linux-kernel - Re: Direct IO for fat

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Message-ID: <20070208173515.GB3780@duck.suse.cz>
Date:	Thu, 8 Feb 2007 18:35:16 +0100
From:	Jan Kara <jack@...e.cz>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Direct IO for fat

On Fri 09-02-07 01:40:31, OGAWA Hirofumi wrote:
> Jan Kara <jack@...e.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.
  I think you misread the code - see below.

> 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;
  Here, create == 1.

> 		if (dio->lock_type == DIO_LOCKING) {
> 			if (dio->block_in_file < (i_size_read(dio->inode) >>
> 							dio->blkbits))
> 				create = 0;
  But here create was reset back to 0 - exactly because
dio->block_in_file > i_size...

									Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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