Messages in this thread | | | Date | Tue, 19 Sep 2023 10:52:56 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH 1/1] mm: lock VMAs skipped by a failed queue_pages_range() |
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On Mon 18-09-23 14:16:08, Suren Baghdasaryan wrote: > When queue_pages_range() encounters an unmovable page, it terminates > its page walk. This walk, among other things, locks the VMAs in the range. > This termination might result in some VMAs being left unlock after > queue_pages_range() completes. Since do_mbind() continues to operate on > these VMAs despite the failure from queue_pages_range(), it will encounter > an unlocked VMA. > This mbind() behavior has been modified several times before and might > need some changes to either finish the page walk even in the presence > of unmovable pages or to error out immediately after the failure to > queue_pages_range(). However that requires more discussions, so to > fix the immediate issue, explicitly lock the VMAs in the range if > queue_pages_range() failed. The added condition does not save much > but is added for documentation purposes to understand when this extra > locking is needed.
The semantic of the walk in this case is really clear as mud. I was trying to reconstruct the whole picture and it really hurts... Then I found http://lkml.kernel.org/r/CAHbLzkrmTaqBRmHVdE2kyW57Uoghqd_E+jAXC9cB5ofkhL-uvw@mail.gmail.com and that helped a lot. Let's keep it a reference at least in the email thread here for future.
> Fixes: 49b0638502da ("mm: enable page walking API to lock vmas during the walk") > Reported-by: syzbot+b591856e0f0139f83023@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/all/000000000000f392a60604a65085@google.com/ > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
I cannot say I like the patch (it looks like a potential double locking unless you realize this lock is special) but considering this might be just temporal I do not mind.
Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!
> --- > mm/mempolicy.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 42b5567e3773..cbc584e9b6ca 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -1342,6 +1342,9 @@ static long do_mbind(unsigned long start, unsigned long len, > vma_iter_init(&vmi, mm, start); > prev = vma_prev(&vmi); > for_each_vma_range(vmi, vma, end) { > + /* If queue_pages_range failed then not all VMAs might be locked */ > + if (ret) > + vma_start_write(vma); > err = mbind_range(&vmi, vma, &prev, start, end, new); > if (err) > break; > -- > 2.42.0.459.ge4e396fd5e-goog
-- Michal Hocko SUSE Labs
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