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    SubjectRe: Linux 2.6.26-rc7

    On Sun, 22 Jun 2008, Linus Torvalds wrote:
    > Hmm. Can you see which part of it broke? Was it the "fix XIP" part ot the
    > ZERO_PAGE part? The easiest way to test is to apply this patch, and see
    > (it just disables the XIP part of fix)

    Side note: considering what the patch fixes, I don't actually think we can
    undo it. But there's a possibility that we've actually uncovered an old
    bug that just was very unusual before, because the FOLL_ANON code was much
    harder to trigger before that patch (it only triggered if you didn't have
    a page table fully set up - now it triggers for even just a single missing
    page rather than a whole page table).

    And I do see something pretty iffy in the logic for whether it can use
    FOLL_ANON or not - it had a nonsensical test for VM_LOCKED (which makes no
    sense at all), and it had never been taught about the ->nopfn() way of
    filling page tables.

    So assuming it's not the XIP fix, you could try this patch instead. It
    replaces the (insane) use of VM_LOCKED with VM_SHARED (which is a lot more
    meaningful for the case of ZERO_PAGE, but strictly speaking probably
    doesn't matter either), and it teaches it about the fact that
    non-anonymous pages can be populated not just with the "->fault" handler,
    but with "->nopfn" too.

    I really don't think it's due to this (nobody sane really uses '->nopfn'),
    but if the XIP disabling patch doesn't make a difference, give it a try.

    (Most of the patch is obviously the fact that I moved the conditionals
    into a helper inline function to make the dang thing more readable). The
    actual change is trivial.


    mm/memory.c | 12 ++++++++++--
    1 files changed, 10 insertions(+), 2 deletions(-)

    diff --git a/mm/memory.c b/mm/memory.c
    index 9aefaae..8c5675f 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -1045,6 +1045,15 @@ no_page_table:
    return page;

    +/* Can we do the FOLL_ANON optimization? */
    +static inline int use_zero_page(struct vm_area_struct *vma)
    + if (vma->vm_flags & VM_SHARED)
    + return 0;
    + return !vma->vm_ops ||
    + (!vma->vm_ops->fault && !vma->vm_ops->nopfn);
    int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
    unsigned long start, int len, int write, int force,
    struct page **pages, struct vm_area_struct **vmas)
    @@ -1119,8 +1128,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
    foll_flags = FOLL_TOUCH;
    if (pages)
    foll_flags |= FOLL_GET;
    - if (!write && !(vma->vm_flags & VM_LOCKED) &&
    - (!vma->vm_ops || !vma->vm_ops->fault))
    + if (!write && use_zero_page(vma))
    foll_flags |= FOLL_ANON;

    do {

     \ /
      Last update: 2008-06-22 19:29    [W:0.022 / U:1.372 seconds]
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