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Message-ID: <19dad66f-4ec8-358c-df7f-35481f855c81@redhat.com>
Date: Wed, 5 Apr 2023 17:54:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: Axel Rasmussen <axelrasmussen@...gle.com>,
Nadav Amit <nadav.amit@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>,
Yang Shi <shy828301@...il.com>,
linux-stable <stable@...r.kernel.org>
Subject: Re: [PATCH] mm/khugepaged: Check again on anon uffd-wp during
isolation
On 05.04.23 17:51, Peter Xu wrote:
> Khugepaged collapse an anonymous thp in two rounds of scans. The 2nd round
> done in __collapse_huge_page_isolate() after hpage_collapse_scan_pmd(),
> during which all the locks will be released temporarily. It means the
> pgtable can change during this phase before 2nd round starts.
>
> It's logically possible some ptes got wr-protected during this phase, and
> we can errornously collapse a thp without noticing some ptes are
> wr-protected by userfault. e1e267c7928f wanted to avoid it but it only did
> that for the 1st phase, not the 2nd phase.
>
> Since __collapse_huge_page_isolate() happens after a round of small page
> swapins, we don't need to worry on any !present ptes - if it existed
> khugepaged will already bail out. So we only need to check present ptes
> with uffd-wp bit set there.
>
> This is something I found only but never had a reproducer, I thought it was
> one caused a bug in Muhammad's recent pagemap new ioctl work, but it turns
> out it's not the cause of that but an userspace bug. However this seems to
> still be a real bug even with a very small race window, still worth to have
> it fixed and copy stable.
>
> Cc: linux-stable <stable@...r.kernel.org>
> Fixes: e1e267c7928f ("khugepaged: skip collapse if uffd-wp detected")
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> mm/khugepaged.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index a19aa140fd52..42ac93b4bd87 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -575,6 +575,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> result = SCAN_PTE_NON_PRESENT;
> goto out;
> }
> + if (pte_uffd_wp(pteval)) {
> + result = SCAN_PTE_UFFD_WP;
> + goto out;
> + }
> page = vm_normal_page(vma, address, pteval);
> if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
> result = SCAN_PAGE_NULL;
Yes, I agree that would be a small race where it could happen.
Reviewed-by: David Hildenbrand <david@...hat.com>
--
Thanks,
David / dhildenb
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