Messages in this thread | | | Subject | Re: ftruncate-mmap: pages are lost after writing to mmaped file. | From | Peter Zijlstra <> | Date | Thu, 19 Mar 2009 17:16:01 +0100 |
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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@linux-foundation.org> 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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