Date:   Thu, 2 Mar 2023 00:53:45 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, michel@lespinasse.org,
        jglisse@google.com, mhocko@suse.com, vbabka@suse.cz,
        hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net,
        willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org,
        ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com,
        will@kernel.org, luto@kernel.org, songliubraving@fb.com,
        peterx@redhat.com, david@redhat.com, dhowells@redhat.com,
        hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev,
        punit.agrawal@bytedance.com, lstoakes@gmail.com,
        peterjung1337@gmail.com, rientjes@google.com, chriscli@google.com,
        axelrasmussen@google.com, joelaf@google.com, minchan@google.com,
        rppt@kernel.org, jannh@google.com, shakeelb@google.com,
        tatashin@google.com, edumazet@google.com, gthelen@google.com,
        gurua@google.com, arjunroy@google.com, soheil@google.com,
        leewalsh@google.com, posk@google.com,
        michalechner92@googlemail.com, linux-mm@kvack.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH v4 18/33] mm: write-lock VMAs before removing them from
 VMA tree
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On Wed, Mar 01, 2023 at 10:42:48AM -0800, Suren Baghdasaryan wrote:
> On Wed, Mar 1, 2023 at 10:34 AM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Tue, Feb 28, 2023 at 11:57 PM Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> > >
> > > On Wed, Mar 01, 2023 at 07:43:33AM +0000, Hyeonggon Yoo wrote:
> > > > On Mon, Feb 27, 2023 at 09:36:17AM -0800, Suren Baghdasaryan wrote:
> > > > > Write-locking VMAs before isolating them ensures that page fault
> > > > > handlers don't operate on isolated VMAs.
> > > > >
> > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > > ---
> > > > >  mm/mmap.c  | 1 +
> > > > >  mm/nommu.c | 5 +++++
> > > > >  2 files changed, 6 insertions(+)
> > > > >
> > > > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > > > index 1f42b9a52b9b..f7ed357056c4 100644
> > > > > --- a/mm/mmap.c
> > > > > +++ b/mm/mmap.c
> > > > > @@ -2255,6 +2255,7 @@ int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > > > >  static inline int munmap_sidetree(struct vm_area_struct *vma,
> > > > >                                struct ma_state *mas_detach)
> > > > >  {
> > > > > +   vma_start_write(vma);
> > > > >     mas_set_range(mas_detach, vma->vm_start, vma->vm_end - 1);
> > > >
> > > > I may be missing something, but have few questions:
> > > >
> > > >       1) Why does a writer need to both write-lock a VMA and mark the VMA detached
> > > >          when unmapping it, isn't it enough to just only write-lock a VMA?
> >
> > We need to mark the VMA detached to avoid handling page fault in a
> > detached VMA. The possible scenario is:
> >
> > lock_vma_under_rcu
> >   vma = mas_walk(&mas)
> >                                                         munmap_sidetree
> >                                                           vma_start_write(vma)
> >
> > mas_store_gfp() // remove VMA from the tree
> >                                                           vma_end_write_all()
> >   vma_start_read(vma)
> >   // we locked the VMA but it is not part of the tree anymore.
> >
> > So, marking the VMA locked before vma_end_write_all() and checking
> 
> Sorry, I should have said "marking the VMA *detached* before
> vma_end_write_all() and checking vma->detached after vma_start_read()
> helps us avoid handling faults in the detached VMA."
> 
> > vma->detached after vma_start_read() helps us avoid handling faults in
> > the detached VMA.

Thank you for explanation, that makes sense!

By the way, if there are no 32bit users of Per-VMA lock (are there?),
"detached" bool could be a VMA flag (i.e. making it depend on 64BIT
and selecting ARCH_USES_HIGH_VMA_FLAGS)

Thanks,
Hyeonggon