Re: [PATCH] mm, dax: fix DAX deadlocks (COW fault)

[Boaz Harrosh]

On 11/16/2015 08:34 PM, Ross Zwisler wrote:
> On Mon, Nov 16, 2015 at 10:15:56AM -0800, Dan Williams wrote:
>> On Mon, Nov 16, 2015 at 4:09 AM, Yigal Korman <yigal@xxxxxxxxxxxxx> wrote:
>>> DAX handling of COW faults has wrong locking sequence:
>>>         dax_fault does i_mmap_lock_read
>>>         do_cow_fault does i_mmap_unlock_write
>>>
>>> Ross's commit[1] missed a fix[2] that Kirill added to Matthew's
>>> commit[3].
>>>
>>> Original COW locking logic was introduced by Matthew here[4].
>>>
>>> This should be applied to v4.3 as well.
>>>
>>> [1] 0f90cc6609c7 mm, dax: fix DAX deadlocks
>>> [2] 52a2b53ffde6 mm, dax: use i_mmap_unlock_write() in do_cow_fault()
>>> [3] 843172978bb9 dax: fix race between simultaneous faults
>>> [4] 2e4cdab0584f mm: allow page fault handlers to perform the COW
>>>
>>> Signed-off-by: Yigal Korman <yigal@xxxxxxxxxxxxx>
>>>
>>> Cc: Stable Tree <stable@xxxxxxxxxxxxxxx>
>>> Cc: Boaz Harrosh <boaz@xxxxxxxxxxxxx>
>>> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>>> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
>>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>>> Cc: Dave Chinner <dchinner@xxxxxxxxxx>
>>> Cc: Jan Kara <jack@xxxxxxxx>
>>> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>>> Cc: Matthew Wilcox <matthew.r.wilcox@xxxxxxxxx>
>>> ---
>>>  mm/memory.c | 8 ++++----
>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index c716913..e5071af 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -3015,9 +3015,9 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>>>                 } else {
>>>                         /*
>>>                          * The fault handler has no page to lock, so it holds
>>> -                        * i_mmap_lock for write to protect against truncate.
>>> +                        * i_mmap_lock for read to protect against truncate.
>>>                          */
>>> -                       i_mmap_unlock_write(vma->vm_file->f_mapping);
>>> +                       i_mmap_unlock_read(vma->vm_file->f_mapping);
>>>                 }
>>>                 goto uncharge_out;
>>>         }
>>> @@ -3031,9 +3031,9 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>>>         } else {
>>>                 /*
>>>                  * The fault handler has no page to lock, so it holds
>>> -                * i_mmap_lock for write to protect against truncate.
>>> +                * i_mmap_lock for read to protect against truncate.
>>>                  */
>>> -               i_mmap_unlock_write(vma->vm_file->f_mapping);
>>> +               i_mmap_unlock_read(vma->vm_file->f_mapping);
>>>         }
>>>         return ret;
>>>  uncharge_out:
>>
>> Looks good to me.  I'll include this with some other DAX fixes I have pending.
> 
> Looks good to me as well.  Thanks for catching this.
> 

Yes. None of the xfstests catch this. It needs a private-mapping mmap in some combination
of other activity on the file at the same time.

Which the linker of gcc does. We have a test of a git clone Kernel-tree and make. which
catches this in the make phase. For some reason on ext4 it is reliable to crash but on xfs 1/2
the runs go through, go figure.

Thanks Yigal for the fast fix
Boaz

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