Re: [PATCH v4 2/3] mm/khugepaged: Fix GUP-fast interaction by sending IPI

[Yang Shi]

On Mon, Nov 28, 2022 at 10:03 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> Since commit 70cbc3cc78a99 ("mm: gup: fix the fast GUP race against THP
> collapse"), the lockless_pages_from_mm() fastpath rechecks the pmd_t to
> ensure that the page table was not removed by khugepaged in between.
>
> However, lockless_pages_from_mm() still requires that the page table is not
> concurrently freed or reused to store non-PTE data. Otherwise, problems
> can occur because:
>
>  - deposited page tables can be freed when a THP page somewhere in the
>    mm is removed
>  - some architectures store non-PTE information inside deposited page
>    tables (see radix__pgtable_trans_huge_deposit())
>
> Additionally, lockless_pages_from_mm() is also somewhat brittle with
> regards to page tables being repeatedly moved back and forth, but
> that shouldn't be an issue in practice.
>
> Fix it by sending IPIs (if the architecture uses
> semi-RCU-style page table freeing) before freeing/reusing page tables.
>
> As noted in mm/gup.c, on configs that define CONFIG_HAVE_FAST_GUP,
> there are two possible cases:
>
>  1. CONFIG_MMU_GATHER_RCU_TABLE_FREE is set, causing
>     tlb_remove_table_sync_one() to send an IPI to synchronize with
>     lockless_pages_from_mm().
>  2. CONFIG_MMU_GATHER_RCU_TABLE_FREE is unset, indicating that all
>     TLB flushes are already guaranteed to send IPIs.
>     tlb_remove_table_sync_one() will do nothing, but we've already
>     run pmdp_collapse_flush(), which did a TLB flush, which must have
>     involved IPIs.

I'm trying to catch up with the discussion after the holiday break. I
understand you switched from always allocating a new page table page
(we decided before) to sending IPIs to serialize against fast-GUP,
this is fine to me.

So the code now looks like:
    pmdp_collapse_flush()
    sending IPI

But the missing part is how we reached "TLB flushes are already
guaranteed to send IPIs" when CONFIG_MMU_GATHER_RCU_TABLE_FREE is
unset? ARM64 doesn't do it IIRC. Or did I miss something?

>
> Cc: stable@xxxxxxxxxx
> Fixes: ba76149f47d8 ("thp: khugepaged")
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
> v4:
>  - added ack from David Hildenbrand
>  - made commit message more verbose
>
>  include/asm-generic/tlb.h | 4 ++++
>  mm/khugepaged.c           | 2 ++
>  mm/mmu_gather.c           | 4 +---
>  3 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> index 492dce43236ea..cab7cfebf40bd 100644
> --- a/include/asm-generic/tlb.h
> +++ b/include/asm-generic/tlb.h
> @@ -222,12 +222,16 @@ extern void tlb_remove_table(struct mmu_gather *tlb, void *table);
>  #define tlb_needs_table_invalidate() (true)
>  #endif
>
> +void tlb_remove_table_sync_one(void);
> +
>  #else
>
>  #ifdef tlb_needs_table_invalidate
>  #error tlb_needs_table_invalidate() requires MMU_GATHER_RCU_TABLE_FREE
>  #endif
>
> +static inline void tlb_remove_table_sync_one(void) { }
> +
>  #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
>
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 674b111a24fa7..c3d3ce596bff7 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1057,6 +1057,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>         _pmd = pmdp_collapse_flush(vma, address, pmd);
>         spin_unlock(pmd_ptl);
>         mmu_notifier_invalidate_range_end(&range);
> +       tlb_remove_table_sync_one();
>
>         spin_lock(pte_ptl);
>         result =  __collapse_huge_page_isolate(vma, address, pte, cc,
> @@ -1415,6 +1416,7 @@ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *v
>                 lockdep_assert_held_write(&vma->anon_vma->root->rwsem);
>
>         pmd = pmdp_collapse_flush(vma, addr, pmdp);
> +       tlb_remove_table_sync_one();
>         mm_dec_nr_ptes(mm);
>         page_table_check_pte_clear_range(mm, addr, pmd);
>         pte_free(mm, pmd_pgtable(pmd));
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index add4244e5790d..3a2c3f8cad2fe 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -153,7 +153,7 @@ static void tlb_remove_table_smp_sync(void *arg)
>         /* Simply deliver the interrupt */
>  }
>
> -static void tlb_remove_table_sync_one(void)
> +void tlb_remove_table_sync_one(void)
>  {
>         /*
>          * This isn't an RCU grace period and hence the page-tables cannot be
> @@ -177,8 +177,6 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch)
>
>  #else /* !CONFIG_MMU_GATHER_RCU_TABLE_FREE */
>
> -static void tlb_remove_table_sync_one(void) { }
> -
>  static void tlb_remove_table_free(struct mmu_table_batch *batch)
>  {
>         __tlb_remove_table_free(batch);
> --
> 2.38.1.584.g0f3c55d4c2-goog
>