Message-ID: <09c356c3-5874-43ac-93e5-4374a386df98@redhat.com>
Date: Wed, 2 Jul 2025 11:09:41 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.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 02.07.25 11:08, David Hildenbrand wrote:
> On 30.06.25 19:59, Zi Yan wrote:
>> 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.
> 
> That would also work, only miss the case where someone would by accident
> flip both pointers :)

... or pass the same pointer twice.

-- 
Cheers,

David / dhildenb

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