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)?
IIUC the issue is you suspect not all archs correctly implemented
pmdp_collapse_flush(), or am I wrong?
> 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?
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.
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@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.
> + */
> + if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
> + unlikely(pte_val(pte) != pte_val(*ptep))) {
> 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);
> spin_unlock(pmd_ptl);
> --
> 2.26.3
>
--
Peter Xu
