linux-kernel - Re: [PATCH 4/5] mm/hmm: hmm_vma_fault() doesn't always call hmm_range

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Date:   Sun, 12 May 2019 11:07:24 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     Souptick Joarder <jrdr.linux@...il.com>,
        Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        John Hubbard <jhubbard@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/5] mm/hmm: hmm_vma_fault() doesn't always call
 hmm_range_unregister()

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@...dia.com> wrote:
> > > 
> > > From: Ralph Campbell <rcampbell@...dia.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@...dia.com>
> > > Cc: John Hubbard <jhubbard@...dia.com>
> > > Cc: Ira Weiny <ira.weiny@...el.com>
> > > Cc: Dan Williams <dan.j.williams@...el.com>
> > > Cc: Arnd Bergmann <arnd@...db.de>
> > > Cc: Balbir Singh <bsingharora@...il.com>
> > > Cc: Dan Carpenter <dan.carpenter@...cle.com>
> > > Cc: Matthew Wilcox <willy@...radead.org>
> > > Cc: Souptick Joarder <jrdr.linux@...il.com>
> > > Cc: Andrew Morton <akpm@...ux-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