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

[Ross Zwisler <zwisler@kernel.org>]

+ 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()
>
> 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>
> ---
>  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;
>  }
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