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

[Yang Shi]

On Mon, Sep 5, 2022 at 12:59 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 05.09.22 00:29, John Hubbard wrote:
> > On 9/1/22 15:27, 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.  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
> >
> > Hi Yang,
> >
> > Thanks for taking the trouble to write down these notes. I always
> > forget which race we are dealing with, and this is a great help. :)
> >
> > More...
> >
> >>
> >> 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.
> >>
> >> 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@xxxxxxxxx>
> >> ---
> >>   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.
> >> +             */
> >
> > As I mentioned in the other thread, it would be a nice touch to move
> > such discussion into the comment header.
> >
> >> +            if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
> >> +                unlikely(pte_val(pte) != pte_val(*ptep))) {
> >
> >
> > That should be READ_ONCE() for the *pmdp and *ptep reads. Because this
> > whole lockless house of cards may fall apart if we try reading the
> > page table values without READ_ONCE().
>
> I came to the conclusion that the implicit memory barrier when grabbing
> a reference on the page is sufficient such that we don't need READ_ONCE
> here.
>
> If we still intend to change that code, we should fixup all GUP-fast
> functions in a similar way. But again, I don't think we need a change here.
>
>
> >> -     * After this gup_fast can't run anymore. This also removes
> >> -     * any huge TLB entry from the CPU so we won't allow
> >> -     * huge and small TLB entries for the same virtual address
> >> -     * to avoid the risk of CPU bugs in that area.
> >> +     * This removes any huge TLB entry from the CPU so we won't allow
> >> +     * huge and small TLB entries for the same virtual address to
> >> +     * avoid the risk of CPU bugs in that area.
> >> +     *
> >> +     * Parallel fast GUP is fine since fast GUP will back off when
> >> +     * it detects PMD is changed.
> >>       */
> >>      _pmd = pmdp_collapse_flush(vma, address, pmd);
> >
> > To follow up on David Hildenbrand's note about this in the nearby thread...
> > I'm also not sure if pmdp_collapse_flush() implies a memory barrier on
> > all arches. It definitely does do an atomic op with a return value on x86,
> > but that's just one arch.
> >
>
> I think a ptep/pmdp clear + TLB flush really has to imply a memory
> barrier, otherwise TLB flushing code might easily mess up with
> surrounding code. But we should better double-check.
>
> s390x executes an IDTE instruction, which performs serialization (->
> memory barrier). arm64 seems to use DSB instructions to enforce memory
> ordering.

IIUC we just need to care about the architectures which support both
THP and fast-GUP, right? If so I got the below list:

Loongarch - I didn't see explicit memory barrier, but it does IPI
MIPS - as same as Loongarch
PowerPC - has memory barrier
x86 - atomic operations imply memory barrier
Arm - I didn't see explicit memory barrier, not sure if ARM's tlb
flush instructions imply memory barrier or not, but it does IPI
Arm64 - uses DSB as David mentioned
s390 - executes IDTE as David mentioned

So I think the questionable architectures are Loongarch, MIPS and ARM.
And IPI is not called if the PMD entry is not present on a remote CPU.
So they may have race against fast GUP IIUC. But we'd better double
check with the maintainers.

>
> --
> Thanks,
>
> David / dhildenb
>