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

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Date:   Thu, 3 Dec 2020 11:40:15 -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

> 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

>
> > What I meant is the users of the interface do it incrementally not in
> > large chunks. For example:
> >
> > vfio_pin_pages_remote
> >    vaddr_get_pfn
> >         ret = pin_user_pages_remote(mm, vaddr, 1, flags |
> > FOLL_LONGTERM, page, NULL, NULL);
> > 1 -> pin only one pages at a time
>
> I don't know why vfio does this, it is why it so ridiculously slow at
> least.

Agreed.

>
> Jason