Re: [RFC PATCH 6/8] fat: add iomap buffered I/O support

[Matheus Alves de Almeida <matheus.aalmeida@inf.ufrgs.br>]

On Thu, Nov 12, 2026 at 02:06:43PM +0900, OGAWA Hirofumi wrote:
>Matheus Alves de Almeida <matheus.aalmeida@inf.ufrgs.br> writes:
>
>> On Mon, Nov 09, 2026 at 12:03:00AM +0900, OGAWA Hirofumi wrote:
>>>Matheus Alves de Almeida <matheus.aalmeida@inf.ufrgs.br> writes:
>>>
>>>> Use iomap for buffered I/O, replacing buffer_heads. Remove
>>>> fat_get_block(), fat_write_begin(), fat_write_failed() as the iomap path
>>>> makes them unnecessary.
>>>>
>>>> Signed-off-by: Matheus Alves de Almeida <matheus.aalmeida@inf.ufrgs.br>
>>>> ---
>>>>  fs/fat/fat.h   |   4 +-
>>>>  fs/fat/file.c  |  72 +++++++++++++++++++----
>>>>  fs/fat/inode.c | 157 +++++--------------------------------------------
>>>>  fs/fat/iomap.c |  84 ++++++++++++++++++++++++++
>>>>  4 files changed, 164 insertions(+), 153 deletions(-)
>>>>
>>>
>>>[...]
>>>
>>>> diff --git a/fs/fat/file.c b/fs/fat/file.c
>>>> index 8689cbbdad13..723b671d5b4b 100644
>>>> --- a/fs/fat/file.c
>>>> +++ b/fs/fat/file.c
>>>
>>>[...]
>>>
>>>> +static int fat_cont_expand(struct inode *inode, loff_t new_size);
>>>> +
>>>> +static ssize_t fat_file_write_iter_buffered(struct kiocb *iocb,
>>>> +					    struct iov_iter *iter)
>>>> +{
>>>> +	struct inode *inode = file_inode(iocb->ki_filp);
>>>> +	struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb);
>>>> +	loff_t offset, old_size, old_alloc, keep_alloc;
>>>> +	ssize_t ret;
>>>> +
>>>> +	inode_lock(inode);
>>>> +	ret = generic_write_checks(iocb, iter);
>>>> +	if (ret <= 0)
>>>> +		goto out;
>>>> +
>>>> +	ret = file_modified(iocb->ki_filp);
>>>> +	if (ret)
>>>> +		goto out;
>>>> +
>>>> +	offset = iocb->ki_pos;
>>>> +	old_size = i_size_read(inode);
>>>> +	if (offset > old_size) {
>>>> +		/* We need to fill this hole */
>>>> +		ret = fat_cont_expand(inode, offset);
>>>> +		if (ret)
>>>> +			goto out;
>>>> +	}
>>>> +
>>>> +	inode_dio_wait(inode);
>>>> +	old_alloc = inode_get_bytes(inode);
>>>> +	ret = iomap_file_buffered_write(iocb, iter, &fat_iomap_write_ops,
>>>> +					NULL, NULL);
>>>> +	keep_alloc = max(old_alloc, round_up(i_size_read(inode),
>>>> +					     sbi->cluster_size));
>>>> +	if (inode_get_bytes(inode) > keep_alloc)
>>>> +		fat_truncate_blocks(inode, keep_alloc);
>>>
>>>What is doing by this fat_truncate_blocks?
>>>
>>
>> It's trimming the clusters past EOF and keeping the fallocate KEEP_SIZE
>> preallocation. I can add a comment stating this.
>
>Hm, I wonder mostly why this truncate without checking "ret"...
>
>Thanks.
>

It is meant as a cleanup for a failed or short write, serving the same
role as fat_write_failed(). I'll add a ret check for readability.

Thanks,
Matheus