Re: [RFCv3 3/4] ext4: start with shared iolock in case of DIO instead of excl. iolock

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

Hello Jan,

Thanks for a thorough review.

On 11/20/19 7:25 PM, Jan Kara wrote:
> On Wed 20-11-19 10:30:23, Ritesh Harjani wrote:
>> Earlier there was no shared lock in DIO read path.
>> But this patch (16c54688592ce: ext4: Allow parallel DIO reads)
>> simplified some of the locking mechanism while still allowing
>> for parallel DIO reads by adding shared lock in inode DIO
>> read path.
>>
>> But this created problem with mixed read/write workload.
>> It is due to the fact that in DIO path, we first start with
>> exclusive lock and only when we determine that it is a ovewrite
>> IO, we downgrade the lock. This causes the problem, since
>> with above patch we have shared locking in DIO reads.
>>
>> So, this patch tries to fix this issue by starting with
>> shared lock and then switching to exclusive lock only
>> when required based on ext4_dio_write_checks().
>>
>> Other than that, it also simplifies below cases:-
>>
>> 1. Simplified ext4_unaligned_aio API to ext4_unaligned_io.
>> Previous API was abused in the sense that it was not really checking
>> for AIO anywhere also it used to check for extending writes.
>> So this API was renamed and simplified to ext4_unaligned_io()
>> which actully only checks if the IO is really unaligned.
>>
>> Now, in case of unaligned direct IO, iomap_dio_rw needs to do
>> zeroing of partial block and that will require serialization
>> against other direct IOs in the same block. So we take a excl iolock
>> for any unaligned DIO.
>> In case of AIO we also need to wait for any outstanding IOs to
>> complete so that conversion from unwritten to written is completed
>> before anyone try to map the overlapping block. Hence we take
>> excl iolock and also wait for inode_dio_wait() for unaligned DIO case.
>> Please note since we are anyway taking an exclusive lock in unaligned IO,
>> inode_dio_wait() becomes a no-op in case of non-AIO DIO.
>>
>> 2. Added ext4_extending_io(). This checks if the IO is extending the file.
>>
>> 3. Added ext4_dio_write_checks().
>> In this we start with shared iolock and only switch to exclusive iolock
>> if required. So in most cases with aligned, non-extening, dioread_nolock
>> overwrites tries to write with a shared locking.
>> If not, then we restart the operation in ext4_dio_write_checks(),
>> after acquiring excl iolock.
>>
>> Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
>> ---
>>   fs/ext4/file.c | 191 ++++++++++++++++++++++++++++++++++++-------------
>>   1 file changed, 142 insertions(+), 49 deletions(-)
> 
> Thanks for the patch! Some comments below...
> 
>> @@ -365,15 +383,110 @@ static const struct iomap_dio_ops ext4_dio_write_ops = {
>>   	.end_io = ext4_dio_write_end_io,
>>   };
>>   
>> +/*
>> + * 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
>> + * whole operation by releasing the shared lock and acquiring exclusive lock.
>> + *
>> + * - For unaligned_io we never take shared lock as it may cause data corruption
>> + *   when two unaligned IO tries to modify the same block e.g. while zeroing.
>> + *
>> + * - 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.
>> + */
>> +static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct iov_iter *from,
>> +				 unsigned int *iolock, bool *unaligned_io,
>> +				 bool *extend)
>> +{
>> +	struct file *file = iocb->ki_filp;
>> +	struct inode *inode = file_inode(file);
>> +	loff_t offset = iocb->ki_pos;
>> +	loff_t final_size;
>> +	size_t count;
>> +	ssize_t ret;
>> +
>> +restart:
>> +	/* Fallback to buffered I/O if the inode does not support direct I/O. */
>> +	if (!ext4_dio_supported(inode)) {
>> +		ext4_iunlock(inode, *iolock);
>> +		return ext4_buffered_write_iter(iocb, from);
>> +	}
> 
> I don't think it is good to hide things like this fallback to buffered IO
> in ext4_dio_write_checks(). Similarly with 'unaligned_io' and 'extend'

Yes, make sense. Yup will move above block of code in
ext4_dio_write_iter() before calling for ext4_dio_write_checks().


> variables below. So I'd rather leave this in ext4_dio_write_iter() and just
> move file_modified() from ext4_write_checks() since that seems to be the

Yes, in this patch itself that has been done. We removed file_modified()
from ext4_write_checks() & renamed it to ext4_generic_write_checks().

> only thing that cannot be always done with shared i_rwsem, can it?

AFAIU, that's right. Exclusive lock must be needed to change the
security info for inode.

> 
>> +
>> +	ret = ext4_generic_write_checks(iocb, from);
>> +	if (ret <= 0) {
>> +		ext4_iunlock(inode, *iolock);
>> +		return ret;
>> +	}
>> +
>> +	/* Recalculate since offset & count may change above. */
>> +	offset = iocb->ki_pos;
>> +	count = iov_iter_count(from);
>> +	final_size = offset + count;
>> +
>> +	if (ext4_unaligned_io(inode, from, offset))
>> +		*unaligned_io = true;
> 
> No need to recheck alignment here. That cannot change over time regardless
> of locks we hold...

hmm. I think I got confused by function iov_iter_truncate() called
in ext4_generic_write_checks().
But looking at it again, I agree that alignment won't change here.
will remove the alignment check from here.

> 
>> +
>> +	if (ext4_extending_io(inode, offset, count))
>> +		*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.
>> +	 *
>> +	 * 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_overwrite_io(inode, offset, count))) {
>> +		ext4_iunlock(inode, *iolock);
>> +		*iolock = EXT4_IOLOCK_EXCL;
>> +		ext4_ilock(inode, *iolock);
>> +		goto restart;
>> +	}
>> +
>> +	ret = file_modified(file);
>> +	if (ret < 0) {
>> +		ext4_iunlock(inode, *iolock);
>> +		return ret;
>> +	}
>> +
>> +	return count;
>> +}
>> +
>>   static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>   {
>>   	ssize_t ret;
>> -	size_t count;
>> -	loff_t offset;
>>   	handle_t *handle;
>>   	struct inode *inode = file_inode(iocb->ki_filp);
>> -	bool extend = false, overwrite = false, unaligned_aio = false;
>> -	unsigned int iolock = EXT4_IOLOCK_EXCL;
>> +	loff_t offset = iocb->ki_pos;
>> +	size_t count = iov_iter_count(from);
>> +	bool extend = false, unaligned_io = false;
>> +	unsigned int iolock = EXT4_IOLOCK_SHARED;
>> +
>> +	/*
>> +	 * We initially start with shared inode lock
>> +	 * unless it is unaligned IO which needs
>> +	 * exclusive lock anyways.
>> +	 */
>> +	if (ext4_unaligned_io(inode, from, offset)) {
>> +		unaligned_io = true;
>> +		iolock = EXT4_IOLOCK_EXCL;
>> +	}
>> +	/*
>> +	 * Extending writes need exclusive lock.
>> +	 */
>> +	if (ext4_extending_io(inode, offset, count)) {
>> +		extend = true;
>> +		iolock = EXT4_IOLOCK_EXCL;
>> +	}
> 
> You cannot read EXT4_I(inode)->i_disksize without some lock (either
> inode->i_rwsem or EXT4_I(inode)->i_data_sem). So I'd just do here a quick
> check with i_size here (probably don't set extend, but just make note to
> start with exclusive i_rwsem) and later when we hold i_rwsem, we can do a
> reliable check.

hmm. my bad. I think I later moved this block of code from 
ext4_dio_write_checks() down here.
Thanks for correcting it.
Will only check against 'i_size' here then along with a
comment that a more reliable check with
shared lock has been done in ext4_dio_write_checks()
via ext4_extending_io().

> 
>> +	if (iolock == EXT4_IOLOCK_SHARED && !ext4_should_dioread_nolock(inode))
>> +		iolock = EXT4_IOLOCK_EXCL;
>>   
>>   	if (iocb->ki_flags & IOCB_NOWAIT) {
>>   		if (!ext4_ilock_nowait(inode, iolock))
>> @@ -382,47 +495,28 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>   		ext4_ilock(inode, iolock);
>>   	}
>>   
>> -	if (!ext4_dio_supported(inode)) {
>> -		ext4_iunlock(inode, iolock);
>> -		/*
>> -		 * Fallback to buffered I/O if the inode does not support
>> -		 * direct I/O.
>> -		 */
>> -		return ext4_buffered_write_iter(iocb, from);
>> -	}
>> -
>> -	ret = ext4_write_checks(iocb, from);
>> -	if (ret <= 0) {
>> -		ext4_iunlock(inode, iolock);
>> +	ret = ext4_dio_write_checks(iocb, from, &iolock, &unaligned_io,
>> +				    &extend);
>> +	if (ret <= 0)
>>   		return ret;
>> -	}
>>   
>> -	/*
>> -	 * Unaligned asynchronous direct I/O must be serialized among each
>> -	 * other as the zeroing of partial blocks of two competing unaligned
>> -	 * asynchronous direct I/O writes can result in data corruption.
>> -	 */
>>   	offset = iocb->ki_pos;
>>   	count = iov_iter_count(from);
>> -	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS) &&
>> -	    !is_sync_kiocb(iocb) && ext4_unaligned_aio(inode, from, offset)) {
>> -		unaligned_aio = true;
>> -		inode_dio_wait(inode);
>> -	}
>>   
>>   	/*
>> -	 * Determine whether the I/O will overwrite allocated and initialized
>> -	 * blocks. If so, check to see whether it is possible to take the
>> -	 * dioread_nolock path.
>> +	 * Unaligned direct IO must be serialized among each other as zeroing
>> +	 * of partial blocks of two competing unaligned IOs can result in data
>> +	 * corruption.
>> +	 *
>> +	 * So we make sure we don't allow any unaligned IO in flight.
>> +	 * For IOs where we need not wait (like unaligned non-AIO DIO),
>> +	 * below inode_dio_wait() may anyway become a no-op, since we start
>> +	 * with exclusive lock.
>>   	 */
>> -	if (!unaligned_aio && ext4_overwrite_io(inode, offset, count) &&
>> -	    ext4_should_dioread_nolock(inode)) {
>> -		overwrite = true;
>> -		ext4_ilock_demote(inode, iolock);
>> -		iolock = EXT4_IOLOCK_SHARED;
>> -	}
>> +	if (unaligned_io)
>> +		inode_dio_wait(inode);
>>   
>> -	if (offset + count > EXT4_I(inode)->i_disksize) {
>> +	if (extend) {
>>   		handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
>>   		if (IS_ERR(handle)) {
>>   			ret = PTR_ERR(handle);
>> @@ -435,12 +529,11 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>   			goto out;
>>   		}
>>   
>> -		extend = true;
>>   		ext4_journal_stop(handle);
>>   	}
>>   
>>   	ret = iomap_dio_rw(iocb, from, &ext4_iomap_ops, &ext4_dio_write_ops,
>> -			   is_sync_kiocb(iocb) || unaligned_aio || extend);
>> +			   is_sync_kiocb(iocb) || unaligned_io || extend);
>>   
>>   	if (extend)
>>   		ret = ext4_handle_inode_extension(inode, offset, ret, count);
> 
> 								Honza
>