Re: [PATCH] userfaultfd: change src_folio after ensuring it's unpinned in UFFDIO_MOVE

From: Suren Baghdasaryan
Date: Thu Apr 04 2024 - 17:08:06 EST

On Thu, Apr 4, 2024 at 2:04 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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.

Yes, I was typing my reply :)
This might have happened simply because lock_vma_under_rcu() was
originally developed to handle only anonymous page faults and then got
expanded to cover file-backed cases as well. Your suggestion seems
fine to me but I would feel much more comfortable after Matthew (who
added file-backed support) reviewed it.

>
> >
> > >
> > > 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
>