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    SubjectRe: [PATCH v6 03/23] mm: Check against orig_pte for finish_fault()
    On Thu, Dec 16, 2021 at 01:38:33PM +0800, Peter Xu wrote:
    > On Thu, Dec 16, 2021 at 04:01:47PM +1100, Alistair Popple wrote:
    > > On Monday, 15 November 2021 6:55:02 PM AEDT Peter Xu wrote:
    > > > We used to check against none pte and in those cases orig_pte should always be
    > > > none pte anyway.
    > >
    > > Is that always true? From what I can see in handle_pte_fault() orig_pte only
    > > gets initialised in the !pmd_none() case so might not be pte_none.
    >
    > I believe it's true, otherwise I must have overlooked.
    >
    > IMHO it's not "when we set orig_pte" that matters - note that finish_fault()
    > (that this patch modifies) is only called for file-backed memories, and it's
    > only called in do_fault() where the pte is not mapped at all.
    >
    > DAX seems to call it too, but still DAX comes from do_fault() too, afaict.
    >
    > The pte will not be mapped in two cases in handle_pte_fault():
    >
    > - When pmd_none
    >
    > - When !pmd_none, however if we find that pte_none==true, that's:
    >
    > if (pte_none(vmf->orig_pte)) {
    > pte_unmap(vmf->pte);
    > vmf->pte = NULL;
    > }
    >
    > So when we're already in do_fault(), afaict, orig_pte must be pte_none().
    > Another side note is that, IIUC pte_none() is a looser check than the
    > pte_val()==0 and it should be arch dependent.

    So one more thing I forgot to mention... Of course above is based on the fact
    that orig_pte will be initialized to zero when creating vmf structure, and
    that's done in __handle_mm_fault():

    struct vm_fault vmf = {
    .vma = vma,
    .address = address & PAGE_MASK,
    .flags = flags,
    .pgoff = linear_page_index(vma, address),
    .gfp_mask = __get_fault_gfp_mask(vma),
    };

    I'm not sure whether I should explicitly set it to pte_val(0), in most C
    programs we'll already assume it's a proper reset of orig_pte value in c99
    initialization format, but if anyone thinks we should do that explicitly plus
    some comments I can do that too.

    >
    > Thanks,
    >
    > >
    > > > 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.
    > > >
    > > > Signed-off-by: Peter Xu <peterx@redhat.com>
    > > > ---
    > > > mm/memory.c | 2 +-
    > > > 1 file changed, 1 insertion(+), 1 deletion(-)
    > > >
    > > > diff --git a/mm/memory.c b/mm/memory.c
    > > > index 04662b010005..d5966d9e24c3 100644
    > > > --- a/mm/memory.c
    > > > +++ b/mm/memory.c
    > > > @@ -4052,7 +4052,7 @@ 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;
    > > >
    > >
    > >
    > >
    > >
    >
    > --
    > Peter Xu

    --
    Peter Xu

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