Re: [PATCH 2/2] mm/khugepaged: skip shmem with userfaultfd

[David Stevens]

On Thu, Feb 16, 2023 at 7:48 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Tue, Feb 14, 2023 at 04:57:10PM +0900, David Stevens wrote:
> > From: David Stevens <stevensd@xxxxxxxxxxxx>
> >
> > Make sure that collapse_file respects any userfaultfds registered with
> > MODE_MISSING. If userspace has any such userfaultfds registered, then
> > for any page which it knows to be missing, it may expect a
> > UFFD_EVENT_PAGEFAULT. This means collapse_file needs to take care when
> > collapsing a shmem range would result in replacing an empty page with a
> > THP, so that it doesn't break userfaultfd.
> >
> > Synchronization when checking for userfaultfds in collapse_file is
> > tricky because the mmap locks can't be used to prevent races with the
> > registration of new userfaultfds. Instead, we provide synchronization by
> > ensuring that userspace cannot observe the fact that pages are missing
> > before we check for userfaultfds. Although this allows registration of a
> > userfaultfd to race with collapse_file, it ensures that userspace cannot
> > observe any pages transition from missing to present after such a race.
> > This makes such a race indistinguishable to the collapse occurring
> > immediately before the userfaultfd registration.
> >
> > The first step to provide this synchronization is to stop filling gaps
> > during the loop iterating over the target range, since the page cache
> > lock can be dropped during that loop. The second step is to fill the
> > gaps with XA_RETRY_ENTRY after the page cache lock is acquired the final
> > time, to avoid races with accesses to the page cache that only take the
> > RCU read lock.
> >
> > This fix is targeted at khugepaged, but the change also applies to
> > MADV_COLLAPSE. MADV_COLLAPSE on a range with a userfaultfd will now
> > return EBUSY if there are any missing pages (instead of succeeding on
> > shmem and returning EINVAL on anonymous memory). There is also now a
> > window during MADV_COLLAPSE where a fault on a missing page will cause
> > the syscall to fail with EAGAIN.
> >
> > The fact that intermediate page cache state can no longer be observed
> > before the rollback of a failed collapse is also technically a
> > userspace-visible change (via at least SEEK_DATA and SEEK_END), but it
> > is exceedingly unlikely that anything relies on being able to observe
> > that transient state.
> >
> > Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
> > ---
> >  mm/khugepaged.c | 66 +++++++++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 58 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index b648f1053d95..8c2e2349e883 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -55,6 +55,7 @@ enum scan_result {
> >       SCAN_CGROUP_CHARGE_FAIL,
> >       SCAN_TRUNCATED,
> >       SCAN_PAGE_HAS_PRIVATE,
> > +     SCAN_PAGE_FILLED,
>
> PS: You may want to also touch SCAN_STATUS in huge_memory.h next time.
>
> >  };
> >
> >  #define CREATE_TRACE_POINTS
> > @@ -1725,8 +1726,8 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
> >   *  - allocate and lock a new huge page;
> >   *  - scan page cache replacing old pages with the new one
> >   *    + swap/gup in pages if necessary;
> > - *    + fill in gaps;
>
> IIUC it's not a complete removal, but just moved downwards:
>
> >   *    + keep old pages around in case rollback is required;
> > + *  - finalize updates to the page cache;
>
>          + fill in gaps with RETRY entries
>          + detect race conditions with userfaultfds
>
> >   *  - if replacing succeeds:
> >   *    + copy data over;
> >   *    + free old pages;
> > @@ -1805,13 +1806,12 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >                                               result = SCAN_TRUNCATED;
> >                                               goto xa_locked;
> >                                       }
> > -                                     xas_set(&xas, index);
> > +                                     xas_set(&xas, index + 1);
> >                               }
> >                               if (!shmem_charge(mapping->host, 1)) {
> >                                       result = SCAN_FAIL;
> >                                       goto xa_locked;
> >                               }
> > -                             xas_store(&xas, hpage);
> >                               nr_none++;
> >                               continue;
> >                       }
> > @@ -1970,6 +1970,56 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >               put_page(page);
> >               goto xa_unlocked;
> >       }
> > +
> > +     if (nr_none) {
> > +             struct vm_area_struct *vma;
> > +             int nr_none_check = 0;
> > +
> > +             xas_unlock_irq(&xas);
> > +             i_mmap_lock_read(mapping);
> > +             xas_lock_irq(&xas);
> > +
> > +             xas_set(&xas, start);
> > +             for (index = start; index < end; index++) {
> > +                     if (!xas_next(&xas)) {
> > +                             xas_store(&xas, XA_RETRY_ENTRY);
> > +                             nr_none_check++;
> > +                     }
> > +             }
> > +
> > +             if (nr_none != nr_none_check) {
> > +                     result = SCAN_PAGE_FILLED;
> > +                     goto immap_locked;
> > +             }
> > +
> > +             /*
> > +              * If userspace observed a missing page in a VMA with an armed
> > +              * userfaultfd, then it might expect a UFFD_EVENT_PAGEFAULT for
> > +              * that page, so we need to roll back to avoid suppressing such
> > +              * an event. Any userfaultfds armed after this point will not be
> > +              * able to observe any missing pages due to the previously
> > +              * inserted retry entries.
> > +              */
> > +             vma_interval_tree_foreach(vma, &mapping->i_mmap, start, start) {
> > +                     if (userfaultfd_missing(vma)) {
> > +                             result = SCAN_EXCEED_NONE_PTE;
> > +                             goto immap_locked;
> > +                     }
> > +             }
> > +
> > +immap_locked:
> > +             i_mmap_unlock_read(mapping);
> > +             if (result != SCAN_SUCCEED) {
> > +                     xas_set(&xas, start);
> > +                     for (index = start; index < end; index++) {
> > +                             if (xas_next(&xas) == XA_RETRY_ENTRY)
> > +                                     xas_store(&xas, NULL);
> > +                     }
> > +
> > +                     goto xa_locked;
> > +             }
> > +     }
> > +
>
> Until here, all look fine to me (ignoring patch 1 for now; assuming the
> hpage is always uptodate).
>
> My question is after here we'll release page cache lock again before
> try_to_unmap_flush(), but is it safe to keep RETRY entries after releasing
> page cache lock?  It means other threads can be spinning.  I assume page
> lock is always safe and sleepable, but not sure about the page cache lock
> here.

We insert the multi-index entry for hpage before releasing the page
cache lock, which should replace all of the XA_RETRY_ENTRYs. So the
page cache will be fully up to date when we release the lock, at least
in terms of which pages it contains.

-David