Re: [PATCH 1/8] mm: munlock use follow_page

KAMEZAWA Hiroyuki
2009-09-08T03:00:31Z

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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