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

[David Hildenbrand]

On 05.09.22 12:24, David Hildenbrand wrote:
> On 05.09.22 12:16, Baolin Wang wrote:
> > On 9/5/2022 3:59 PM, David Hildenbrand 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.
> >
> > IMHO the compiler may optimize the code 'pte_val(*ptep)' to be always
> > get from a register, then we can get an old value if other thread did
> > set_pte(). I am not sure how the implicit memory barrier can pervent the
> > compiler optimization? Please correct me if I missed something.
>
> IIUC, an memory barrier always implies a compiler barrier.

To clarify what I mean, Documentation/atomic_t.txt documents

NOTE: when the atomic RmW ops are fully ordered, they should also imply 
a compiler barrier.

--
Thanks,

David / dhildenb