linux-kernel - Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped

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Date:   Mon, 21 Aug 2023 17:26:41 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
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        Minchan Kim <minchan@...nel.org>,
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        kernel list <linux-kernel@...r.kernel.org>,
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Subject: Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp()
 versus uffd

On Mon, Aug 21, 2023 at 12:51:20PM -0700, Hugh Dickins wrote:
> Jann Horn demonstrated how userfaultfd ioctl UFFDIO_COPY into a private
> shmem mapping can add valid PTEs to page table collapse_pte_mapped_thp()
> thought it had emptied: page lock on the huge page is enough to protect
> against WP faults (which find the PTE has been cleared), but not enough
> to protect against userfaultfd.  "BUG: Bad rss-counter state" followed.
> 
> retract_page_tables() protects against this by checking !vma->anon_vma;
> but we know that MADV_COLLAPSE needs to be able to work on private shmem
> mappings, even those with an anon_vma prepared for another part of the
> mapping; and we know that MADV_COLLAPSE needs to work on shared shmem
> mappings which are userfaultfd_armed().  Whether it needs to work on
> private shmem mappings which are userfaultfd_armed(), I'm not so sure:
> but assume that it does.
> 
> Just for this case, take the pmd_lock() two steps earlier: not because
> it gives any protection against this case itself, but because ptlock
> nests inside it, and it's the dropping of ptlock which let the bug in.
> In other cases, continue to minimize the pmd_lock() hold time.
> 
> Reported-by: Jann Horn <jannh@...gle.com>
> Closes: https://lore.kernel.org/linux-mm/CAG48ez0FxiRC4d3VTu_a9h=rg5FW-kYD5Rg5xo_RDBM0LTTqZQ@mail.gmail.com/
> Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

The locking is indeed slightly complicated.. but I didn't spot anything
wrong.

Acked-by: Peter Xu <peterx@...hat.com>

Thanks,

-- 
Peter Xu