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

[Jerome Glisse <jglisse@redhat.com>]

On Sun, May 12, 2019 at 11:07:24AM -0400, Jerome Glisse wrote:
> On Tue, May 07, 2019 at 11:12:14AM -0700, Ralph Campbell wrote:
> > 
> > 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.
> 
> NAK for the patch see below why
> 
> > 
> > > > 
> > > > 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.
> 
> So this is not the usage pattern with HMM usage pattern is:
> 
> hmm_range_register()
> hmm_range_fault()
> hmm_range_unregister()
> 
> The hmm_vma_fault() is gonne so this patch here break thing.
> 
> See https://cgit.freedesktop.org/~glisse/linux/log/?h=hmm-5.2-v3

Sorry not enough coffee on sunday morning, so yeah this patch
looks good except that you do not need to move it up.

Note that hmm_vma_fault() is a gonner once ODP to HMM is upstream
and i converted nouveau/amd to new API then we can remove that
one.

Cheers,
Jérôme