Subject: Re: [PATCH v3 4/6] mm/vmalloc: Extend page table walk to support larger page_shift sizes and eliminate page table rewalk

From: Dev Jain <[email-masked]>
Date: Fri, 29 May 2026 10:58:06 +0530

On 28/05/26 9:09 am, Wen Jiang wrote:
> On Wed, 27 May 2026 at 13:59, Dev Jain <[email-masked]> wrote:
>>
>>
>>
>> On 22/05/26 11:01 am, Wen Jiang wrote:
>> From: "Barry Song (Xiaomi)" <[email-masked]>
>>
>> vmap_pages_range_noflush_walk() (formerly vmap_small_pages_range_noflush())
>> provides a clean interface by taking struct page **pages and mapping them
>> via direct PTE iteration. This avoids the page table rewalk seen when
>> using vmap_range_noflush() for page_shift values other than PAGE_SHIFT.
>>
>> Extend it to support larger page_shift values, and add PMD- and
>> contiguous-PTE mappings as well. Rename it to vmap_pages_range_noflush_walk()
>> since it now handles more than just small pages.
>>
>> For vmalloc() allocations with VM_ALLOW_HUGE_VMAP, we no longer need to
>> iterate over pages one by one via vmap_range_noflush(), which would
>> otherwise lead to page table rewalk. The code is now unified with the
>> PAGE_SHIFT case by simply calling vmap_pages_range_noflush_walk().
>>
>> Signed-off-by: Barry Song (Xiaomi) <[email-masked]>
>> Signed-off-by: Wen Jiang <[email-masked]>
>> Tested-by: Xueyuan Chen <[email-masked]>
>> ---
>>  mm/vmalloc.c | 71 +++++++++++++++++++++++++++++-----------------------
>>  1 file changed, 40 insertions(+), 31 deletions(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 53fd4ee460ea4..deb764abc0571 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -543,8 +543,10 @@ void vunmap_range(unsigned long addr, unsigned long end)
>>
>>  static int vmap_pages_pte_range(pmd_t *pmd, unsigned long addr,
>>               unsigned long end, pgprot_t prot, struct page **pages, int *nr,
>> -             pgtbl_mod_mask *mask)
>> +             pgtbl_mod_mask *mask, unsigned int shift)
>>  {
>> +     unsigned long pfn, size;
>> +     unsigned int steps;
>>       int err = 0;
>>       pte_t *pte;
>>
>> @@ -575,9 +577,10 @@ static int vmap_pages_pte_range(pmd_t *pmd, unsigned long addr,
>>                       break;
>>               }
>>
>> -             set_pte_at(&init_mm, addr, pte, mk_pte(page, prot));
>> -             (*nr)++;
>> -     } while (pte++, addr += PAGE_SIZE, addr != end);
>> +             pfn = page_to_pfn(page);
>> +             size = vmap_set_ptes(pte, addr, end, pfn, prot, shift);
>> +             steps = PFN_DOWN(size);
>> +     } while (pte += steps, *nr += steps, addr += size, addr != end);
>>
>>       lazy_mmu_mode_disable();
>>       *mask |= PGTBL_PTE_MODIFIED;
>> @@ -587,7 +590,7 @@ static int vmap_pages_pte_range(pmd_t *pmd, unsigned long addr,
>>
>>  static int vmap_pages_pmd_range(pud_t *pud, unsigned long addr,
>>               unsigned long end, pgprot_t prot, struct page **pages, int *nr,
>> -             pgtbl_mod_mask *mask)
>> +             pgtbl_mod_mask *mask, unsigned int shift)
>>  {
>>       pmd_t *pmd;
>>       unsigned long next;
>>> @@ -597,7 +600,27 @@ static int vmap_pages_pmd_range(pud_t *pud, unsigned long addr,
>>>               return -ENOMEM;
>>>       do {
>>>               next = pmd_addr_end(addr, end);
>>> -             if (vmap_pages_pte_range(pmd, addr, next, prot, pages, nr, mask))
>>> +
>>> +             if (shift == PMD_SHIFT) {
>>> +                     struct page *page = pages[*nr];
>>> +                     phys_addr_t phys_addr;
>>> +
>>> +                     if (WARN_ON(!page))
>>> +                             return -ENOMEM;
>>> +                     if (WARN_ON(!pfn_valid(page_to_pfn(page))))
>>> +                             return -EINVAL;
>>
>>
>> So I know these !page and !pfn_valid checks have been copied from vmap_pages_pte_range,
>> but do they mean anything?
>>
>> I think pfn_valid() makes sense in that someone may take a random VA/PA, convert it into a struct
>> page and pass to vmap layer. But I don't see how anyone would pass page == NULL? At the
>> very least, returning ENOMEM does not make sense because the pages are not being
>> allocated by vmap() but have already been allocated.
> 
> Hi Dev,
> 
> vmap() is EXPORT_SYMBOL with many callers across drivers, each
> constructing the pages array differently. The !page check guards
> against malformed arrays at this API boundary.
> 
> The same -ENOMEM issue also exists in vmap_pages_pte_range().
> Should I fix both in this patchset or leave it as a separate cleanup?

Hmm - I think this is the issue of who validates what. I think the change
should be done but this is not an urgent issue, so let us leave this
for now.

> 
>>
>>> +
>>> +                     phys_addr = page_to_phys(page);
>>> +
>>> +                     if (vmap_try_huge_pmd(pmd, addr, next, phys_addr, prot,
>>> +                                             shift)) {
>>> +                             *mask |= PGTBL_PMD_MODIFIED;
>>> +                             *nr += 1 << (shift - PAGE_SHIFT);
>>> +                             continue;
>>> +                     }
>>> +             }
>>> +
>>> +             if (vmap_pages_pte_range(pmd, addr, next, prot, pages, nr, mask, shift))
>>>                       return -ENOMEM;
>>>       } while (pmd++, addr = next, addr != end);
>>>       return 0;
>>> @@ -605,7 +628,7 @@ static int vmap_pages_pmd_range(pud_t *pud, unsigned long addr,
>>>
>>>  static int vmap_pages_pud_range(p4d_t *p4d, unsigned long addr,
>>>               unsigned long end, pgprot_t prot, struct page **pages, int *nr,
>>> -             pgtbl_mod_mask *mask)
>>> +             pgtbl_mod_mask *mask, unsigned int shift)
>>>  {
>>>       pud_t *pud;
>>>       unsigned long next;
>>> @@ -615,7 +638,7 @@ static int vmap_pages_pud_range(p4d_t *p4d, unsigned long addr,
>>>               return -ENOMEM;
>>>       do {
>>>               next = pud_addr_end(addr, end);
>>> -             if (vmap_pages_pmd_range(pud, addr, next, prot, pages, nr, mask))
>>> +             if (vmap_pages_pmd_range(pud, addr, next, prot, pages, nr, mask, shift))
>>>                       return -ENOMEM;
>>>       } while (pud++, addr = next, addr != end);
>>>       return 0;
>>> @@ -623,7 +646,7 @@ static int vmap_pages_pud_range(p4d_t *p4d, unsigned long addr,
>>>
>>>  static int vmap_pages_p4d_range(pgd_t *pgd, unsigned long addr,
>>>               unsigned long end, pgprot_t prot, struct page **pages, int *nr,
>>> -             pgtbl_mod_mask *mask)
>>> +             pgtbl_mod_mask *mask, unsigned int shift)
>>>  {
>>>       p4d_t *p4d;
>>>       unsigned long next;
>>> @@ -633,14 +656,14 @@ static int vmap_pages_p4d_range(pgd_t *pgd, unsigned long addr,
>>>               return -ENOMEM;
>>>       do {
>>>               next = p4d_addr_end(addr, end);
>>> -             if (vmap_pages_pud_range(p4d, addr, next, prot, pages, nr, mask))
>>> +             if (vmap_pages_pud_range(p4d, addr, next, prot, pages, nr, mask, shift))
>>>                       return -ENOMEM;
>>>       } while (p4d++, addr = next, addr != end);
>>>       return 0;
>>>  }
>>>
>>> -static int vmap_small_pages_range_noflush(unsigned long addr, unsigned long end,
>>> -             pgprot_t prot, struct page **pages)
>>> +static int vmap_pages_range_noflush_walk(unsigned long addr, unsigned long end,
>>> +             pgprot_t prot, struct page **pages, unsigned int shift)
>>>  {
>>>       unsigned long start = addr;
>>>       pgd_t *pgd;
>>> @@ -655,7 +678,7 @@ static int vmap_small_pages_range_noflush(unsigned long addr, unsigned long end,
>>>               next = pgd_addr_end(addr, end);
>>>               if (pgd_bad(*pgd))
>>>                       mask |= PGTBL_PGD_MODIFIED;
>>> -             err = vmap_pages_p4d_range(pgd, addr, next, prot, pages, &nr, &mask);
>>> +             err = vmap_pages_p4d_range(pgd, addr, next, prot, pages, &nr, &mask, shift);
>>>               if (err)
>>>                       break;
>>>       } while (pgd++, addr = next, addr != end);
>>> @@ -678,27 +701,13 @@ static int vmap_small_pages_range_noflush(unsigned long addr, unsigned long end,
>>>  int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
>>>               pgprot_t prot, struct page **pages, unsigned int page_shift)
>>>  {
>>> -     unsigned int i, nr = (end - addr) >> PAGE_SHIFT;
>>> -
>>>       WARN_ON(page_shift < PAGE_SHIFT);
>>>
>>> -     if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
>>> -                     page_shift == PAGE_SHIFT)
>>> -             return vmap_small_pages_range_noflush(addr, end, prot, pages);
>>> +     if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC))
>>> +             page_shift = PAGE_SHIFT;
>>>
>>> -     for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
>>> -             int err;
>>> -
>>> -             err = vmap_range_noflush(addr, addr + (1UL << page_shift),
>>> -                                     page_to_phys(pages[i]), prot,
>>> -                                     page_shift);
>>> -             if (err)
>>> -                     return err;
>>> -
>>> -             addr += 1UL << page_shift;
>>> -     }
>>> -
>>> -     return 0;
>>> +     return vmap_pages_range_noflush_walk(addr, end, prot, pages,
>>> +                     min(page_shift, PMD_SHIFT));
>>
>>
>> We can easily extend to PUD huge mappings right? Not sure whether we
>> should keep everything symmetric to how vmap_range_noflush() operates
>> right now, since P4D mappings don't exist, but PUD looks worthwhile.
>>
> 
> PUD mapping requires 1GB of contiguous physical memory, but the buddy
> allocator's MAX_PAGE_ORDER is 10 (4MB on 4K pages). So page_shift
> passed to vmap_pages_range_noflush_walk() never exceeds PMD_SHIFT.

Ah okay. It is really confusing to see PUD helpers everywhere when
they are basically dead code ...

> 
> Thanks,
> Wen
>>>  }
>>>
>>>  int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
>>