Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse

[Peter Xu <peterx@redhat.com>]

On Thu, Sep 01, 2022 at 04:50:45PM -0700, Yang Shi wrote:
> On Thu, Sep 1, 2022 at 4:26 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > Hi, Yang,
> >
> > On Thu, Sep 01, 2022 at 03:27:07PM -0700, Yang Shi wrote:
> > > Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm:
> > > introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer
> > > sufficient to handle concurrent GUP-fast in all cases, it only handles
> > > traditional IPI-based GUP-fast correctly.
> >
> > If TLB flush (or, IPI broadcasts) used to work to protect against gup-fast,
> > I'm kind of confused why it's not sufficient even if with RCU gup?  Isn't
> > that'll keep working as long as interrupt disabled (which current fast-gup
> > will still do)?
> 
> Actually the wording was copied from David's commit log for his
> PageAnonExclusive fix. My understanding is the IPI broadcast still
> works, but it may not be supported by all architectures and not
> preferred anymore. So we should avoid depending on IPI broadcast IIUC.
> 
> >
> > IIUC the issue is you suspect not all archs correctly implemented
> > pmdp_collapse_flush(), or am I wrong?
> 
> This is a possible fix, please see below for details.
> 
> >
> > > On architectures that send
> > > an IPI broadcast on TLB flush, it works as expected.  But on the
> > > architectures that do not use IPI to broadcast TLB flush, it may have
> > > the below race:
> > >
> > >    CPU A                                          CPU B
> > > THP collapse                                     fast GUP
> > >                                               gup_pmd_range() <-- see valid pmd
> > >                                                   gup_pte_range() <-- work on pte
> > > pmdp_collapse_flush() <-- clear pmd and flush
> > > __collapse_huge_page_isolate()
> > >     check page pinned <-- before GUP bump refcount
> > >                                                       pin the page
> > >                                                       check PTE <-- no change
> > > __collapse_huge_page_copy()
> > >     copy data to huge page
> > >     ptep_clear()
> > > install huge pmd for the huge page
> > >                                                       return the stale page
> > > discard the stale page
> > >
> > > The race could be fixed by checking whether PMD is changed or not after
> > > taking the page pin in fast GUP, just like what it does for PTE.  If the
> > > PMD is changed it means there may be parallel THP collapse, so GUP
> > > should back off.
> >
> > Could the race also be fixed by impl pmdp_collapse_flush() correctly for
> > the archs that are missing? Do you know which arch(s) is broken with it?
> 
> Yes, and this was suggested by me in the first place, but per the
> suggestion from John and David, this is not the preferred way. I think
> it is because:
> 
> Firstly, using IPI to serialize against fast GUP is not recommended
> anymore since fast GUP does check PTE then back off so we should avoid
> it.
> Secondly, if checking PMD then backing off could solve the problem,
> why do we still need broadcast IPI? It doesn't sound performant.
> 
> >
> > It's just not clear to me whether this patch is an optimization or a fix,
> > if it's a fix whether the IPI broadcast in ppc pmdp_collapse_flush() would
> > still be needed.
> 
> It is a fix and the fix will make IPI broadcast not useful anymore.

How about another patch to remove the ppc impl too?  Then it can be a two
patches series.

So that ppc developers can be copied and maybe it helps to have the ppc
people looking at current approach too.

Then the last piece of it is the s390 pmdp_collapse_flush().  I'm wondering
whether generic pmdp_collapse_flush() would be good enough, since the only
addition comparing to the s390 one will be flush_tlb_range() (which is a
further __tlb_flush_mm_lazy).  David may have some thoughts.

The patch itself looks good to me, one trivial nit below.

> 
> >
> > Thanks,
> >
> > >
> > > Also update the stale comment about serializing against fast GUP in
> > > khugepaged.
> > >
> > > Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()")
> > > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > > ---
> > >  mm/gup.c        | 30 ++++++++++++++++++++++++------
> > >  mm/khugepaged.c | 10 ++++++----
> > >  2 files changed, 30 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/mm/gup.c b/mm/gup.c
> > > index f3fc1f08d90c..4365b2811269 100644
> > > --- a/mm/gup.c
> > > +++ b/mm/gup.c
> > > @@ -2380,8 +2380,9 @@ static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
> > >  }
> > >
> > >  #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
> > > -static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> > > -                      unsigned int flags, struct page **pages, int *nr)
> > > +static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
> > > +                      unsigned long end, unsigned int flags,
> > > +                      struct page **pages, int *nr)
> > >  {
> > >       struct dev_pagemap *pgmap = NULL;
> > >       int nr_start = *nr, ret = 0;
> > > @@ -2423,7 +2424,23 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> > >                       goto pte_unmap;
> > >               }
> > >
> > > -             if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> > > +             /*
> > > +              * THP collapse conceptually does:
> > > +              *   1. Clear and flush PMD
> > > +              *   2. Check the base page refcount
> > > +              *   3. Copy data to huge page
> > > +              *   4. Clear PTE
> > > +              *   5. Discard the base page
> > > +              *
> > > +              * So fast GUP may race with THP collapse then pin and
> > > +              * return an old page since TLB flush is no longer sufficient
> > > +              * to serialize against fast GUP.
> > > +              *
> > > +              * Check PMD, if it is changed just back off since it
> > > +              * means there may be parallel THP collapse.

Would you mind rewording this comment a bit?  I feel it a bit weird to
suddenly mention about thp collapse especially its details.

Maybe some statement on the whole history of why check pte, and in what
case pmd check is needed (where the thp collapse example can be moved to,
imho)?

One of my attempt for reference..

		/*
		 * Fast-gup relies on pte change detection to avoid
		 * concurrent pgtable operations.
		 *
		 * To pin the page, fast-gup needs to do below in order:
		 * (1) pin the page (by prefetching pte), then (2) check
		 * pte not changed.
		 *
		 * For the rest of pgtable operations where pgtable updates
		 * can be racy with fast-gup, we need to do (1) clear pte,
		 * then (2) check whether page is pinned.
		 *
		 * Above will work for all pte-level operations, including
		 * thp split.
		 *
		 * For thp collapse, it's a bit more complicated because
		 * with RCU pgtable free fast-gup can be walking a pgtable
		 * page that is being freed (so pte is still valid but pmd
		 * can be cleared already).  To avoid race in such
		 * condition, we need to also check pmd here to make sure
		 * pmd doesn't change (corresponds to pmdp_collapse_flush()
		 * in the thp collide code path).
		 */

If you agree with the comment change, feel free to add:

Acked-by: Peter Xu <peterx@redhat.com>

Thanks,

-- 
Peter Xu