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

[Nick Piggin]

On Friday 20 March 2009 03:16:01 Peter Zijlstra wrote:
> 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.

I think there might be issues with having a clean page but dirty buffers
if you do it that way... At any rate, if we can solve the race without
swapping the order, I think that would be safer.

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