LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Jan Kara <jack@suse.cz>
To: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Direct IO for fat
Date: Thu, 8 Feb 2007 17:10:01 +0100	[thread overview]
Message-ID: <20070208161001.GB3767@duck.suse.cz> (raw)
In-Reply-To: <87k5ysfyd5.fsf@duaron.myhome.or.jp>

On Fri 09-02-07 00:44:06, OGAWA Hirofumi wrote:
> Jan Kara <jack@suse.cz> writes:
  Hello,

> >   I've noticed that extending a file using direct IO fails for FAT with
> > EINVAL. It's basically because of the following code in fat_direct_IO():
> >
> > if (rw == WRITE) {
> >         /*
> >          * FIXME: blockdev_direct_IO() doesn't use
> >          * ->prepare_write(),
> >          * so we need to update the ->mmu_private to block
> >          * boundary.
> >          *
> >          * But we must fill the remaining area or hole by nul for
> >          * updating ->mmu_private.
> >          */
> >         loff_t size = offset + iov_length(iov, nr_segs);
> >         if (MSDOS_I(inode)->mmu_private < size)
> >                 return -EINVAL;
> > }
> >
> >   But isn't this check bogus? blockdev_direct_IO writes only to space that
> > is already allocated and stops as soon as it needs to extend the file
> > (further extension is then handled by buffered writes). So it should
> > already do what it needed for FAT. Thanks for an answer in advance.
> 
> 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.

								Honza
> 
> Thanks.
> -- 
> OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs

  reply	other threads:[~2007-02-08 16:07 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-08 11:28 Jan Kara
2007-02-08 15:44 ` OGAWA Hirofumi
2007-02-08 16:10   ` Jan Kara [this message]
2007-02-08 16:40     ` OGAWA Hirofumi
2007-02-08 17:35       ` Jan Kara
2007-02-08 18:41         ` Jan Kara
2007-02-08 19:53           ` OGAWA Hirofumi
2007-02-09  7:19             ` Jan Kara
2007-02-09 13:42               ` OGAWA Hirofumi

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20070208161001.GB3767@duck.suse.cz \
    --to=jack@suse.cz \
    --cc=hirofumi@mail.parknet.co.jp \
    --cc=linux-kernel@vger.kernel.org \
    --subject='Re: Direct IO for fat' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).