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    SubjectRe: Ack to merge through DRM? WAS Re: [PATCH v2 1/5] mm: Add write-protect and clean utilities for address space ranges
    On Thu, Sep 26, 2019 at 03:20:42PM -0700, Linus Torvalds wrote:
    > On Thu, Sep 26, 2019 at 1:55 PM Thomas Hellström (VMware)
    > <thomas_os@shipmail.org> wrote:
    > >
    > > Well, we're working on supporting huge puds and pmds in the graphics
    > > VMAs, although in the write-notify cases we're looking at here, we would
    > > probably want to split them down to PTE level.
    >
    > Well, that's what the existing walker code does if you don't have that
    > "pud_entry()" callback.
    >
    > That said, I assume you would *not* want to do that if the huge
    > pud/pmd is already clean and read-only, but just continue.
    >
    > So you may want to have a special pud_entry() that handles that case.
    > Eventually. Maybe. Although honestly, if you're doing dirty tracking,
    > I doubt it makes much sense to use largepages.
    >
    > > Looking at zap_pud_range() which when called from unmap_mapping_pages()
    > > uses identical locking (no mmap_sem), it seems we should be able to get
    > > away with i_mmap_lock(), making sure the whole page table doesn't
    > > disappear under us. So it's not clear to me why the mmap_sem is strictly
    > > needed here. Better to sort those restrictions out now rather than when
    > > huge entries start appearing.
    >
    > zap_pud_range()actually does have that
    >
    > VM_BUG_ON_VMA(!rwsem_is_locked(&tlb->mm->mmap_sem), vma);

    The VM_BUG is a blind copy from PMD layer and it's bogus. i_mmap_lock()
    works fine for file mappings.

    The PMD was intended for THP case at the time when there were only
    anon-THP. The check was relaxed and later dropped for file-THP on PMD
    level. It has to be dropped on PUD too. We don't have anon-THP on PUD
    level at all, only DAX played with them.

    --
    Kirill A. Shutemov

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