From: Mina Almasry <almasrymina@google.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>,
Peter Xu <peterx@redhat.com>, Linux-MM <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY
Date: Thu, 13 May 2021 17:23:00 -0700 [thread overview]
Message-ID: <CAHS8izPFc+bSrKN-6gRguGefAqrj6kXaMUvgeUL5U7QxhXfWDw@mail.gmail.com> (raw)
In-Reply-To: <09dc0712-48e8-8ba2-f170-4c2febcfff83@oracle.com>
On Thu, May 13, 2021 at 5:14 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 5/13/21 4:49 PM, Mina Almasry wrote:
> > On Thu, May 13, 2021 at 4:43 PM Mina Almasry <almasrymina@google.com> wrote:
> >>
> >> When hugetlb_mcopy_atomic_pte() is called with:
> >> - mode==MCOPY_ATOMIC_NORMAL and,
> >> - we already have a page in the page cache corresponding to the
> >> associated address,
> >>
> >> We will allocate a huge page from the reserves, and then fail to insert it
> >> into the cache and return -EEXIST. In this case, we need to return -EEXIST
> >> without allocating a new page as the page already exists in the cache.
> >> Allocating the extra page causes the resv_huge_pages to underflow temporarily
> >> until the extra page is freed.
> >>
> >> To fix this we check if a page exists in the cache, and allocate it and
> >> insert it in the cache immediately while holding the lock. After that we
> >> copy the contents into the page.
> >>
> >> As a side effect of this, pages may exist in the cache for which the
> >> copy failed and for these pages PageUptodate(page) == false. Modify code
> >> that query the cache to handle this correctly.
> >>
> >
> > To be honest, I'm not sure I've done this bit correctly. Please take a
> > look and let me know what you think. It may be too overly complicated
> > to have !PageUptodate() pages in the cache and ask the rest of the
> > code to handle that edge case correctly, but I'm not sure how else to
> > fix this issue.
> >
>
> I think you just moved the underflow from hugetlb_mcopy_atomic_pte to
> hugetlb_no_page. Why?
>
> Consider the case where there is only one reserve left and someone does
> the MCOPY_ATOMIC_NORMAL for the address. We will allocate the page and
> consume the reserve (reserve count == 0) and insert the page into the
> cache. Now, if the copy_huge_page_from_user fails we must drop the
> locks/fault mutex to do the copy. While locks are dropped, someone
> faults on the address and ends up in hugetlb_no_page. The page is in
> the cache but not up to date, so we go down the allocate new page path
> and will decrement the reserve count again to cause underflow.
>
For what it's worth, I think I fixed the underflow with this patch,
not moved it. I added a check in hugetlb_no_page() such that if we
find a page in the cache with !PageUptodate(page), we will reuse that
page instead of allocating a new one and decrementing the count again.
Running the test with the WARN_ONCE_ON locally shows no warnings
again.
> How about this approach?
I'll give it a shot for sure. FWIW on first glance it looks more
complicated that what I have here, but my guess I'm not doing the
!PageUptodate() handling correctly and that's why it seems this
solution is simpler. I'll give it a shot though.
> - Keep the check for hugetlbfs_pagecache_present in hugetlb_mcopy_atomic_pte
> that you added. That will catch the race where the page was added to
> the cache before entering the routine.
> - With the above check in place, we only need to worry about the case
> where copy_huge_page_from_user fails and we must drop locks. In this
> case we:
> - Free the page previously allocated.
> - Allocate a 'temporary' huge page without consuming reserves. I'm
> thinking of something similar to page migration.
> - Drop the locks and let the copy_huge_page_from_user be done to the
> temporary page.
> - When reentering hugetlb_mcopy_atomic_pte after dropping locks (the
> *pagep case) we need to once again check
> hugetlbfs_pagecache_present.
> - We then try to allocate the huge page which will consume the
> reserve. If successful, copy contents of temporary page to newly
> allocated page. Free temporary page.
>
> There may be issues with this, and I have not given it deep thought. It
> does abuse the temporary huge page concept, but perhaps no more than
> page migration. Things do slow down if the extra page allocation and
> copy is required, but that would only be the case if copy_huge_page_from_user
> needs to be done without locks. Not sure, but hoping that is rare.
> --
> Mike Kravetz
next prev parent reply other threads:[~2021-05-14 0:23 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-13 23:43 [PATCH] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY Mina Almasry
2021-05-13 23:49 ` Mina Almasry
2021-05-14 0:14 ` Mike Kravetz
2021-05-14 0:23 ` Mina Almasry [this message]
2021-05-14 4:02 ` Mike Kravetz
2021-05-14 12:31 ` Peter Xu
2021-05-14 17:56 ` Mike Kravetz
2021-05-14 18:30 ` Axel Rasmussen
2021-05-14 19:16 ` Peter Xu
2021-05-20 19:18 ` Mina Almasry
2021-05-20 19:21 ` Mina Almasry
2021-05-20 20:00 ` Mike Kravetz
2021-05-20 20:31 ` Mina Almasry
2021-05-21 2:05 ` Mina Almasry
-- strict thread matches above, loose matches on Subject: below --
2021-05-12 3:06 resv_huge_page underflow with userfaultfd test Mike Kravetz
[not found] ` <20210512065813.89270-1-almasrymina@google.com>
2021-05-12 7:44 ` [PATCH] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY Mina Almasry
[not found] ` <CAJHvVch0ZMapPVEc0Ge5V4KDgNDNhECbqwDi0y9XxsxFXQZ-gg@mail.gmail.com>
[not found] ` <c455d241-11f6-95a6-eb29-0ddd94eedbd7@oracle.com>
2021-05-12 19:42 ` Mina Almasry
2021-05-12 20:14 ` Peter Xu
2021-05-12 21:31 ` Mike Kravetz
2021-05-12 21:52 ` Mina Almasry
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