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

[Mike Kravetz]

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@xxxxxxxxxxxxxxx
>> 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@xxxxxxxxxxxxxxx [4.3]

Will add.

> 
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> ---
>>  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@xxxxxxxxxxxxx>

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
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