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From:   Song Liu <songliubraving@fb.com>
To:     Hugh Dickins <hughd@google.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [PATCH] uprobes: __replace_page() avoid BUG in munlock_vma_page()
Thread-Topic: [PATCH] uprobes: __replace_page() avoid BUG in
 munlock_vma_page()
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Date:   Mon, 17 Aug 2020 05:43:06 +0000
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> On Aug 16, 2020, at 1:44 PM, Hugh Dickins <hughd@google.com> wrote:
>=20
> syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(),
> when called from uprobes __replace_page().  Which of many ways to fix it?
> Settled on not calling when PageCompound (since Head and Tail are equals
> in this context, PageCompound the usual check in uprobes.c, and the prior
> use of FOLL_SPLIT_PMD will have cleared PageMlocked already).
>=20
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: stable@vger.kernel.org # v5.4+
> ---
> This one is not a 5.9-rc regression, but still good to fix.
>=20
> kernel/events/uprobes.c |    2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>=20
> --- v5.9-rc/kernel/events/uprobes.c	2020-08-12 19:46:50.851196584 -0700
> +++ linux/kernel/events/uprobes.c	2020-08-16 13:18:35.292821674 -0700
> @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area
> 		try_to_free_swap(old_page);
> 	page_vma_mapped_walk_done(&pvmw);
>=20
> -	if (vma->vm_flags & VM_LOCKED)
> +	if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page))

Do we need munlock_vma_page() for THP page head?=20

Thanks,
Song

> 		munlock_vma_page(old_page);
> 	put_page(old_page);
>=20