On Thu, Sep 14, 2023 at 08:53:59PM +0000, Suren Baghdasaryan wrote: > On Thu, Sep 14, 2023 at 8:00 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > On Thu, Sep 14, 2023 at 7:09 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > On Thu, Sep 14, 2023 at 06:20:56PM +0000, Suren Baghdasaryan wrote: > > > > I think I found the problem and the explanation is much simpler. While > > > > walking the page range, queue_folios_pte_range() encounters an > > > > unmovable page and queue_folios_pte_range() returns 1. That causes a > > > > break from the loop inside walk_page_range() and no more VMAs get > > > > locked. After that the loop calling mbind_range() walks over all VMAs, > > > > even the ones which were skipped by queue_folios_pte_range() and that > > > > causes this BUG assertion. > > > > > > > > Thinking what's the right way to handle this situation (what's the > > > > expected behavior here)... > > > > I think the safest way would be to modify walk_page_range() and make > > > > it continue calling process_vma_walk_lock() for all VMAs in the range > > > > even when __walk_page_range() returns a positive err. Any objection or > > > > alternative suggestions? > > > > > > So we only return 1 here if MPOL_MF_MOVE* & MPOL_MF_STRICT were > > > specified. That means we're going to return an error, no matter what, > > > and there's no point in calling mbind_range(). Right? > > > > > > +++ b/mm/mempolicy.c > > > @@ -1334,6 +1334,8 @@ static long do_mbind(unsigned long start, unsigned long len, > > > ret = queue_pages_range(mm, start, end, nmask, > > > flags | MPOL_MF_INVERT, &pagelist, true); > > > > > > + if (ret == 1) > > > + ret = -EIO; > > > if (ret < 0) { > > > err = ret; > > > goto up_out; > > > > > > (I don't really understand this code, so it can't be this simple, can > > > it? Why don't we just return -EIO from queue_folios_pte_range() if > > > this is the right answer?) > > > > Yeah, I'm trying to understand the expected behavior of this function > > to make sure we are not missing anything. I tried a simple fix that I > > suggested in my previous email and it works but I want to understand a > > bit more about this function's logic before posting the fix. > > So, current functionality is that after queue_pages_range() encounters > an unmovable page, terminates the loop and returns 1, mbind_range() > will still be called for the whole range > (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1345), > all pages in the pagelist will be migrated > (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1355) > and only after that the -EIO code will be returned > (https://elixir.bootlin.com/linux/latest/source/mm/mempolicy.c#L1362). > So, if we follow Matthew's suggestion we will be altering the current > behavior which I assume is not what we want to do. Right, I'm intentionally changing the behaviour. My thinking is that mbind(MPOL_MF_MOVE | MPOL_MF_STRICT) is going to fail. Should such a failure actually move the movable pages before reporting that it failed? I don't know. > The simple fix I was thinking about that would not alter this behavior > is smth like this: I don't like it, but can we run it past syzbot to be sure it solves the issue and we're not chasing a ghost here?