Re: [RFC PATCH v4 5/8] xfs: refactor the truncating order

[Zhang Yi]

On 2024/6/3 6:46, Dave Chinner wrote:
> On Wed, May 29, 2024 at 05:52:03PM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>>
>> When truncating down an inode, we call xfs_truncate_page() to zero out
>> the tail partial block that beyond new EOF, which prevents exposing
>> stale data. But xfs_truncate_page() always assumes the blocksize is
>> i_blocksize(inode), it's not always true if we have a large allocation
>> unit for a file and we should aligned to this unitsize, e.g. realtime
>> inode should aligned to the rtextsize.
>>
>> Current xfs_setattr_size() can't support zeroing out a large alignment
>> size on trucate down since the process order is wrong. We first do zero
>> out through xfs_truncate_page(), and then update inode size through
>> truncate_setsize() immediately. If the zeroed range is larger than a
>> folio, the write back path would not write back zeroed pagecache beyond
>> the EOF folio, so it doesn't write zeroes to the entire tail extent and
>> could expose stale data after an appending write into the next aligned
>> extent.
>>
>> We need to adjust the order to zero out tail aligned blocks, write back
>> zeroed or cached data, update i_size and drop cache beyond aligned EOF
>> block, preparing for the fix of realtime inode and supporting the
>> upcoming forced alignment feature.
>>
>> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
>> ---
> .....
>> @@ -853,30 +854,7 @@ xfs_setattr_size(
>>  	 * the transaction because the inode cannot be unlocked once it is a
>>  	 * part of the transaction.
>>  	 *
>> -	 * Start with zeroing any data beyond EOF that we may expose on file
>> -	 * extension, or zeroing out the rest of the block on a downward
>> -	 * truncate.
>> -	 */
>> -	if (newsize > oldsize) {
>> -		trace_xfs_zero_eof(ip, oldsize, newsize - oldsize);
>> -		error = xfs_zero_range(ip, oldsize, newsize - oldsize,
>> -				&did_zeroing);
>> -	} else if (newsize != oldsize) {
>> -		error = xfs_truncate_page(ip, newsize, &did_zeroing);
>> -	}
>> -
>> -	if (error)
>> -		return error;
>> -
>> -	/*
>> -	 * We've already locked out new page faults, so now we can safely remove
>> -	 * pages from the page cache knowing they won't get refaulted until we
>> -	 * drop the XFS_MMAP_EXCL lock after the extent manipulations are
>> -	 * complete. The truncate_setsize() call also cleans partial EOF page
>> -	 * PTEs on extending truncates and hence ensures sub-page block size
>> -	 * filesystems are correctly handled, too.
>> -	 *
>> -	 * We have to do all the page cache truncate work outside the
>> +	 * And we have to do all the page cache truncate work outside the
>>  	 * transaction context as the "lock" order is page lock->log space
>>  	 * reservation as defined by extent allocation in the writeback path.
>>  	 * Hence a truncate can fail with ENOMEM from xfs_trans_alloc(), but
> ......
> 
> Lots of new logic for zeroing here. That makes xfs_setattr_size()
> even longer than it already is. Can you lift this EOF zeroing logic
> into it's own helper function so that it is clear that it is a
> completely independent operation to the actual transaction that
> changes the inode size. That would also allow the operations to be
> broken up into:
> 
> 	if (newsize >= oldsize) {
> 		/* do the simple stuff */
> 		....
> 		return error;
> 	}
> 	/* do the complex size reduction stuff without additional indenting */
> 

Sure, I will try to factor them out.

Thanks,
Yi.