Re: [PATCH v4 0/2] ext4: fix DAX dma vs truncate/hole-punch

[Jan Kara <jack@suse.cz>]

On Fri 27-07-18 10:28:51, Ross Zwisler wrote:
> + fsdevel and the xfs list.
> 
> On Wed, Jul 25, 2018 at 4:28 PM Ross Zwisler
> <ross.zwisler@linux.intel.com> wrote:
> > On Wed, Jul 11, 2018 at 10:17:41AM +0200, Jan Kara wrote:
> > > On Tue 10-07-18 13:10:29, Ross Zwisler wrote:
> > > > Changes since v3:
> > > >  * Added an ext4_break_layouts() call to ext4_insert_range() to ensure
> > > >    that the {ext4,xfs}_break_layouts() calls have the same meaning.
> > > >    (Dave, Darrick and Jan)
> > >
> > > How about the occasional WARN_ON_ONCE you mention below. Were you able to
> > > hunt them down?
> >
> > The root cause of this issue is that while the ei->i_mmap_sem provides
> > synchronization between ext4_break_layouts() and page faults, it doesn't
> > provide synchronize us with the direct I/O path.  This exact same issue exists
> > in XFS AFAICT, with the synchronization tool there being the XFS_MMAPLOCK.
> >
> > This allows the direct I/O path to do I/O and raise & lower page->_refcount
> > while we're executing a truncate/hole punch.  This leads to us trying to free
> > a page with an elevated refcount.
> >
> > Here's one instance of the race:
> >
> > CPU 0                                   CPU 1
> > -----                                   -----
> > ext4_punch_hole()
> >   ext4_break_layouts() # all pages have refcount=1
> >
> >                                         ext4_direct_IO()
> >                                           ... lots of layers ...
> >                                           follow_page_pte()
> >                                             get_page() # elevates refcount
> >
> >   truncate_pagecache_range()
> >    ... a few layers ...
> >    dax_disassociate_entry() # sees elevated refcount, WARN_ON_ONCE()
> >

So this is a very different race from the one below. And it should be
impossible to happen. This race is exactly the reason why
dax_layout_busy_page() has unmap_mapping_range() call to force GUP to fault
which blocks on ei->i_mmap_sem / XFS_MMAPLOCK and thus avoids the race.

> > A similar race occurs when the refcount is being dropped while we're running
> > ext4_break_layouts(), and this is the one that my test was actually hitting:
> >
> > CPU 0                                   CPU 1
> > -----                                   -----
> >                                         ext4_direct_IO()
> >                                           ... lots of layers ...
> >                                           follow_page_pte()
> >                                             get_page()
> >                                             # elevates refcount of page X
> > ext4_punch_hole()
> >   ext4_break_layouts() # two pages, X & Y, have refcount == 2
> >     __wait_var_event() # called for page X
> >
> >                                           __put_devmap_managed_page()
> >                                           # drops refcount of X to 1
> >
> >    # __wait_var_events() checks X's refcount in "if (condition)", and breaks.
> >    # We never actually called ext4_wait_dax_page(), so 'retry' in
> >    # ext4_break_layouts() is still false.  Exit do/while loop in
> >    # ext4_break_layouts, never attempting to wait on page Y which still has an
> >    # elevated refcount of 2.
> >
> >   truncate_pagecache_range()
> >    ... a few layers ...
> >    dax_disassociate_entry() # sees elevated refcount for Y, WARN_ON_ONCE()
> >
> > This second race can be fixed with the patch at the end of this function,
> > which I think should go in, unless there is a benfit to the current retry
> > scheme which relies on the 'retry' variable in {ext4,xfs}_break_layouts()?
> > With this patch applied I've been able to run my unit test through
> > thousands of iterations, where it used to failed consistently within 10 or
> > so.
> >
> > Even so, I wonder if the real solution is to add synchronization between
> > the direct I/O path and {ext4,xfs}_break_layouts()?  Other ideas on how
> > this should be handled?
> >
> > --- >8 ---
> >
> > From a4519b0f40362f0a63ae96acaf986092aff0f0d3 Mon Sep 17 00:00:00 2001
> > From: Ross Zwisler <zwisler@kernel.org>
> > Date: Wed, 25 Jul 2018 16:16:05 -0600
> > Subject: [PATCH] ext4: Close race between direct IO and ext4_break_layouts()
> >
> > If the refcount of a page is lowered between the time that it is returned
> > by dax_busy_page() and when the refcount is again checked in
> > ext4_break_layouts() => ___wait_var_event(), the waiting function
> > ext4_wait_dax_page() will never be called.  This means that
> > ext4_break_layouts() will still have 'retry' set to false, so we'll stop
> > looping and never check the refcount of other pages in this inode.
> >
> > Instead, always continue looping as long as dax_layout_busy_page() gives us
> > a page which it found with an elevated refcount.
> >
> > Note that this works around the race exposed by my unit test, but I think
> > that there is another race that needs to be addressed, probably with
> > additional synchronization added between direct I/O and
> > {ext4,xfs}_break_layouts().
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>

OK, this is a good catch and the patch looks good. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

Also please post this fix officially to Ted to include it in his tree (I
can see that he has all your other patches queued for the merge window).

AFAICS XFS implementation of xfs_break_dax_layouts() has the same problem.
Can you please fix it? Thanks for working on this!

								Honza

> > ---
> >  fs/ext4/inode.c | 9 +++------
> >  1 file changed, 3 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 27e9643bc13b..fedb88104bbf 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -4193,9 +4193,8 @@ int ext4_update_disksize_before_punch(struct inode *inode, loff_t offset,
> >         return 0;
> >  }
> >
> > -static void ext4_wait_dax_page(struct ext4_inode_info *ei, bool *did_unlock)
> > +static void ext4_wait_dax_page(struct ext4_inode_info *ei)
> >  {
> > -       *did_unlock = true;
> >         up_write(&ei->i_mmap_sem);
> >         schedule();
> >         down_write(&ei->i_mmap_sem);
> > @@ -4205,14 +4204,12 @@ int ext4_break_layouts(struct inode *inode)
> >  {
> >         struct ext4_inode_info *ei = EXT4_I(inode);
> >         struct page *page;
> > -       bool retry;
> >         int error;
> >
> >         if (WARN_ON_ONCE(!rwsem_is_locked(&ei->i_mmap_sem)))
> >                 return -EINVAL;
> >
> >         do {
> > -               retry = false;
> >                 page = dax_layout_busy_page(inode->i_mapping);
> >                 if (!page)
> >                         return 0;
> > @@ -4220,8 +4217,8 @@ int ext4_break_layouts(struct inode *inode)
> >                 error = ___wait_var_event(&page->_refcount,
> >                                 atomic_read(&page->_refcount) == 1,
> >                                 TASK_INTERRUPTIBLE, 0, 0,
> > -                               ext4_wait_dax_page(ei, &retry));
> > -       } while (error == 0 && retry);
> > +                               ext4_wait_dax_page(ei));
> > +       } while (error == 0);
> >
> >         return error;
> >  }
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR