Re: [PATCH v2 08/24] ext4: support large block size in ext4_readdir()

[Baokun Li <libaokun1@huawei.com>]

On 2025-11-10 20:26, Pankaj Raghav wrote:
> On 11/7/25 15:42, libaokun@huaweicloud.com wrote:
>> From: Baokun Li <libaokun1@huawei.com>
>>
>> In ext4_readdir(), page_cache_sync_readahead() is used to readahead mapped
>> physical blocks. With LBS support, this can lead to a negative right shift.
>>
>> To fix this, the page index is now calculated by first converting the
>> physical block number (pblk) to a file position (pos) before converting
>> it to a page index. Also, the correct number of pages to readahead is now
>> passed.
>>
>> Signed-off-by: Baokun Li <libaokun1@huawei.com>
>> Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
>> Reviewed-by: Jan Kara <jack@suse.cz>
>> ---
> Minor general comments below.
>
> Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>
Thanks for the review!
>
>>  fs/ext4/dir.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
>> index d4164c507a90..256fe2c1d4c1 100644
>> --- a/fs/ext4/dir.c
>> +++ b/fs/ext4/dir.c
>> @@ -192,13 +192,13 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
>>  			continue;
>>  		}
>>  		if (err > 0) {
>> -			pgoff_t index = map.m_pblk >>
>> -					(PAGE_SHIFT - inode->i_blkbits);
>> +			pgoff_t index = map.m_pblk << inode->i_blkbits >>
>> +					PAGE_SHIFT;
> It will be nice to have some common helper for this operation. We do something
> similar in filemap.c as well.

In patch 10, we introduced a macro to handle the conversion from lblk
to page index. In this particular case, though, it is pblk, which has
a different value range compared to lblk. As this is the only instance
in the code, I applied a standalone modification here.

>
>>  			if (!ra_has_index(&file->f_ra, index))
>>  				page_cache_sync_readahead(
>>  					sb->s_bdev->bd_mapping,
>> -					&file->f_ra, file,
>> -					index, 1);
>> +					&file->f_ra, file, index,
>> +					1 << EXT4_SB(sb)->s_min_folio_order);
> Just a personal opinion but it would be nice to have some variable for this instead of doing it
> inline? It could be defined along with index.
>
> unsigned long min_nr_pages = 1UL << EXT4_SB(sb)->s_min_folio_order;
>
Thank you for the suggestion. This is indeed a bit easier to understand,
but the space here is limited and the variable is only used once,
so I prefer the current direct style.


Cheers,
Baokun

>>  			file->f_ra.prev_pos = (loff_t)index << PAGE_SHIFT;
>>  			bh = ext4_bread(NULL, inode, map.m_lblk, 0);
>>  			if (IS_ERR(bh)) {
>