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.
>
> 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
>
