Re: [PATCH v2] mm/hugetlbfs Fix bugs in fallocate hole punch of areas with holes

Naoya Horiguchi
2015-11-11T03:03:37Z

Hello Mike,

On Tue, Nov 10, 2015 at 05:38:01PM -0800, Mike Kravetz wrote:
> This is against linux-stable 4.3. Will send to stable@vger.kernel.org
> when Ack'ed here.

This is not what stable stuff works, please see
Documentation/stable_kernel_rules.txt.

> Hugh Dickins pointed out problems with the new hugetlbfs fallocate
> hole punch code. These problems are in the routine remove_inode_hugepages
> and mostly occur in the case where there are holes in the range of
> pages to be removed. These holes could be the result of a previous hole
> punch or simply sparse allocation.
>
> remove_inode_hugepages handles both hole punch and truncate operations.
> Page index handling was fixed/cleaned up so that the loop index always
> matches the page being processed. The code now only makes a single pass
> through the range of pages as it was determined page faults could not
> race with truncate. A cond_resched() was added after removing up to
> PAGEVEC_SIZE pages.
>
> Some totally unnecessary code in hugetlbfs_fallocate() that remained from
> early development was also removed.
>
> v2:
> Make remove_inode_hugepages simpler after verifying truncate can not
> race with page faults here.
>
> Fixes: b5cec28d36f5 ("hugetlbfs: truncate_hugepages() takes a range of pages")

Cc: stable@vger.kernel.org [4.3]

> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
> fs/hugetlbfs/inode.c | 57 ++++++++++++++++++++++++++--------------------------
> 1 file changed, 29 insertions(+), 28 deletions(-)
>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 316adb9..8290f39 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -332,12 +332,15 @@ static void remove_huge_page(struct page *page)
> * truncation is indicated by end of range being LLONG_MAX
> * In this case, we first scan the range and release found pages.
> * After releasing pages, hugetlb_unreserve_pages cleans up region/reserv
> - * maps and global counts.
> + * maps and global counts. Page faults can not race with truncation
> + * in this routine. hugetlb_no_page() prevents page faults in the
> + * truncated range.

Could you be specific about how/why? Maybe hugetlb_fault_mutex_hash and/or
i_size check should be mentioned, because it's not so obvious.

> * hole punch is indicated if end is not LLONG_MAX
> * In the hole punch case we scan the range and release found pages.
> * Only when releasing a page is the associated region/reserv map
> * deleted. The region/reserv map for ranges without associated
> - * pages are not modified.
> + * pages are not modified. Page faults can race with hole punch.
> + * This is indicated if we find a mapped page.
> * Note: If the passed end of range value is beyond the end of file, but
> * not LLONG_MAX this routine still performs a hole punch operation.
> */
> @@ -361,44 +364,38 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
> next = start;
> while (next < end) {
> /*
> - * Make sure to never grab more pages that we
> - * might possibly need.
> + * Don't grab more pages than the number left in the range.
> */
> if (end - next < lookup_nr)
> lookup_nr = end - next;
>
> /*
> - * This pagevec_lookup() may return pages past 'end',
> - * so we must check for page->index > end.
> + * When no more pages are found, we are done.
> */
> - if (!pagevec_lookup(&pvec, mapping, next, lookup_nr)) {
> - if (next == start)
> - break;
> - next = start;
> - continue;
> - }
> + if (!pagevec_lookup(&pvec, mapping, next, lookup_nr))
> + break;
>
> for (i = 0; i < pagevec_count(&pvec); ++i) {
> struct page *page = pvec.pages[i];
> u32 hash;
>
> + /*
> + * The page (index) could be beyond end. This is
> + * only possible in the punch hole case as end is
> + * max page offset in the truncate case.
> + */
> + next = page->index;
> + if (next >= end)
> + break;
> +
> hash = hugetlb_fault_mutex_hash(h, current->mm,
> &pseudo_vma,
> mapping, next, 0);
> mutex_lock(&hugetlb_fault_mutex_table[hash]);
>
> lock_page(page);
> - if (page->index >= end) {
> - unlock_page(page);
> - mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> - next = end; /* we are done */
> - break;
> - }
> -
> /*
> - * If page is mapped, it was faulted in after being
> - * unmapped. Do nothing in this race case. In the
> - * normal case page is not mapped.
> + * In the normal case the page is not mapped.
> */
> if (!page_mapped(page)) {

I feel that doing like "likely(!page_mapped(page))" without comment is enough
and self-descriptive.

> bool rsv_on_error = !PagePrivate(page);
> @@ -421,17 +418,24 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
> hugetlb_fix_reserve_counts(
> inode, rsv_on_error);
> }
> + } else {
> + /*
> + * If page is mapped, it was faulted in after
> + * being unmapped. It indicates a race between
> + * hole punch and page fault. Do nothing in
> + * this case. Getting here in a truncate
> + * operation is a bug.
> + */
> + BUG_ON(truncate_op);
> }
>
> - if (page->index > next)
> - next = page->index;
> -
> ++next;

My comment was ignored for some reason?
http://marc.info/?l=linux-kernel&m=144705235903057&w=2

Anyway, I think the patch's idea is OK, so

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Thanks,
Naoya Horiguchi

> unlock_page(page);
>
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> }
> huge_pagevec_release(&pvec);
> + cond_resched();
> }
>
> if (truncate_op)
> @@ -647,9 +651,6 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
> if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + len > inode->i_size)
> i_size_write(inode, offset + len);
> inode->i_ctime = CURRENT_TIME;
> - spin_lock(&inode->i_lock);
> - inode->i_private = NULL;
> - spin_unlock(&inode->i_lock);
> out:
> mutex_unlock(&inode->i_mutex);
> return error;
> --
> 2.4.3
>