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Date: Thu, 4 Apr 2024 17:04:36 -0400
From: Peter Xu <peterx@redhat.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	Lokesh Gidra <lokeshgidra@google.com>, akpm@linux-foundation.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kernel-team@android.com, aarcange@redhat.com, david@redhat.com,
	zhengqi.arch@bytedance.com, kaleshsingh@google.com,
	ngeoffray@google.com
Subject: Re: [PATCH] userfaultfd: change src_folio after ensuring it's
 unpinned in UFFDIO_MOVE
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References: <20240404171726.2302435-1-lokeshgidra@google.com>
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On Thu, Apr 04, 2024 at 01:55:07PM -0700, Suren Baghdasaryan wrote:
> On Thu, Apr 4, 2024 at 1:32 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Thu, Apr 04, 2024 at 06:21:50PM +0100, Matthew Wilcox wrote:
> > > On Thu, Apr 04, 2024 at 10:17:26AM -0700, Lokesh Gidra wrote:
> > > > -           folio_move_anon_rmap(src_folio, dst_vma);
> > > > -           WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> > > > -
> > > >             src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
> > > >             /* Folio got pinned from under us. Put it back and fail the move. */
> > > >             if (folio_maybe_dma_pinned(src_folio)) {
> > > > @@ -2270,6 +2267,9 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
> > > >                     goto unlock_ptls;
> > > >             }
> > > >
> > > > +           folio_move_anon_rmap(src_folio, dst_vma);
> > > > +           WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> > > > +
> > >
> > > This use of WRITE_ONCE scares me.  We hold the folio locked.  Why do
> > > we need to use WRITE_ONCE?  Who's looking at folio->index without
> > > holding the folio lock?
> >
> > Seems true, but maybe suitable for a separate patch to clean it even so?
> > We also have the other pte level which has the same WRITE_ONCE(), so if we
> > want to drop we may want to drop both.
> 
> Yes, I'll do that separately and will remove WRITE_ONCE() in both places.

Thanks, Suren.  Besides, any comment on below?

It's definely a generic per-vma question too (besides my willingness to
remove that userfault specific code..), so comments welcomed.

> 
> >
> > I just got to start reading some the new move codes (Lokesh, apologies on
> > not be able to provide feedbacks previously..), but then I found one thing
> > unclear, on special handling of private file mappings only in userfault
> > context, and I didn't know why:
> >
> > lock_vma():
> >         if (vma) {
> >                 /*
> >                  * lock_vma_under_rcu() only checks anon_vma for private
> >                  * anonymous mappings. But we need to ensure it is assigned in
> >                  * private file-backed vmas as well.
> >                  */
> >                 if (!(vma->vm_flags & VM_SHARED) && unlikely(!vma->anon_vma))
> >                         vma_end_read(vma);
> >                 else
> >                         return vma;
> >         }
> >
> > AFAIU even for generic users of lock_vma_under_rcu(), anon_vma must be
> > stable to be used.  Here it's weird to become an userfault specific
> > operation to me.
> >
> > I was surprised how it worked for private file maps on faults, then I had a
> > check and it seems we postponed such check until vmf_anon_prepare(), which
> > is the CoW path already, so we do as I expected, but seems unnecessary to
> > that point?
> >
> > Would something like below make it much cleaner for us?  As I just don't
> > yet see why userfault is special here.
> >
> > Thanks,
> >
> > ===8<===
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 984b138f85b4..d5cf1d31c671 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3213,10 +3213,8 @@ vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
> >
> >         if (likely(vma->anon_vma))
> >                 return 0;
> > -       if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > -               vma_end_read(vma);
> > -               return VM_FAULT_RETRY;
> > -       }
> > +       /* We shouldn't try a per-vma fault at all if anon_vma isn't solid */
> > +       WARN_ON_ONCE(vmf->flags & FAULT_FLAG_VMA_LOCK);
> >         if (__anon_vma_prepare(vma))
> >                 return VM_FAULT_OOM;
> >         return 0;
> > @@ -5817,9 +5815,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> >          * find_mergeable_anon_vma uses adjacent vmas which are not locked.
> >          * This check must happen after vma_start_read(); otherwise, a
> >          * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
> > -        * from its anon_vma.
> > +        * from its anon_vma.  This applies to both anon or private file maps.
> >          */
> > -       if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
> > +       if (unlikely(!(vma->vm_flags & VM_SHARED) && !vma->anon_vma))
> >                 goto inval_end_read;
> >
> >         /* Check since vm_start/vm_end might change before we lock the VMA */
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index f6267afe65d1..61f21da77dcd 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -72,17 +72,8 @@ static struct vm_area_struct *lock_vma(struct mm_struct *mm,
> >         struct vm_area_struct *vma;
> >
> >         vma = lock_vma_under_rcu(mm, address);
> > -       if (vma) {
> > -               /*
> > -                * lock_vma_under_rcu() only checks anon_vma for private
> > -                * anonymous mappings. But we need to ensure it is assigned in
> > -                * private file-backed vmas as well.
> > -                */
> > -               if (!(vma->vm_flags & VM_SHARED) && unlikely(!vma->anon_vma))
> > -                       vma_end_read(vma);
> > -               else
> > -                       return vma;
> > -       }
> > +       if (vma)
> > +               return vma;
> >
> >         mmap_read_lock(mm);
> >         vma = find_vma_and_prepare_anon(mm, address);
> > --
> > 2.44.0
> >
> >
> > --
> > Peter Xu
> >
> 

-- 
Peter Xu