linux-kernel - Re: [PATCH v1 4/4] mm: remove boolean output parameters from folio_pte_batch

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Message-ID: <EB391EDC-4CDC-4B18-BE95-7DEFE43109DF@nvidia.com>
Date: Mon, 30 Jun 2025 13:59:52 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Matthew Brost <matthew.brost@...el.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Pedro Falcato <pfalcato@...e.de>,
 Rik van Riel <riel@...riel.com>, Harry Yoo <harry.yoo@...cle.com>
Subject: Re: [PATCH v1 4/4] mm: remove boolean output parameters from
 folio_pte_batch_ext()

On 27 Jun 2025, at 7:55, David Hildenbrand wrote:

> Instead, let's just allow for specifying through flags whether we want
> to have bits merged into the original PTE.
>
> For the madvise() case, simplify by having only a single parameter for
> merging young+dirty. For madvise_cold_or_pageout_pte_range() merging the
> dirty bit is not required, but also not harmful. This code is not that
> performance critical after all to really force all micro-optimizations.
>
> As we now have two pte_t * parameters, use PageTable() to make sure we
> are actually given a pointer at a copy of the PTE, not a pointer into
> an actual page table.
>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/internal.h | 58 +++++++++++++++++++++++++++++++--------------------
>  mm/madvise.c  | 26 +++++------------------
>  mm/memory.c   |  8 ++-----
>  mm/util.c     |  2 +-
>  4 files changed, 43 insertions(+), 51 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 6000b683f68ee..fe69e21b34a24 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -208,6 +208,18 @@ typedef int __bitwise fpb_t;
>  /* Compare PTEs honoring the soft-dirty bit. */
>  #define FPB_HONOR_SOFT_DIRTY		((__force fpb_t)BIT(1))
>
> +/*
> + * Merge PTE write bits: if any PTE in the batch is writable, modify the
> + * PTE at @ptentp to be writable.
> + */
> +#define FPB_MERGE_WRITE			((__force fpb_t)BIT(2))
> +
> +/*
> + * Merge PTE young and dirty bits: if any PTE in the batch is young or dirty,
> + * modify the PTE at @ptentp to be young or dirty, respectively.
> + */
> +#define FPB_MERGE_YOUNG_DIRTY		((__force fpb_t)BIT(3))
> +
>  static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>  {
>  	if (!(flags & FPB_HONOR_DIRTY))
> @@ -220,16 +232,11 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>  /**
>   * folio_pte_batch_ext - detect a PTE batch for a large folio
>   * @folio: The large folio to detect a PTE batch for.
> + * @vma: The VMA. Only relevant with FPB_MERGE_WRITE, otherwise can be NULL.
>   * @ptep: Page table pointer for the first entry.
> - * @pte: Page table entry for the first page.
> + * @ptentp: Pointer at a copy of the first page table entry.
>   * @max_nr: The maximum number of table entries to consider.
>   * @flags: Flags to modify the PTE batch semantics.
> - * @any_writable: Optional pointer to indicate whether any entry except the
> - *		  first one is writable.
> - * @any_young: Optional pointer to indicate whether any entry except the
> - *		  first one is young.
> - * @any_dirty: Optional pointer to indicate whether any entry except the
> - *		  first one is dirty.
>   *
>   * Detect a PTE batch: consecutive (present) PTEs that map consecutive
>   * pages of the same large folio in a single VMA and a single page table.
> @@ -242,28 +249,26 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>   * must be limited by the caller so scanning cannot exceed a single VMA and
>   * a single page table.
>   *
> + * Depending on the FPB_MERGE_* flags, the pte stored at @ptentp will
> + * be modified.
> + *
>   * This function will be inlined to optimize based on the input parameters;
>   * consider using folio_pte_batch() instead if applicable.
>   *
>   * Return: the number of table entries in the batch.
>   */
>  static inline unsigned int folio_pte_batch_ext(struct folio *folio,
> -		pte_t *ptep, pte_t pte, unsigned int max_nr, fpb_t flags,
> -		bool *any_writable, bool *any_young, bool *any_dirty)
> +		struct vm_area_struct *vma, pte_t *ptep, pte_t *ptentp,
> +		unsigned int max_nr, fpb_t flags)
>  {
> +	bool any_writable = false, any_young = false, any_dirty = false;
> +	pte_t expected_pte, pte = *ptentp;
>  	unsigned int nr, cur_nr;
> -	pte_t expected_pte;
> -
> -	if (any_writable)
> -		*any_writable = false;
> -	if (any_young)
> -		*any_young = false;
> -	if (any_dirty)
> -		*any_dirty = false;
>
>  	VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>  	VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
>  	VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio);
> +	VM_WARN_ON(virt_addr_valid(ptentp) && PageTable(virt_to_page(ptentp)));

Why not just VM_WARN_ON(!pte_same(*ptep, *ptentp)) ?

ptep points to the first page table entry and ptentp points to the copy of it.
I assume ptep should point to a valid page table entry to begin with.

Best Regards,
Yan, Zi