linux-ext4 - RE: [PATCH 3/5] ext4: No need to truncate pagecache twice in collapse range

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Date:	Thu, 17 Apr 2014 10:07:43 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Namjae Jeon <namjae.jeon@...sung.com>
cc:	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: RE: [PATCH 3/5] ext4: No need to truncate pagecache twice in collapse
 range

On Thu, 17 Apr 2014, Namjae Jeon wrote:

> Date: Thu, 17 Apr 2014 09:41:45 +0900
> From: Namjae Jeon <namjae.jeon@...sung.com>
> To: Lukáš Czerner <lczerner@...hat.com>
> Cc: linux-ext4 <linux-ext4@...r.kernel.org>
> Subject: RE: [PATCH 3/5] ext4: No need to truncate pagecache twice in collapse
>      range
> 
> > 
> > We're already calling truncate_pagecache_range() before we attempt to
> > do any actual job so there is not need to truncate pagecache once more
> > using truncate_setsize() after we're finished.
> > 
> > Remove truncate_setsize() and replace it just with i_size_write() note
> > that we're holding appropriate locks.
> > 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> 
> Hi Lukas.
> 
> I added this code by getting rewiew from Hugh.
> Plz see the disscusion beween Hugh and Dave.
> 
> Hugh: But your case is different: collapse is much closer to truncation,
>  and if you do not unmap the private COW'ed pages, then pages left
>  behind beyond the EOF will break the spec that requires SIGBUS when
>  touching there, and pages within EOF will be confusingly derived
>  from file data now belonging to another offset or none (move these
>  pages within the user address space? no, I don't think anon_vmas
>  would allow that, and there may be no right place to move them).
>  
> Dave: See above - we never leave pages beyond the new EOF because setting
>  the new EOF is a truncate operation that calls
>  truncate_setsize(inode, newsize).
>  
> Hugh: Right, thanks, I now see the truncate_setsize() in the xfs case -
>  though not in the ext4 case, which looks as if it's just doing an
>  i_size_write() afterwards.
> 
> Dave: So that's a bug in the ext4 code ;)
> 
> truncate_setsize is not needed in case Hugh pointed out ?
> 
> Thanks!

That is true, we need to make sure that the page cache is coherent
with what's on disk. But we've already done that before releasing
the blocks. As I mention in the comment we're doing
truncate_pagecache_range() before removing any space. That's exactly
how it's supposed to be used. See comment in
truncate_pagecache_range().

However as I noticed we do not actually need to use
truncate_pagecache_range(), but rather truncate_pagecache() so I can
change that in my patch.

Does that make sense to everyone ?

Thanks!
-Lukas