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    SubjectRe: [PATCH 1/6] mlock: only hold mmap_sem in shared mode when faulting in pages
    On Thu, Dec 9, 2010 at 10:11 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
    > On Wednesday, December 8, 2010, Michel Lespinasse <walken@google.com> wrote:
    >>
    >> Yes, patch 1/6 changes the long hold time to be in read mode instead
    >> of write mode, which is only a band-aid. But, this prepares for patch
    >> 5/6, which releases mmap_sem whenever there is contention on it or
    >> when blocking on disk reads.
    >
    > I have to say that I'm not a huge fan of that horribly kludgy
    > contention check case.
    >
    > The "move page-in to read-locked sequence" and the changes to
    > get_user_pages look fine, but the contention thing is just disgusting.
    > I'd really like to see some other approach if at all possible.

    Andrew, should I amend my patches to remove the rwsem_is_contended() code ?
    This would involve:
    - remove rwsem-implement-rwsem_is_contended.patch and
    x86-rwsem-more-precise-rwsem_is_contended-implementation.patch
    - in mlock-do-not-hold-mmap_sem-for-extended-periods-of-time.patch,
    drop the one hunk making use of rwsem_is_contended (rest of the patch
    would still work without it)
    - optionally, follow up patch to limit batch size to a constant
    in do_mlock_pages():

    diff --git a/mm/mlock.c b/mm/mlock.c
    index 569ae6a..a505a7e 100644
    --- a/mm/mlock.c
    +++ b/mm/mlock.c
    @@ -457,15 +457,23 @@ static int do_mlock_pages(unsigned long start, size_t len)
    continue;
    if (nstart < vma->vm_start)
    nstart = vma->vm_start;
    + /*
    + * Constrain batch size to limit mmap_sem hold time.
    + */
    + if (nend > nstart + 1024 * PAGE_SIZE)
    + nend = nstart + 1024 * PAGE_SIZE;
    /*
    * Now fault in a range of pages. __mlock_vma_pages_range()
    * double checks the vma flags, so that it won't mlock pages
    * if the vma was already munlocked.
    */
    ret = __mlock_vma_pages_range(vma, nstart, nend, &locked);
    if (ret < 0) {
    ret = __mlock_posix_error_return(ret);
    break;
    + } else if (locked) {
    + locked = 0;
    + up_read(&mm->mmap_sem);
    }
    nend = nstart + ret * PAGE_SIZE;
    ret = 0;

    I don't really prefer using a constant, but I'm not sure how else to make
    Linus happy :)

    --
    Michel "Walken" Lespinasse
    A program is never fully debugged until the last user dies.



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