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Re: [PATCH] xfs_io: actually issue 0 size writes

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: [PATCH] xfs_io: actually issue 0 size writes
From: Lachlan McIlroy <lmcilroy@xxxxxxxxxx>
Date: Thu, 13 Aug 2009 21:55:59 -0400 (EDT)
Cc: xfs mailing list <xfs@xxxxxxxxxxx>
In-reply-to: <148381776.1936161250214905902.JavaMail.root@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
Reply-to: Lachlan McIlroy <lmcilroy@xxxxxxxxxx>
----- "Eric Sandeen" <sandeen@xxxxxxxxxxx> wrote:

> Lachlan McIlroy wrote:
> > ----- "Eric Sandeen" <sandeen@xxxxxxxxxxx> wrote:
> > 
> >> Felix Blyakher wrote:
> >>> On Aug 13, 2009, at 5:15 PM, Eric Sandeen wrote:
> >>>
> >>>> While testing some stuff in generic_write_checks() in the
> >>>> kernel I realized that you can't actually use xfs_io to send
> >>>> a 0-byte write in.  This is actually a condition worth testing:
> >>>>
> >>>>       If  count  is zero and fd refers to a regular file,
> >>>>       then write() may return a failure status if one  of
> >>>>       the  errors  below  is  detected.  If no errors are
> >>>>       detected, 0 will be returned  without  causing  any
> >>>>       other  effect.
> >>> As I understand the desire to be able to issue 0 size writes
> >>> from xfs_io is to test the possibility of writing to a given fd.
> >>> What kind of errors would you expect to test for?
> >> In general EFBIG or ENOSPC.
> >>
> >> This sort of thing in generic_write_checks():
> >>
> >>         if (unlikely(*pos >= inode->i_sb->s_maxbytes)) {
> >>                 if (*count || *pos > inode->i_sb->s_maxbytes) {
> >>                      return -EFBIG;
> >>                 }
> >>                 /* zero-length writes at ->s_maxbytes are OK */
> >>         }
> >>
> >> Although I'm a little confused about why "*pos == s_maxbytes" is
> ok;
> >> I
> >> thought s_maxbytes was a count/size whereas pos is an offset, so
> it
> >> seems to me that pos == s_maxbytes is one past the max.  But
> anyway,
> >> that's mostly unrelated to the patch in this thread.  :)
> 
> > pos == s_maxbytes is only okay if count == 0 also.  So even though
> we
> > are writing at the limit we are not actually going to write
> anything.
> > At s_maxbytes-1 we are allowed to write one byte and at s_maxbytes
> we
> > are allowed to write nothing - literally.
> 
> I think my confusion over maxbytes is whether it's a size or an
> offset.
> 
> The comment says ... max size.
Yes it's a size.

> 
> Also in the above function it does i_size_read on the block device -
> again a size.
Yes.

> 
> If it's a max offset you're right; if it's a max -size- then pos ==
> s_maxbytes is already off the end, one past the limit.
Technically it's only off the end if you try to read something.

> 
> -eric
> 
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