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    Subject[PATCH v8 03/23] mm: Check against orig_pte for finish_fault()
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    We used to check against none pte in finish_fault(), with the assumption
    that the orig_pte is always none pte.

    This change prepares us to be able to call do_fault() on !none ptes. For
    example, we should allow that to happen for pte marker so that we can restore
    information out of the pte markers.

    Let's change the "pte_none" check into detecting changes since we fetched
    orig_pte. One trivial thing to take care of here is, when pmd==NULL for
    the pgtable we may not initialize orig_pte at all in handle_pte_fault().

    By default orig_pte will be all zeros however the problem is not all
    architectures are using all-zeros for a none pte. pte_clear() will be the
    right thing to use here so that we'll always have a valid orig_pte value
    for the whole handle_pte_fault() call.

    Signed-off-by: Peter Xu <peterx@redhat.com>
    ---
    mm/memory.c | 9 ++++++++-
    1 file changed, 8 insertions(+), 1 deletion(-)

    diff --git a/mm/memory.c b/mm/memory.c
    index 3f396241a7db..b1af996b09ca 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -4241,7 +4241,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
    vmf->address, &vmf->ptl);
    ret = 0;
    /* Re-check under ptl */
    - if (likely(pte_none(*vmf->pte)))
    + if (likely(pte_same(*vmf->pte, vmf->orig_pte)))
    do_set_pte(vmf, page, vmf->address);
    else
    ret = VM_FAULT_NOPAGE;
    @@ -4709,6 +4709,13 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
    * concurrent faults and from rmap lookups.
    */
    vmf->pte = NULL;
    + /*
    + * Always initialize orig_pte. This matches with below
    + * code to have orig_pte to be the none pte if pte==NULL.
    + * This makes the rest code to be always safe to reference
    + * it, e.g. in finish_fault() we'll detect pte changes.
    + */
    + pte_clear(vmf->vma->vm_mm, vmf->address, &vmf->orig_pte);
    } else {
    /*
    * If a huge pmd materialized under us just retry later. Use
    --
    2.32.0
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