Re: [PATCH] ext2: Use page_mkwrite vma_operations to get mmap write notification.

[Jan Kara <jack@suse.cz>]

On Wed 11-06-08 12:07:49, Andrew Morton wrote:
> On Wed, 11 Jun 2008 20:38:45 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
> 
> > On Thu, Jun 05, 2008 at 12:30:45PM -0700, Andrew Morton wrote:
> > > On Thu,  5 Jun 2008 22:35:12 +0530
> > > "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
> > > 
> > > > We would like to get notified when we are doing a write on mmap
> > > > section.  The changes are needed to handle ENOSPC when writing to an
> > > > mmap section of files with holes.
> > > > 
> > > 
> > > Whoa.  You didn't copy anything like enough mailing lists for a change
> > > of this magnitude.  I added some.
> > > 
> > > This is a large change in behaviour!
> > > 
> > > a) applications will now get a synchronous SIGBUS when modifying a
> > >    page over an ENOSPC filesystem.  Whereas previously they could have
> > >    proceeded to completion and then detected the error via an fsync().
> > 
> > Or not detect the error at all if we don't call fsync() right ? Isn't a
> > synchronous SIGBUS the right behaviour ?
> >
> 
> Not according to POSIX.  Or at least posix-several-years-ago, when this
> last was discussed.  The spec doesn't have much useful to say about any
> of this.
> 
> It's a significant change in the userspace interface.
> 
> > 
> > > 
> > >    It's going to take more than one skimpy little paragraph to
> > >    justify this, and to demonstrate that it is preferable, and to
> > >    convince us that nothing will break from this user-visible behaviour
> > >    change.
> > > 
> > > b) we're now doing fs operations (and some I/O) in the pagefault
> > >    code.  This has several implications:
> > > 
> > >    - performance changes
> > > 
> > >    - potential for deadlocks when a process takes the fault from
> > >      within a copy_to_user() in, say, mm/filemap.c
> > > 
> > >    - performing additional memory allocations within that
> > >      copy_to_user().  Possibility that these will reenter the
> > >      filesystem.
> > > 
> > > And that's just ext2.
> > > 
> > > For ext3 things are even more complex, because we have the
> > > journal_start/journal_end pair which is effectively another "lock" for
> > > ranking/deadlock purposes.  And now we're taking i_alloc_sem and
> > > lock_page and we're doing ->writepage() and its potential
> > > journal_start(), all potentially within the context of a
> > > copy_to_user().
> > 
> > One of the reason why we would need this in ext3/ext4 is that we cannot
> > do block allocation in the writepage with the recent locking changes.
> 
> Perhaps those recent locking changes were wrong.
  Well, the locking changes are those reverting locking ordering of
transaction start and page lock - we have them in ext4 and Aneesh seems 
to be looking into porting them to ext3 (at least ordered mode rewrite
needs them). I wouldn't say they are wrong in principle.
  It's easier to use page_mkwrite() to allocate blocks so that
later in writepage() we don't have to do block allocation which needs to
start a transaction (because that means unlocking the page which gets
quickly nasty to handle properly...).
  BTW: XFS, OCFS2 or GFS2 define page_mkwrite() in this manner so they do
return SIGBUS when you run out of space when writing to mmapped hole. So
it's not like this change is introducing completely new behavior... I can
understand that we might not want to change the behavior for ext2 or ext3
but ext4 is IMO definitely free to choose.

> > The locking changes involve changing the locking order of journal_start
> > and page_lock. With writepage we are already called with page_lock and
> > we can't start new transaction needed for block allocation.
> 
> ext3_write_begin() has journal_start() nesting inside the lock_page().
> 
> > But if we agree that we should not do block allocation in page_mkwrite
> > we need to add writepages and allocate blocks in writepages.
> 
> I'm not sure what writepages has to do with pagefaults?

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

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