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

Mike Kravetz
2015-11-11T03:39:59Z

On 11/10/2015 06:58 PM, Naoya Horiguchi wrote:
> 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.

Ok. I'll resend with the Cc.

>
>> 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]

Will add.

>
>> 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.

The long explanation is here:
http://marc.info/?l=linux-mm&m=144719585221409&w=2
I will include a brief summary here.

>
>> * 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.
>

Ok, makes sense

>> 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

My apologies. I somehow overlooked that e-mail. It was not my intention
to ignore your comments.

>From that comment, I agree than the ++next should be moved outside
the for look.

>
> Anyway, I think the patch's idea is OK, so
>
> Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Thanks for your comments. I'll respin shortly and incorporate your
comments.

--
Mike Kravetz

>
> 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