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Date: Tue, 6 Sep 2022 11:50:56 -0700
From: Yang Shi <shy828301@...il.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: david@...hat.com, peterx@...hat.com,
kirill.shutemov@...ux.intel.com, jgg@...dia.com, hughd@...gle.com,
akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse
On Sun, Sep 4, 2022 at 3:29 PM John Hubbard <jhubbard@...dia.com> 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. :)
My pleasure, I'm glad it is helpful.
>
> 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@...il.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.
> > + */
>
> As I mentioned in the other thread, it would be a nice touch to move
> such discussion into the comment header.
Sure, you mean the comment before gup_pte_range() so that the real
code stays succinct, right?
>
> > + 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().
>
> That's a rather vague statement, and in fact, the READ_ONCE() should
> be paired with a page table write somewhere else, to make that claim
> more precise.
Thanks for the suggestion. Per the discussion later (mainly from David
and Jason), I think we are going to have a separate patch to clean up
all the page table access for GUP.
>
>
> > gup_put_folio(folio, 1, flags);
> > goto pte_unmap;
> > }
> > @@ -2470,8 +2487,9 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> > * get_user_pages_fast_only implementation that can pin pages. Thus it's still
> > * useful to have gup_huge_pmd even if we can't operate on ptes.
> > */
> > -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)
> > {
> > return 0;
> > }
> > @@ -2791,7 +2809,7 @@ static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned lo
> > if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr,
> > PMD_SHIFT, next, flags, pages, nr))
> > return 0;
> > - } else if (!gup_pte_range(pmd, addr, next, flags, pages, nr))
> > + } else if (!gup_pte_range(pmd, pmdp, addr, next, flags, pages, nr))
> > return 0;
> > } while (pmdp++, addr = next, addr != end);
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 2d74cf01f694..518b49095db3 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1049,10 +1049,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >
> > pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> > /*
> > - * 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.
Will reply in detail to David's thread.
>
>
> thanks,
>
> --
> John Hubbard
> NVIDIA
>
> > spin_unlock(pmd_ptl);
>
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