Re: [PATCH 09/12] mm/khugepaged: retract_page_tables() without mmap or vma lock

Hugh Dickins
2023-06-06T06:19:27Z

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On Wed, 31 May 2023, Jann Horn wrote:
> On Mon, May 29, 2023 at 8:25 AM Hugh Dickins <hughd@google.com> wrote:
> > +static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
...
> > + * Note that vma->anon_vma check is racy: it can be set after
> > + * the check, but page locks (with XA_RETRY_ENTRYs in holes)
> > + * prevented establishing new ptes of the page. So we are safe
> > + * to remove page table below, without even checking it's empty.
>
> This "we are safe to remove page table below, without even checking
> it's empty" assumes that the only way to create new anonymous PTEs is
> to use existing file PTEs, right? What about private shmem VMAs that
> are registered with userfaultfd as VM_UFFD_MISSING? I think for those,
> the UFFDIO_COPY ioctl lets you directly insert anonymous PTEs without
> looking at the mapping and its pages (except for checking that the
> insertion point is before end-of-file), protected only by mmap_lock
> (shared) and pte_offset_map_lock().

Right, from your comments and Peter's, thank you both, I can see that
userfaultfd breaks the usual assumptions here: so I'm putting an
if (unlikely(vma->anon_vma || userfaultfd_wp(vma)))
check in once we've got the ptlock; with a comment above it to point
the blame at uffd, though I gave up on describing all the detail.
And deleted this earlier "we are safe" paragraph.

You did suggest, in another mail, that perhaps there should be a scan
checking all pte_none() when we get the ptlock. I wasn't keen on yet
another debug scan for bugs and didn't add that, thinking I was going
to add a patch on the end to do so in page_table_check_pte_clear_range().

But when I came to write that patch, found that I'd been misled by its
name: it's about checking or adjusting some accounting, not really a
suitable place to check for pte_none() at all; so just scrapped it.

...
> > - collapse_and_free_pmd(mm, vma, addr, pmd);
>
> The old code called collapse_and_free_pmd(), which involves MMU
> notifier invocation...

...
> > + pml = pmd_lock(mm, pmd);
> > + ptl = pte_lockptr(mm, pmd);
> > + if (ptl != pml)
> > + spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
> > + pgt_pmd = pmdp_collapse_flush(vma, addr, pmd);
>
> ... while the new code only does pmdp_collapse_flush(), which clears
> the pmd entry and does a TLB flush, but AFAICS doesn't use MMU
> notifiers. My understanding is that that's problematic - maybe (?) it
> is sort of okay with regards to classic MMU notifier users like KVM,
> but it's probably wrong for IOMMUv2 users, where an IOMMU directly
> consumes the normal page tables?

Right, I intentionally left out the MMU notifier invocation, knowing
that we have already done an MMU notifier invocation when unmapping
any PTEs which were mapped: it was necessary for collapse_and_free_pmd()
in the collapse_pte_mapped_thp() case, but there was no notifier in this
case for many years, and I was glad to be rid of it.

However, I now see that you were adding it intentionally even for this
case in your f268f6cf875f; and from later comments in this thread, it
looks like there is still uncertainty about whether it is needed here,
but safer to assume that it is needed: I'll add it back.

>
> (FWIW, last I looked, there also seemed to be some other issues with
> MMU notifier usage wrt IOMMUv2, see the thread
> <https://lore.kernel.org/linux-mm/Yzbaf9HW1%2FreKqR8@nvidia.com/>.)