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Message-ID: <e53b7fe9-d2f7-4e4b-9313-3ea015128d74@arm.com>
Date: Thu, 16 Nov 2023 10:26:33 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Ard Biesheuvel <ardb@...nel.org>,
Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
James Morse <james.morse@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Zenghui Yu <yuzenghui@...wei.com>,
Andrey Ryabinin <ryabinin.a.a@...il.com>,
Alexander Potapenko <glider@...gle.com>,
Andrey Konovalov <andreyknvl@...il.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Vincenzo Frascino <vincenzo.frascino@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
Anshuman Khandual <anshuman.khandual@....com>,
Matthew Wilcox <willy@...radead.org>,
Yu Zhao <yuzhao@...gle.com>,
Mark Rutland <mark.rutland@....com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()
On 16/11/2023 10:03, David Hildenbrand wrote:
> On 15.11.23 17:30, Ryan Roberts wrote:
>> Convert copy_pte_range() to copy a set of ptes in a batch. A given batch
>> maps a physically contiguous block of memory, all belonging to the same
>> folio, with the same permissions, and for shared mappings, the same
>> dirty state. This will likely improve performance by a tiny amount due
>> to batching the folio reference count management and calling set_ptes()
>> rather than making individual calls to set_pte_at().
>>
>> However, the primary motivation for this change is to reduce the number
>> of tlb maintenance operations that the arm64 backend has to perform
>> during fork, as it is about to add transparent support for the
>> "contiguous bit" in its ptes. By write-protecting the parent using the
>> new ptep_set_wrprotects() (note the 's' at the end) function, the
>> backend can avoid having to unfold contig ranges of PTEs, which is
>> expensive, when all ptes in the range are being write-protected.
>> Similarly, by using set_ptes() rather than set_pte_at() to set up ptes
>> in the child, the backend does not need to fold a contiguous range once
>> they are all populated - they can be initially populated as a contiguous
>> range in the first place.
>>
>> This change addresses the core-mm refactoring only, and introduces
>> ptep_set_wrprotects() with a default implementation that calls
>> ptep_set_wrprotect() for each pte in the range. A separate change will
>> implement ptep_set_wrprotects() in the arm64 backend to realize the
>> performance improvement as part of the work to enable contpte mappings.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>> include/linux/pgtable.h | 13 +++
>> mm/memory.c | 175 +++++++++++++++++++++++++++++++---------
>> 2 files changed, 150 insertions(+), 38 deletions(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index af7639c3b0a3..1c50f8a0fdde 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -622,6 +622,19 @@ static inline void ptep_set_wrprotect(struct mm_struct
>> *mm, unsigned long addres
>> }
>> #endif
>> +#ifndef ptep_set_wrprotects
>> +struct mm_struct;
>> +static inline void ptep_set_wrprotects(struct mm_struct *mm,
>> + unsigned long address, pte_t *ptep,
>> + unsigned int nr)
>> +{
>> + unsigned int i;
>> +
>> + for (i = 0; i < nr; i++, address += PAGE_SIZE, ptep++)
>> + ptep_set_wrprotect(mm, address, ptep);
>> +}
>> +#endif
>> +
>> /*
>> * On some architectures hardware does not set page access bit when accessing
>> * memory page, it is responsibility of software setting this bit. It brings
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 1f18ed4a5497..b7c8228883cf 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -921,46 +921,129 @@ copy_present_page(struct vm_area_struct *dst_vma,
>> struct vm_area_struct *src_vma
>> /* Uffd-wp needs to be delivered to dest pte as well */
>> pte = pte_mkuffd_wp(pte);
>> set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte);
>> - return 0;
>> + return 1;
>> +}
>> +
>> +static inline unsigned long page_cont_mapped_vaddr(struct page *page,
>> + struct page *anchor, unsigned long anchor_vaddr)
>> +{
>> + unsigned long offset;
>> + unsigned long vaddr;
>> +
>> + offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT;
>> + vaddr = anchor_vaddr + offset;
>> +
>> + if (anchor > page) {
>> + if (vaddr > anchor_vaddr)
>> + return 0;
>> + } else {
>> + if (vaddr < anchor_vaddr)
>> + return ULONG_MAX;
>> + }
>> +
>> + return vaddr;
>> +}
>> +
>> +static int folio_nr_pages_cont_mapped(struct folio *folio,
>> + struct page *page, pte_t *pte,
>> + unsigned long addr, unsigned long end,
>> + pte_t ptent, bool *any_dirty)
>> +{
>> + int floops;
>> + int i;
>> + unsigned long pfn;
>> + pgprot_t prot;
>> + struct page *folio_end;
>> +
>> + if (!folio_test_large(folio))
>> + return 1;
>> +
>> + folio_end = &folio->page + folio_nr_pages(folio);
>> + end = min(page_cont_mapped_vaddr(folio_end, page, addr), end);
>> + floops = (end - addr) >> PAGE_SHIFT;
>> + pfn = page_to_pfn(page);
>> + prot = pte_pgprot(pte_mkold(pte_mkclean(ptent)));
>> +
>> + *any_dirty = pte_dirty(ptent);
>> +
>> + pfn++;
>> + pte++;
>> +
>> + for (i = 1; i < floops; i++) {
>> + ptent = ptep_get(pte);
>> + ptent = pte_mkold(pte_mkclean(ptent));
>> +
>> + if (!pte_present(ptent) || pte_pfn(ptent) != pfn ||
>> + pgprot_val(pte_pgprot(ptent)) != pgprot_val(prot))
>> + break;
>> +
>> + if (pte_dirty(ptent))
>> + *any_dirty = true;
>> +
>> + pfn++;
>> + pte++;
>> + }
>> +
>> + return i;
>> }
>> /*
>> - * Copy one pte. Returns 0 if succeeded, or -EAGAIN if one preallocated page
>> - * is required to copy this pte.
>> + * Copy set of contiguous ptes. Returns number of ptes copied if succeeded
>> + * (always gte 1), or -EAGAIN if one preallocated page is required to copy the
>> + * first pte.
>> */
>> static inline int
>> -copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>> - pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss,
>> - struct folio **prealloc)
>> +copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct
>> *src_vma,
>> + pte_t *dst_pte, pte_t *src_pte,
>> + unsigned long addr, unsigned long end,
>> + int *rss, struct folio **prealloc)
>> {
>> struct mm_struct *src_mm = src_vma->vm_mm;
>> unsigned long vm_flags = src_vma->vm_flags;
>> pte_t pte = ptep_get(src_pte);
>> struct page *page;
>> struct folio *folio;
>> + int nr = 1;
>> + bool anon;
>> + bool any_dirty = pte_dirty(pte);
>> + int i;
>> page = vm_normal_page(src_vma, addr, pte);
>> - if (page)
>> + if (page) {
>> folio = page_folio(page);
>> - if (page && folio_test_anon(folio)) {
>> - /*
>> - * If this page may have been pinned by the parent process,
>> - * copy the page immediately for the child so that we'll always
>> - * guarantee the pinned page won't be randomly replaced in the
>> - * future.
>> - */
>> - folio_get(folio);
>> - if (unlikely(page_try_dup_anon_rmap(page, false, src_vma))) {
>> - /* Page may be pinned, we have to copy. */
>> - folio_put(folio);
>> - return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
>> - addr, rss, prealloc, page);
>> + anon = folio_test_anon(folio);
>> + nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr,
>> + end, pte, &any_dirty);
>> +
>> + for (i = 0; i < nr; i++, page++) {
>> + if (anon) {
>> + /*
>> + * If this page may have been pinned by the
>> + * parent process, copy the page immediately for
>> + * the child so that we'll always guarantee the
>> + * pinned page won't be randomly replaced in the
>> + * future.
>> + */
>> + if (unlikely(page_try_dup_anon_rmap(
>> + page, false, src_vma))) {
>> + if (i != 0)
>> + break;
>> + /* Page may be pinned, we have to copy. */
>> + return copy_present_page(
>> + dst_vma, src_vma, dst_pte,
>> + src_pte, addr, rss, prealloc,
>> + page);
>> + }
>> + rss[MM_ANONPAGES]++;
>> + VM_BUG_ON(PageAnonExclusive(page));
>> + } else {
>> + page_dup_file_rmap(page, false);
>> + rss[mm_counter_file(page)]++;
>> + }
>> }
>> - rss[MM_ANONPAGES]++;
>> - } else if (page) {
>> - folio_get(folio);
>> - page_dup_file_rmap(page, false);
>> - rss[mm_counter_file(page)]++;
>> +
>> + nr = i;
>> + folio_ref_add(folio, nr);
>
> You're changing the order of mapcount vs. refcount increment. Don't. Make sure
> your refcount >= mapcount.
Ouch - good spot.
>
> You can do that easily by doing the folio_ref_add(folio, nr) first and then
> decrementing in case of error accordingly. Errors due to pinned pages are the
> corner case.
Yep, propose this for v3:
diff --git a/mm/memory.c b/mm/memory.c
index b7c8228883cf..98373349806e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1014,6 +1014,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct
vm_area_struct *src_vma
anon = folio_test_anon(folio);
nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr,
end, pte, &any_dirty);
+ folio_ref_add(folio, nr);
for (i = 0; i < nr; i++, page++) {
if (anon) {
@@ -1029,6 +1030,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct
vm_area_struct *src_vma
if (i != 0)
break;
/* Page may be pinned, we have to copy. */
+ folio_ref_sub(folio, nr);
return copy_present_page(
dst_vma, src_vma, dst_pte,
src_pte, addr, rss, prealloc,
@@ -1042,8 +1044,10 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct
vm_area_struct *src_vma
}
}
- nr = i;
- folio_ref_add(folio, nr);
+ if (i < nr) {
+ folio_ref_sub(folio, nr - i);
+ nr = i;
+ }
}
>
> I'll note that it will make a lot of sense to have batch variants of
> page_try_dup_anon_rmap() and page_dup_file_rmap().
>
> Especially, the batch variant of page_try_dup_anon_rmap() would only check once
> if the folio maybe pinned, and in that case, you can simply drop all references
> again. So you either have all or no ptes to process, which makes that code easier.
>
> But that can be added on top, and I'll happily do that.
That's very kind - thanks for the offer! I'll leave it to you then.
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