linux-ext4 - Re: [PATCH 4/4] ext3: Implement delayed allocation on page

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Date:	Tue, 3 May 2011 00:20:20 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 4/4] ext3: Implement delayed allocation on page_mkwrite
 time

On Mon 02-05-11 14:12:30, Andrew Morton wrote:
> On Mon,  2 May 2011 22:56:56 +0200
> Jan Kara <jack@...e.cz> wrote:
> 
> > So far, ext3 was allocating necessary blocks for mmapped writes when
> > writepage() was called. There are several issues with this. The worst
> > being that user is allowed to arbitrarily exceed disk quotas because
> > writepage() is called from flusher thread context (which is root) and thus
> > quota limits are ignored. Another bad consequence is that data is just lost
> > if we find there's no space on the filesystem during ->writepage() time.
> > 
> > We solve these issues by implementing block reservation in page_mkwrite()
> > callback. We don't want to really allocate blocks on page_mkwrite() time
> > because for random writes via mmap (as seen for example with applications using
> > BerkeleyDB) it results in much more fragmented files and thus much worse
> > performance. So we allocate indirect blocks and reserve space for data block in
> > page_mkwrite() and do the allocation of data block from writepage().
> 
> Yes, instantiating the metadata and accounting the data is a good
> approach.  The file layout will be a bit suboptimal, but surely that
> will be a minor thing.
> 
> But boy, it's a complicated patch!  Are we really sure that we want to
> make changes this extensive to our antiquated old fs?  Or do we just
> say "yeah, it's broken with quotas - use ext4"?
  The patch isn't trivial, I agree (although it's mostly straightforward).
Regarding telling users to switch to ext4 - it seems a bit harsh to me
to ask people to switch to ext4 as a response to a (possibly security)
issue they uncover. Because for most admins switching to ext4 will require
some non-trivial testing I presume. Of course, the counterweight is the
possibility of new bugs introduced to the code by my patch. But after some
considerations I've decided it's worth it and and fixed the bug...

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