Re: [PATCH 4/5] mm/hmm: hmm_vma_fault() doesn't always call hmm_range_unregister()

[Ralph Campbell <rcampbell@nvidia.com>]

On 5/7/19 6:15 AM, Souptick Joarder wrote:
> On Tue, May 7, 2019 at 5:00 AM <rcampbell@nvidia.com> wrote:
>>
>> From: Ralph Campbell <rcampbell@nvidia.com>
>>
>> The helper function hmm_vma_fault() calls hmm_range_register() but is
>> missing a call to hmm_range_unregister() in one of the error paths.
>> This leads to a reference count leak and ultimately a memory leak on
>> struct hmm.
>>
>> Always call hmm_range_unregister() if hmm_range_register() succeeded.
> 
> How about * Call hmm_range_unregister() in error path if
> hmm_range_register() succeeded* ?

Sure, sounds good.
I'll include that in v2.

>>
>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>> Cc: John Hubbard <jhubbard@nvidia.com>
>> Cc: Ira Weiny <ira.weiny@intel.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Arnd Bergmann <arnd@arndb.de>
>> Cc: Balbir Singh <bsingharora@gmail.com>
>> Cc: Dan Carpenter <dan.carpenter@oracle.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Souptick Joarder <jrdr.linux@gmail.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> ---
>>   include/linux/hmm.h | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
>> index 35a429621e1e..fa0671d67269 100644
>> --- a/include/linux/hmm.h
>> +++ b/include/linux/hmm.h
>> @@ -559,6 +559,7 @@ static inline int hmm_vma_fault(struct hmm_range *range, bool block)
>>                  return (int)ret;
>>
>>          if (!hmm_range_wait_until_valid(range, HMM_RANGE_DEFAULT_TIMEOUT)) {
>> +               hmm_range_unregister(range);
>>                  /*
>>                   * The mmap_sem was taken by driver we release it here and
>>                   * returns -EAGAIN which correspond to mmap_sem have been
>> @@ -570,13 +571,13 @@ static inline int hmm_vma_fault(struct hmm_range *range, bool block)
>>
>>          ret = hmm_range_fault(range, block);
>>          if (ret <= 0) {
>> +               hmm_range_unregister(range);
> 
> what is the reason to moved it up ?

I moved it up because the normal calling pattern is:
     down_read(&mm->mmap_sem)
     hmm_vma_fault()
         hmm_range_register()
         hmm_range_fault()
         hmm_range_unregister()
     up_read(&mm->mmap_sem)

I don't think it is a bug to unlock mmap_sem and then unregister,
it is just more consistent nesting.

>>                  if (ret == -EBUSY || !ret) {
>>                          /* Same as above, drop mmap_sem to match old API. */
>>                          up_read(&range->vma->vm_mm->mmap_sem);
>>                          ret = -EBUSY;
>>                  } else if (ret == -EAGAIN)
>>                          ret = -EBUSY;
>> -               hmm_range_unregister(range);
>>                  return ret;
>>          }
>>          return 0;
>> --
>> 2.20.1
>>