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    SubjectRe: [PATCH 1/8] mm: munlock use follow_page
    On Mon, 7 Sep 2009 22:29:55 +0100 (BST)
    Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

    > Hiroaki Wakabayashi points out that when mlock() has been interrupted
    > by SIGKILL, the subsequent munlock() takes unnecessarily long because
    > its use of __get_user_pages() insists on faulting in all the pages
    > which mlock() never reached.
    >
    > It's worse than slowness if mlock() is terminated by Out Of Memory kill:
    > the munlock_vma_pages_all() in exit_mmap() insists on faulting in all the
    > pages which mlock() could not find memory for; so innocent bystanders are
    > killed too, and perhaps the system hangs.
    >
    > __get_user_pages() does a lot that's silly for munlock(): so remove the
    > munlock option from __mlock_vma_pages_range(), and use a simple loop of
    > follow_page()s in munlock_vma_pages_range() instead; ignoring absent
    > pages, and not marking present pages as accessed or dirty.
    >
    > (Change munlock() to only go so far as mlock() reached? That does not
    > work out, given the convention that mlock() claims complete success even
    > when it has to give up early - in part so that an underlying file can be
    > extended later, and those pages locked which earlier would give SIGBUS.)
    >
    > Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
    > Cc: stable@kernel.org
    > ---
    >
    > mm/mlock.c | 99 ++++++++++++++++++++-------------------------------
    > 1 file changed, 40 insertions(+), 59 deletions(-)
    >
    > --- mm0/mm/mlock.c 2009-06-25 05:18:10.000000000 +0100
    > +++ mm1/mm/mlock.c 2009-09-07 13:16:15.000000000 +0100
    > @@ -139,49 +139,36 @@ static void munlock_vma_page(struct page
    > }
    >
    > /**
    > - * __mlock_vma_pages_range() - mlock/munlock a range of pages in the vma.
    > + * __mlock_vma_pages_range() - mlock a range of pages in the vma.
    > * @vma: target vma
    > * @start: start address
    > * @end: end address
    > - * @mlock: 0 indicate munlock, otherwise mlock.
    > *
    > - * If @mlock == 0, unlock an mlocked range;
    > - * else mlock the range of pages. This takes care of making the pages present ,
    > - * too.
    > + * This takes care of making the pages present too.
    > *
    > * return 0 on success, negative error code on error.
    > *
    > * vma->vm_mm->mmap_sem must be held for at least read.
    > */
    > static long __mlock_vma_pages_range(struct vm_area_struct *vma,
    > - unsigned long start, unsigned long end,
    > - int mlock)
    > + unsigned long start, unsigned long end)
    > {
    > struct mm_struct *mm = vma->vm_mm;
    > unsigned long addr = start;
    > struct page *pages[16]; /* 16 gives a reasonable batch */
    > int nr_pages = (end - start) / PAGE_SIZE;
    > int ret = 0;
    > - int gup_flags = 0;
    > + int gup_flags;
    >
    > VM_BUG_ON(start & ~PAGE_MASK);
    > VM_BUG_ON(end & ~PAGE_MASK);
    > VM_BUG_ON(start < vma->vm_start);
    > VM_BUG_ON(end > vma->vm_end);
    > - VM_BUG_ON((!rwsem_is_locked(&mm->mmap_sem)) &&
    > - (atomic_read(&mm->mm_users) != 0));
    > -
    > - /*
    > - * mlock: don't page populate if vma has PROT_NONE permission.
    > - * munlock: always do munlock although the vma has PROT_NONE
    > - * permission, or SIGKILL is pending.
    > - */
    > - if (!mlock)
    > - gup_flags |= GUP_FLAGS_IGNORE_VMA_PERMISSIONS |
    > - GUP_FLAGS_IGNORE_SIGKILL;
    > + VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
    >
    > + gup_flags = 0;
    > if (vma->vm_flags & VM_WRITE)
    > - gup_flags |= GUP_FLAGS_WRITE;
    > + gup_flags = GUP_FLAGS_WRITE;
    >
    > while (nr_pages > 0) {
    > int i;
    > @@ -201,19 +188,10 @@ static long __mlock_vma_pages_range(stru
    > * This can happen for, e.g., VM_NONLINEAR regions before
    > * a page has been allocated and mapped at a given offset,
    > * or for addresses that map beyond end of a file.
    > - * We'll mlock the the pages if/when they get faulted in.
    > + * We'll mlock the pages if/when they get faulted in.
    > */
    > if (ret < 0)
    > break;
    > - if (ret == 0) {
    > - /*
    > - * We know the vma is there, so the only time
    > - * we cannot get a single page should be an
    > - * error (ret < 0) case.
    > - */
    > - WARN_ON(1);
    > - break;
    > - }
    >
    > lru_add_drain(); /* push cached pages to LRU */
    >
    > @@ -224,28 +202,22 @@ static long __mlock_vma_pages_range(stru
    > /*
    > * Because we lock page here and migration is blocked
    > * by the elevated reference, we need only check for
    > - * page truncation (file-cache only).
    > + * file-cache page truncation. This page->mapping
    > + * check also neatly skips over the ZERO_PAGE(),
    > + * though if that's common we'd prefer not to lock it.
    > */
    > - if (page->mapping) {
    > - if (mlock)
    > - mlock_vma_page(page);
    > - else
    > - munlock_vma_page(page);
    > - }
    > + if (page->mapping)
    > + mlock_vma_page(page);
    > unlock_page(page);
    > - put_page(page); /* ref from get_user_pages() */
    > -
    > - /*
    > - * here we assume that get_user_pages() has given us
    > - * a list of virtually contiguous pages.
    > - */
    > - addr += PAGE_SIZE; /* for next get_user_pages() */
    > - nr_pages--;
    > + put_page(page); /* ref from get_user_pages() */
    > }
    > +
    > + addr += ret * PAGE_SIZE;
    > + nr_pages -= ret;
    > ret = 0;
    > }
    >
    > - return ret; /* count entire vma as locked_vm */
    > + return ret; /* 0 or negative error code */
    > }
    >
    > /*
    > @@ -289,7 +261,7 @@ long mlock_vma_pages_range(struct vm_are
    > is_vm_hugetlb_page(vma) ||
    > vma == get_gate_vma(current))) {
    >
    > - __mlock_vma_pages_range(vma, start, end, 1);
    > + __mlock_vma_pages_range(vma, start, end);
    >
    > /* Hide errors from mmap() and other callers */
    > return 0;
    > @@ -310,7 +282,6 @@ no_mlock:
    > return nr_pages; /* error or pages NOT mlocked */
    > }
    >
    > -
    > /*
    > * munlock_vma_pages_range() - munlock all pages in the vma range.'
    > * @vma - vma containing range to be munlock()ed.
    > @@ -330,10 +301,24 @@ no_mlock:
    > * free them. This will result in freeing mlocked pages.
    > */
    > void munlock_vma_pages_range(struct vm_area_struct *vma,
    > - unsigned long start, unsigned long end)
    > + unsigned long start, unsigned long end)
    > {
    > + unsigned long addr;
    > +
    > + lru_add_drain();
    > vma->vm_flags &= ~VM_LOCKED;
    > - __mlock_vma_pages_range(vma, start, end, 0);
    > +
    > + for (addr = start; addr < end; addr += PAGE_SIZE) {
    > + struct page *page = follow_page(vma, addr, FOLL_GET);
    > + if (page) {
    > + lock_page(page);
    > + if (page->mapping)
    > + munlock_vma_page(page);

    Could you add "please see __mlock_vma_pages_range() to see why" or some here ?

    Thanks,
    -Kame

    > + unlock_page(page);
    > + put_page(page);
    > + }
    > + cond_resched();
    > + }
    > }
    >
    > /*
    > @@ -400,18 +385,14 @@ success:
    > * It's okay if try_to_unmap_one unmaps a page just after we
    > * set VM_LOCKED, __mlock_vma_pages_range will bring it back.
    > */
    > - vma->vm_flags = newflags;
    >
    > if (lock) {
    > - ret = __mlock_vma_pages_range(vma, start, end, 1);
    > -
    > - if (ret > 0) {
    > - mm->locked_vm -= ret;
    > - ret = 0;
    > - } else
    > - ret = __mlock_posix_error_return(ret); /* translate if needed */
    > + vma->vm_flags = newflags;
    > + ret = __mlock_vma_pages_range(vma, start, end);
    > + if (ret < 0)
    > + ret = __mlock_posix_error_return(ret);
    > } else {
    > - __mlock_vma_pages_range(vma, start, end, 0);
    > + munlock_vma_pages_range(vma, start, end);
    > }
    >
    > out:
    > --
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