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

[Ralph Campbell]

On 5/12/19 8:07 AM, 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@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.
>
> NAK for the patch see below why
>
> > > > 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.
>
> 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

The patch series is on top of v5.1-rc6-mmotm-2019-04-25-16-30.
hmm_vma_fault() is defined there and in your hmm-5.2-v3 branch as
a backward compatibility transition function in include/linux/hmm.h.
So I agree the new API is to use hmm_range_register(), etc.
This is intended to cover the transition period.
Note that hmm_vma_fault() is being called from
drivers/gpu/drm/nouveau/nouveau_svm.c in both trees.