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Message-ID: <938c4726-b93e-46df-bceb-65c7574714a6@linux.dev>
Date: Wed, 25 Jun 2025 18:47:55 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: 21cnbao@...il.com, akpm@...ux-foundation.org,
baolin.wang@...ux.alibaba.com, chrisl@...nel.org, kasong@...cent.com,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
lorenzo.stoakes@...cle.com, ryan.roberts@....com, v-songbaohua@...o.com,
x86@...nel.org, ying.huang@...el.com, zhengtangquan@...o.com,
Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large
folios during reclamation
On 2025/6/25 18:00, David Hildenbrand wrote:
> On 24.06.25 18:25, Lance Yang wrote:
>> On 2025/6/24 23:34, David Hildenbrand wrote:
>>> On 24.06.25 17:26, Lance Yang wrote:
>>>> On 2025/6/24 20:55, David Hildenbrand wrote:
>>>>> On 14.02.25 10:30, Barry Song wrote:
>>>>>> From: Barry Song <v-songbaohua@...o.com>
>>>> [...]
>>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>>> index 89e51a7a9509..8786704bd466 100644
>>>>>> --- a/mm/rmap.c
>>>>>> +++ b/mm/rmap.c
>>>>>> @@ -1781,6 +1781,25 @@ void folio_remove_rmap_pud(struct folio
>>>>>> *folio,
>>>>>> struct page *page,
>>>>>> #endif
>>>>>> }
>>>>>> +/* We support batch unmapping of PTEs for lazyfree large folios */
>>>>>> +static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
>>>>>> + struct folio *folio, pte_t *ptep)
>>>>>> +{
>>>>>> + const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
>>>>>> FPB_IGNORE_SOFT_DIRTY;
>>>>>> + int max_nr = folio_nr_pages(folio);
>>>>>
>>>>> Let's assume we have the first page of a folio mapped at the last page
>>>>> table entry in our page table.
>>>>
>>>> Good point. I'm curious if it is something we've seen in practice ;)
>>>
>>> I challenge you to write a reproducer :P I assume it might be doable
>>> through simple mremap().
>>>
>>>>
>>>>>
>>>>> What prevents folio_pte_batch() from reading outside the page table?
>>>>
>>>> Assuming such a scenario is possible, to prevent any chance of an
>>>> out-of-bounds read, how about this change:
>>>>
>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>> index fb63d9256f09..9aeae811a38b 100644
>>>> --- a/mm/rmap.c
>>>> +++ b/mm/rmap.c
>>>> @@ -1852,6 +1852,25 @@ static inline bool
>>>> can_batch_unmap_folio_ptes(unsigned long addr,
>>>> const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
>>>> FPB_IGNORE_SOFT_DIRTY;
>>>> int max_nr = folio_nr_pages(folio);
>>>> pte_t pte = ptep_get(ptep);
>>>> + unsigned long end_addr;
>>>> +
>>>> + /*
>>>> + * To batch unmap, the entire folio's PTEs must be contiguous
>>>> + * and mapped within the same PTE page table, which corresponds to
>>>> + * a single PMD entry. Before calling folio_pte_batch(), which
>>>> does
>>>> + * not perform boundary checks itself, we must verify that the
>>>> + * address range covered by the folio does not cross a PMD
>>>> boundary.
>>>> + */
>>>> + end_addr = addr + (max_nr * PAGE_SIZE) - 1;
>>>> +
>>>> + /*
>>>> + * A fast way to check for a PMD boundary cross is to align both
>>>> + * the start and end addresses to the PMD boundary and see if they
>>>> + * are different. If they are, the range spans across at least two
>>>> + * different PMD-managed regions.
>>>> + */
>>>> + if ((addr & PMD_MASK) != (end_addr & PMD_MASK))
>>>> + return false;
>>>
>>> You should not be messing with max_nr = folio_nr_pages(folio) here at
>>> all. folio_pte_batch() takes care of that.
>>>
>>> Also, way too many comments ;)
>>>
>>> You may only batch within a single VMA and within a single page table.
>>>
>>> So simply align the addr up to the next PMD, and make sure it does not
>>> exceed the vma end.
>>>
>>> ALIGN and friends can help avoiding excessive comments.
>>
>> Thanks! How about this updated version based on your suggestion:
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index fb63d9256f09..241d55a92a47 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1847,12 +1847,25 @@ void folio_remove_rmap_pud(struct folio
>> *folio, struct page *page,
>> /* We support batch unmapping of PTEs for lazyfree large folios */
>> static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
>> - struct folio *folio, pte_t *ptep)
>> + struct folio *folio, pte_t *ptep,
>> + struct vm_area_struct *vma)
>> {
>> const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> + unsigned long next_pmd, vma_end, end_addr;
>> int max_nr = folio_nr_pages(folio);
>> pte_t pte = ptep_get(ptep);
>> + /*
>> + * Limit the batch scan within a single VMA and within a single
>> + * page table.
>> + */
>> + vma_end = vma->vm_end;
>> + next_pmd = ALIGN(addr + 1, PMD_SIZE);
>> + end_addr = addr + (unsigned long)max_nr * PAGE_SIZE;
>> +
>> + if (end_addr > min(next_pmd, vma_end))
>> + return false;
>
> May I suggest that we clean all that up as we fix it?
Yeah, that looks much better. Thanks for the suggestion!
>
> Maybe something like this:
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 3b74bb19c11dd..11fbddc6ad8d6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1845,23 +1845,38 @@ void folio_remove_rmap_pud(struct folio *folio,
> struct page *page,
> #endif
> }
>
> -/* We support batch unmapping of PTEs for lazyfree large folios */
> -static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
> - struct folio *folio, pte_t *ptep)
> +static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
> + struct page_vma_mapped_walk *pvmw, enum ttu_flags flags,
> + pte_t pte)
> {
> const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> - int max_nr = folio_nr_pages(folio);
> - pte_t pte = ptep_get(ptep);
> + struct vm_area_struct *vma = pvmw->vma;
> + unsigned long end_addr, addr = pvmw->address;
> + unsigned int max_nr;
> +
> + if (flags & TTU_HWPOISON)
> + return 1;
> + if (!folio_test_large(folio))
> + return 1;
> +
> + /* We may only batch within a single VMA and a single page
> table. */
> + end_addr = min_t(unsigned long, ALIGN(addr + 1, PMD_SIZE), vma-
> >vm_end);
> + max_nr = (end_addr - addr) >> PAGE_SHIFT;
>
> + /* We only support lazyfree batching for now ... */
> if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
> - return false;
> + return 1;
> if (pte_unused(pte))
> - return false;
> - if (pte_pfn(pte) != folio_pfn(folio))
> - return false;
> + return 1;
> + /* ... where we must be able to batch the whole folio. */
> + if (pte_pfn(pte) != folio_pfn(folio) || max_nr !=
> folio_nr_pages(folio))
> + return 1;
> + max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr,
> fpb_flags,
> + NULL, NULL, NULL);
>
> - return folio_pte_batch(folio, addr, ptep, pte, max_nr,
> fpb_flags, NULL,
> - NULL, NULL) == max_nr;
> + if (max_nr != folio_nr_pages(folio))
> + return 1;
> + return max_nr;
> }
>
> /*
> @@ -2024,9 +2039,7 @@ static bool try_to_unmap_one(struct folio *folio,
> struct vm_area_struct *vma,
> if (pte_dirty(pteval))
> folio_mark_dirty(folio);
> } else if (likely(pte_present(pteval))) {
> - if (folio_test_large(folio) && !(flags &
> TTU_HWPOISON) &&
> - can_batch_unmap_folio_ptes(address, folio,
> pvmw.pte))
> - nr_pages = folio_nr_pages(folio);
> + nr_pages = folio_unmap_pte_batch(folio, &pvmw,
> flags, pteval);
> end_addr = address + nr_pages * PAGE_SIZE;
> flush_cache_range(vma, address, end_addr);
>
>
> Note that I don't quite understand why we have to batch the whole thing
> or fallback to
> individual pages. Why can't we perform other batches that span only some
> PTEs? What's special
> about 1 PTE vs. 2 PTEs vs. all PTEs?
That's a good point about the "all-or-nothing" batching logic ;)
It seems the "all-or-nothing" approach is specific to the lazyfree use
case, which needs to unmap the entire folio for reclamation. If that's
not possible, it falls back to the single-page slow path.
Also, supporting partial batches would be useful, but not common case
I guess, so let's leave it as is ;p
Thanks,
Lance
>
>
> Can someone enlighten me why that is required?
>
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