Re: [RFCv3 4/4] ext4: Move to shared iolock even without dioread_nolock mount opt

[Ritesh Harjani <riteshh@linux.ibm.com>]

Hello Jan,

Sorry about getting a little late on this.

On 11/20/19 8:02 PM, Jan Kara wrote:
> On Wed 20-11-19 10:30:24, Ritesh Harjani wrote:
>> We were using shared locking only in case of dioread_nolock
>> mount option in case of DIO overwrites. This mount condition
>> is not needed anymore with current code, since:-
>>
>> 1. No race between buffered writes & DIO overwrites.
>> Since buffIO writes takes exclusive locks & DIO overwrites
>> will take share locking. Also DIO path will make sure
>> to flush and wait for any dirty page cache data.
>>
>> 2. No race between buffered reads & DIO overwrites, since there
>> is no block allocation that is possible with DIO overwrites.
>> So no stale data exposure should happen. Same is the case
>> between DIO reads & DIO overwrites.
>>
>> 3. Also other paths like truncate is protected,
>> since we wait there for any DIO in flight to be over.
>>
>> 4. In case of buffIO writes followed by DIO reads:
>> Since here also we take exclusive locks in ext4_write_begin/end().
>> There is no risk of exposing any stale data in this case.
>> Since after ext4_write_end, iomap_dio_rw() will wait to flush &
>> wait for any dirty page cache data.
>>
>> Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
> 
> There's one more case to consider here as I mentioned in [1]. There can be

Yes, I should have mentioned about this in cover letter and about my
thoughts on that.
I was of the opinion that since the race is already existing
and it may not be caused due to this patch, so we should handle that in 
incremental fashion and as a separate patch series after this one.
Let me know your thoughts on above.

Also, I wanted to have some more discussions on this race before
making the changes.
But nevertheless, it's the right time to discuss those changes here.

> mmap write instantiating dirty page and then someone starting writeback
> against that page while DIO read is running still theoretically leading to
> stale data exposure. Now this patch does not have influence on that race
> but:

Yes, agreed.

> 
> 1) We need to close the race mentioned above. Maybe we could do that by
> proactively allocating unwritten blocks for a page being faulted when there
> is direct IO running against the file - the one who fills holes through
> mmap write while direct IO is running on the file deserves to suffer the
> performance penalty...

I was giving this a thought. So even if we try to penalize mmap
write as you mentioned above, what I am not sure about it, is that, how 
can we reliably detect that the DIO is in progress?

Say even if we try to check for atomic_read(&inode->i_dio_count) in mmap
ext4_page_mkwrite path, it cannot be reliable unless there is some sort 
of a lock protection, no?
Because after the check the DIO can still snoop in, right?


2. Also what about the delalloc opt. in that case? Even for delalloc
should we go ahead and allocate the unwritten blocks? That may even need
to start/stop the journal which could add more latency, no?


> 
> 2) After this patch there's no point in having dioread_nolock at all so we
> can just make that mount option no-op and drop all the precautions from the
> buffered IO path connected with dioread_nolock.

Yes, with some careful review we should be able to drop those
precautions related to dioread_nolock, after getting above race fixed.



> 
> [1] https://lore.kernel.org/linux-ext4/20190925092339.GB23277@quack2.suse.cz
> 
> 								Honza
> 
>> ---
>>   fs/ext4/file.c | 25 +++++++++++++++++++------
>>   1 file changed, 19 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>> index 18cbf9fa52c6..b97efc89cd63 100644
>> --- a/fs/ext4/file.c
>> +++ b/fs/ext4/file.c
>> @@ -383,6 +383,17 @@ static const struct iomap_dio_ops ext4_dio_write_ops = {
>>   	.end_io = ext4_dio_write_end_io,
>>   };
>>   
>> +static bool ext4_dio_should_shared_lock(struct inode *inode)
>> +{
>> +	if (!S_ISREG(inode->i_mode))
> 
> This cannot happen for DIO so no point in checking here.
> 
>> +		return false;
>> +	if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
> 
> Why this?
> 
>> +		return false;
>> +	if (ext4_should_journal_data(inode))
> 
> We don't do DIO when journalling data so no point in checking here
> (dio_supported() already checked this).
> 
> 								Honza
>> +		return false;
>> +	return true;
>> +}
>> +

Yes, agreed we don't need this function (ext4_dio_should_shared_lock)
anyways.


>>   /*
>>    * The intention here is to start with shared lock acquired then see if any
>>    * condition requires an exclusive inode lock. If yes, then we restart the
>> @@ -394,8 +405,8 @@ static const struct iomap_dio_ops ext4_dio_write_ops = {
>>    * - For extending writes case we don't take the shared lock, since it requires
>>    *   updating inode i_disksize and/or orphan handling with exclusive lock.
>>    *
>> - * - shared locking will only be true mostly in case of overwrites with
>> - *   dioread_nolock mode. Otherwise we will switch to excl. iolock mode.
>> + * - shared locking will only be true mostly in case of overwrites.
>> + *   Otherwise we will switch to excl. iolock mode.
>>    */
>>   static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct iov_iter *from,
>>   				 unsigned int *iolock, bool *unaligned_io,
>> @@ -433,15 +444,14 @@ static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct iov_iter *from,
>>   		*extend = true;
>>   	/*
>>   	 * Determine whether the IO operation will overwrite allocated
>> -	 * and initialized blocks. If so, check to see whether it is
>> -	 * possible to take the dioread_nolock path.
>> +	 * and initialized blocks.
>>   	 *
>>   	 * We need exclusive i_rwsem for changing security info
>>   	 * in file_modified().
>>   	 */
>>   	if (*iolock == EXT4_IOLOCK_SHARED &&
>>   	    (!IS_NOSEC(inode) || *unaligned_io || *extend ||
>> -	     !ext4_should_dioread_nolock(inode) ||
>> +	     !ext4_dio_should_shared_lock(inode) ||
>>   	     !ext4_overwrite_io(inode, offset, count))) {
>>   		ext4_iunlock(inode, *iolock);
>>   		*iolock = EXT4_IOLOCK_EXCL;
>> @@ -485,7 +495,10 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>   		iolock = EXT4_IOLOCK_EXCL;
>>   	}
>>   
>> -	if (iolock == EXT4_IOLOCK_SHARED && !ext4_should_dioread_nolock(inode))
>> +	/*
>> +	 * Check if we should continue with shared iolock
>> +	 */
>> +	if (iolock == EXT4_IOLOCK_SHARED && !ext4_dio_should_shared_lock(inode))
>>   		iolock = EXT4_IOLOCK_EXCL;
>>   
>>   	if (iocb->ki_flags & IOCB_NOWAIT) {
>> -- 
>> 2.21.0
>>