Re: ftruncate-mmap: pages are lost after writing to mmaped file.

[Peter Zijlstra]

On Fri, 2009-03-20 at 02:48 +1100, Nick Piggin wrote:
> On Thursday 19 March 2009 10:54:33 Ying Han wrote:
> > On Wed, Mar 18, 2009 at 4:36 PM, Linus Torvalds
> >
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > On Wed, 18 Mar 2009, Ying Han wrote:
> > >> > Can you say what filesystem, and what mount-flags you use? Iirc, last
> > >> > time we had MAP_SHARED lost writes it was at least partly triggered by
> > >> > the filesystem doing its own flushing independently of the VM (ie ext3
> > >> > with "data=journal", I think), so that kind of thing does tend to
> > >> > matter.
> > >>
> > >> /etc/fstab
> > >> "/dev/hda1 / ext2 defaults 1 0"
> > >
> > > Sadly, /etc/fstab is not necessarily accurate for the root filesystem. At
> > > least Fedora will ignore the flags in it.
> > >
> > > What does /proc/mounts say? That should be a more reliable indication of
> > > what the kernel actually does.
> >
> > "/dev/root / ext2 rw,errors=continue 0 0"
> 
> No luck with finding the problem yet.
> 
> But I think we do have a race in __set_page_dirty_buffers():
> 
> The page may not have buffers between the mapping->private_lock
> critical section and the __set_page_dirty call there. So between
> them, another thread might do a create_empty_buffers which can
> see !PageDirty and thus it will create clean buffers. The page
> will get dirtied by the original thread, but if the buffers are
> clean it can be cleaned without writing out buffers.
> 
> Holding mapping->private_lock over the __set_page_dirty should
> fix it, although I guess you'd want to release it before calling
> __mark_inode_dirty so as not to put inode_lock under there. I
> have a patch for this if it sounds reasonable.

When I first did those dirty tracking patches someone (I think Andrew)
commented no the fact that I did set_page_dirty() under one of these
inner locks..

/me frobs around in archives for a bit..

 - fs/buffers.c try_to_free_buffers(): remove clear_page_dirty() from under
   ->private_lock. This seems to be save, since ->private_lock is used to
   serialize access to the buffers, not the page itself.

Hmm, that's a slightly different issue...

But yeah, your scenario makes heaps of sense.

Can't we do the TestSetPageDirty() before private_lock ? It's currently
done before tree_lock as well.


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