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    SubjectRe: [PATCH v1] fs/proc/task_mmu: Fix loss of young/dirty bits during pagemap scan
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    Thanks!

    On 4/29/24 4:40 PM, Ryan Roberts wrote:
    > make_uffd_wp_pte() was previously doing:
    >
    > pte = ptep_get(ptep);
    > ptep_modify_prot_start(ptep);
    > pte = pte_mkuffd_wp(pte);
    > ptep_modify_prot_commit(ptep, pte);
    >
    > But if another thread accessed or dirtied the pte between the first 2
    > calls, this could lead to loss of that information. Since
    > ptep_modify_prot_start() gets and clears atomically, the following is
    > the correct pattern and prevents any possible race. Any access after the
    > first call would see an invalid pte and cause a fault:
    >
    > pte = ptep_modify_prot_start(ptep);
    > pte = pte_mkuffd_wp(pte);
    > ptep_modify_prot_commit(ptep, pte);
    >
    > Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
    > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
    Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
    Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>

    > ---
    > fs/proc/task_mmu.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    > index 23fbab954c20..af4bc1da0c01 100644
    > --- a/fs/proc/task_mmu.c
    > +++ b/fs/proc/task_mmu.c
    > @@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma,
    > pte_t old_pte;
    >
    > old_pte = ptep_modify_prot_start(vma, addr, pte);
    > - ptent = pte_mkuffd_wp(ptent);
    > + ptent = pte_mkuffd_wp(old_pte);
    > ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
    > } else if (is_swap_pte(ptent)) {
    > ptent = pte_swp_mkuffd_wp(ptent);
    > --
    > 2.25.1
    >

    --
    BR,
    Muhammad Usama Anjum

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