Re: [PATCH v2 1/6] ext4: move partial block zeroing earlier in ext4_zero_range()

[Jan Kara]

On Tue 07-07-26 13:01:41, Zhang Yi wrote:
> On 7/7/2026 10:13 AM, Zhang Yi wrote:
> > In ext4_zero_range(), move the ext4_zero_partial_blocks() call, which
> > handles unaligned edges, into the same branch where the unaligned range
> > is preallocated, immediately after ext4_alloc_file_blocks(). This is
> > safe because there is no dependency between partial block handling and
> > the subsequent full block handling.
> > 
> > This change will be used by later patches that handle unaligned
> > FALLOC_FL_WRITE_ZEROES operations, which will need to check the partial
> > zeroed result.
> > 
> > Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > ---
> >  fs/ext4/extents.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index 125f628e738a..27ca641e701b 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -4734,10 +4734,16 @@ static long ext4_zero_range(struct file *file, loff_t offset,
> >  	}
> >  
> >  	flags = EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT;
> > -	/* Preallocate the range including the unaligned edges */
> > +	/*
> > +	 * Preallocate the range including the unaligned edges, and zero
> > +	 * out partial blocks if they already contain data.
> > +	 */
> >  	if (!IS_ALIGNED(offset | end, blocksize)) {
> >  		ret = ext4_alloc_file_blocks(file, offset, len, new_size,
> >  					     flags);
> > +		if (!ret)
> > +			ret = ext4_zero_partial_blocks(inode, offset, len,
> > +						       &partial_zeroed);
> >  		if (ret)
> >  			return ret;
> 
> Sashiko pointed out below:
> 
> >Is it safe to modify these folios before calling
> >ext4_truncate_page_cache_block_range()?
> 
> >Previously, ext4_zero_range() called ext4_truncate_page_cache_block_range()
> >first, which waits for writeback on the boundary folios via
> >truncate_pagecache_range() -> truncate_inode_pages_range() ->
> >truncate_inode_partial_folio().
> 
> >By zeroing the partial blocks earlier, ext4_block_do_zero_range() might
> >acquire the folio lock and mutate the folio data while PG_writeback is still
> >set, potentially causing data corruption or checksum mismatches if the block
> >layer is actively DMA-ing it to disk.
> 
> I think this issue is confined to stable devices. The proposed fix is
> to modify the fgf_flags passed to the folio lookup function in
> ext4_load_tail_bh() by setting them to FGP_WRITEBEGIN | FGP_ACCESSED.

Yes, that makes sense.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR