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SubjectRe: Direct IO for fat
From
Date
Jan Kara <jack@suse.cz> writes:

>> FAT has to fill the hole completely, but DIO doesn't seems to do.
>>
>> e.g.
>> fd = open("file", O_WRONLY | O_CREAT | O_TRUNC);
>> write(fd, buf, 512);
>> lseek(fd, 10000, SEEK_SET);
>> write(fd, buf, 512);
>>
>> We need to allocate the blocks on 512 ~ 10000, and fill it with zero.
>> However, I think DIO doesn't fill it. If I'm missing something, please
>> let me know, I'll kill that check.
> I know. DIO doesn't do it. But the point is that if blockdev_direct_IO
> finds out it should allocate new blocks, it exits without allocating them.
> Then in __generic_file_aio_write_nolock() if we find out that we did not
> write everything in generic_file_direct_write(), we just call
> generic_file_buffered_write() to write the unwritten part.
> Hence, in case you describe above, the second write() finds out that
> block is not allocated and eventually everything falls back to calling
> generic_file_buffered_write() which calls prepare_write() and everything is
> happy.

I see. But sorry, I can't see where is preventing it... Finally, I
think we do the following on second write(2).

This is write, so create == 1, and ->lock_type == DIO_LOCKING,
and dio->block_in_file > ->i_size, so DIO callback fat_get_block() with
create == 1.
Then fat_get_block() seems to allocate block without fill hole,
because it can't know caller is prepre_write or not...
Well, anyway I'll test it on weekend. Thanks.

-> blockdev_direct_IO()
-> direct_io_worker()
-> do_direct_IO()
-> get_more_blocks()

create = dio->rw & WRITE;
if (dio->lock_type == DIO_LOCKING) {
if (dio->block_in_file < (i_size_read(dio->inode) >>
dio->blkbits))
create = 0;
} else if (dio->lock_type == DIO_NO_LOCKING) {
create = 0;
}
/*
* For writes inside i_size we forbid block creations: only
* overwrites are permitted. We fall back to buffered writes
* at a higher level for inside-i_size block-instantiating
* writes.
*/
ret = (*dio->get_block)(dio->inode, fs_startblk,
map_bh, create);
--
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
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