Re: [v2 PATCH 1/2] mm: gup: fix the fast GUP race against THP collapse

[John Hubbard <jhubbard@nvidia.com>]

On 9/7/22 11:01, 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
> 
> 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()")
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
> v2: * Incorporated the comment from Peter about the comment.
>      * Moved the comment right before gup_pte_range() instead of in the
>        body of the function, per John.
>      * Added patch 2/2 per Aneesh.
> 
>   mm/gup.c        | 34 ++++++++++++++++++++++++++++------
>   mm/khugepaged.c | 10 ++++++----
>   2 files changed, 34 insertions(+), 10 deletions(-)
> 

Looks good.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

> diff --git a/mm/gup.c b/mm/gup.c
> index f3fc1f08d90c..40aa1c937212 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2380,8 +2380,28 @@ 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)
> +/*
> + * Fast-gup relies on pte change detection to avoid concurrent pgtable
> + * operations.
> + *
> + * To pin the page, fast-gup needs to do below in order:
> + * (1) pin the page (by prefetching pte), then (2) check pte not changed.
> + *
> + * For the rest of pgtable operations where pgtable updates can be racy
> + * with fast-gup, we need to do (1) clear pte, then (2) check whether page
> + * is pinned.
> + *
> + * Above will work for all pte-level operations, including THP split.
> + *
> + * For THP collapse, it's a bit more complicated because fast-gup may be
> + * walking a pgtable page that is being freed (pte is still valid but pmd
> + * can be cleared already).  To avoid race in such condition, we need to
> + * also check pmd here to make sure pmd doesn't change (corresponds to
> + * pmdp_collapse_flush() in the THP collapse code path).
> + */
> +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 +2443,8 @@ 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))) {
> +		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 +2491,9 @@ 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 +2813,7 @@ 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);