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Re: [PATCH 1/4] xfs: remove xfs_tosspages

To: Andrew Dahl <adahl@xxxxxxx>
Subject: Re: [PATCH 1/4] xfs: remove xfs_tosspages
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 13 Nov 2012 10:00:29 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <50A15F57.3080900@xxxxxxx>
References: <1352455804-17045-1-git-send-email-david@xxxxxxxxxxxxx> <1352455804-17045-2-git-send-email-david@xxxxxxxxxxxxx> <50A15F57.3080900@xxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Mon, Nov 12, 2012 at 02:43:03PM -0600, Andrew Dahl wrote:
> On 11/09/2012 04:10 AM, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > 
> > It's a buggy, unnecessary wrapper that is duplicating
> > truncate_pagecache_range().
> > 
> > When replacing the call in xfs_change_file_space(), also ensure that
> > the length being allocated/freed is always positive before making
> > any changes. These checks are done in the lower extent manipulation
> > functions, too, but we need to do them before any page cache
> > operations.
> > 
> > Reported-by: Andrew Dahl <adahl@xxxxxxx>
> > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
....
> > @@ -2169,7 +2186,8 @@ xfs_change_file_space(
> >     switch (cmd) {
> >     case XFS_IOC_ZERO_RANGE:
> >             prealloc_type |= XFS_BMAPI_CONVERT;
> > -           xfs_tosspages(ip, startoffset, startoffset + bf->l_len, 0);
> > +           truncate_pagecache_range(VFS_I(ip), startoffset,
> > +                    round_down(startoffset + bf->l_len, PAGE_SIZE) - 1);
> 
> When calling XFS_IOC_ZERO_RANGE with a range [0, 4095] or [0,1] it's
> tossing pages because the call to round_down() is returning 0 and
> passing -1 in for the end will toss all pages.  So, we need to make sure
> round_down() isn't going to return a 0, or that (startoffset +
> bf->l_len) > PAGE_SIZE.

Right, which was the original bug. I didn't think that through
fully....

> So, something like...
> 
> 
> xfs_off_t       end;
> 
> [...]
> 
> if((end = round_down(startoffset + bf->l_len, PAGE_SIZE)) > 0) {
>       truncate_pagecache_range(VFS_I(ip), startoffset, end - 1);
> }

Actually, it is more complex than that. Think of the range [4096,
4097]. That would result in passing 4096, 4095 to
truncate_pagecache_range(), which is also wrong but implicitly
handled correctly in truncate_inode_pages_range(). Hence it should
be:

                end = round_down(startoffset + bf->l_len, PAGE_SIZE) - 1;
                if (startoffset > end)
                        truncate_pagecache_range(VFS_I(ip), startoffset, end);

That's similar to your original fix, which had a last < first check
in it. I'll post a fix in a few minutes...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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