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

[Souptick Joarder]

On Tue, May 7, 2019 at 11:42 PM Ralph Campbell <rcampbell@xxxxxxxxxx> wrote:
>
>
> On 5/7/19 6:15 AM, Souptick Joarder wrote:
> > On Tue, May 7, 2019 at 5:00 AM <rcampbell@xxxxxxxxxx> wrote:
> >>
> >> From: Ralph Campbell <rcampbell@xxxxxxxxxx>
> >>
> >> 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.

Thanks.
>
> >>
> >> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> >> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> >> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> >> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> >> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> >> Cc: Balbir Singh <bsingharora@xxxxxxxxx>
> >> Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> >> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> >> Cc: Souptick Joarder <jrdr.linux@xxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> ---
> >>   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.

Ok. I think, adding it in change log will be helpful :)
>
> >>                  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
> >>