correct?
> > >
> > > 1633                            if (!isolate_lru_page(head)) {
> > > 1634                                    list_add_tail(&head->lru, &cma_page_list);
> > >
> > > Here we are only running under the read side of the mmap sem so multiple
> > > GUPs can be calling that sequence in parallel. I don't see any
> > > obvious exclusion that will prevent corruption of head->lru. The first
> > > GUP thread to do isolate_lru_page() will ClearPageLRU() and the second
> > > GUP thread will be a NOP for isolate_lru_page().
> > >
> > > They will both race list_add_tail and other list ops. That is not OK.
> >
> > Good question. I studied it, and I do not see how this is OK. Worse,
> > this race is also exposable as a syscall instead of via driver: two
> > move_pages() run simultaneously. Perhaps in other places?
> >
> > move_pages()
> >   kernel_move_pages()
> >     mmget()
> >     do_pages_move()
> >       add_page_for_migratio()
> >          mmap_read_lock(mm);
> >          list_add_tail(&head->lru, pagelist); <- Not protected
>
> When this was CMA only it might have been rarer to trigger, but this
> move stuff sounds like it makes it much more broadly, eg on typical
> servers with RDMA exposed/etc
>
> Seems like it needs fixing as part of this too :\

Just to clarify the stack that I showed above is outside of gup, it is
the same issue that you pointed out that happens elsewhere. I suspect
there might be more. All of them should be addressed together.

Pasha

>
> Page at a time inside the gup loop could address both concerns, unsure
> about batching performance here though..
>
> Jason