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    SubjectRe: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"
    On Thu, Aug 17, 2023 at 10:47 AM Yang Shi <shy828301@gmail.com> wrote:
    >
    > On Wed, Aug 16, 2023 at 2:48 PM Zach O'Keefe <zokeefe@google.com> wrote:
    > >
    > > > We have a out of tree driver that maps huge pages through a file handle and
    > > > relies on -> huge_fault. It used to work in 5.19 kernels but 6.1 changed this
    > > > behaviour.
    > > >
    > > > I don’t think reverting the earlier behaviour of fault_path for huge pages should
    > > > impact kernel negatively.
    > > >
    > > > Do you think we can restore this earlier behaviour of kernel to allow page fault
    > > > for huge pages via ->huge_fault.
    > >
    > > That seems reasonable to me. I think using the existence of a
    > > ->huge_fault() handler as a predicate to return "true" makes sense to
    > > me. The "normal" flow for file-backed memory along fault path still
    > > needs to return "false", so that we correctly fallback to ->fault()
    > > handler. Unless there are objections, I can do that in a v2.
    >
    > Sorry for chiming in late. I'm just back from vacation and trying to catch up...
    >
    > IIUC the out-of-tree driver tries to allocate huge page and install
    > PMD mapping via huge_fault() handler, but the cleanup of
    > hugepage_vma_check() prevents this due to the check to
    > VM_NO_KHUGEPAGED?
    >
    > So you would like to check whether a huge_fault() handler existed
    > instead of vma_is_dax()?

    Sorry for the multiple threads here. There are two problems: (a) the
    VM_NO_KHUGEPAGED check along fault path, and (b) we don't give
    ->huge_fault() a fair shake, if it exists, along fault path. The
    current code assumes vma_is_dax() iff ->huge_fault() exists.

    (a) is easy enough to fix. For (b), I'm currently looking at the
    possibility of not worrying about ->huge_fault() in
    hugepage_vma_check(), and just letting create_huge_pud() /
    create_huge_pmd() check and fallback as necessary. I think we'll need
    the explicit DAX check still, since we want to keep khugepaged and
    MADV_COLLAPSE away, and the presence / absence of ->huge_fault() isn't
    enough to know that (well.. today it kind of is, but we shouldn't
    depend on it).

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