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

[Dave Chinner <david@fromorbit.com>]

On Wed, May 29, 2024 at 05:52:03PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> 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@huawei.com>
> ---
.....
> @@ -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 */

-Dave.

-- 
Dave Chinner
david@fromorbit.com