Re: [PATCH v2 0/7] Optimize mprotect for large folios

[Lance Yang <lance.yang@linux.dev>]

Hey Dev,

Hmm... I also hit the same compilation errors:

In file included from ./include/linux/kasan.h:37,
                  from ./include/linux/slab.h:260,
                  from ./include/linux/crypto.h:19,
                  from arch/x86/kernel/asm-offsets.c:9:
./include/linux/pgtable.h: In function ‘modify_prot_start_ptes’:
./include/linux/pgtable.h:905:15: error: implicit declaration of 
function ‘ptep_modify_prot_start’ [-Werror=implicit-function-declaration]
   905 |         pte = ptep_modify_prot_start(vma, addr, ptep);
       |               ^~~~~~~~~~~~~~~~~~~~~~
./include/linux/pgtable.h:905:15: error: incompatible types when 
assigning to type ‘pte_t’ from type ‘int’
./include/linux/pgtable.h:909:27: error: incompatible types when 
assigning to type ‘pte_t’ from type ‘int’
   909 |                 tmp_pte = ptep_modify_prot_start(vma, addr, ptep);
       |                           ^~~~~~~~~~~~~~~~~~~~~~
./include/linux/pgtable.h: In function ‘modify_prot_commit_ptes’:
./include/linux/pgtable.h:925:17: error: implicit declaration of 
function ‘ptep_modify_prot_commit’ [-Werror=implicit-function-declaration]
   925 |                 ptep_modify_prot_commit(vma, addr, ptep, 
old_pte, pte);
       |                 ^~~~~~~~~~~~~~~~~~~~~~~
./include/linux/pgtable.h: At top level:
./include/linux/pgtable.h:1360:21: error: conflicting types for 
‘ptep_modify_prot_start’; have ‘pte_t(struct vm_area_struct *, long 
unsigned int,  pte_t *)’
  1360 | static inline pte_t ptep_modify_prot_start(struct 
vm_area_struct *vma,
       |                     ^~~~~~~~~~~~~~~~~~~~~~
./include/linux/pgtable.h:905:15: note: previous implicit declaration of 
‘ptep_modify_prot_start’ with type ‘int()’
   905 |         pte = ptep_modify_prot_start(vma, addr, ptep);
       |               ^~~~~~~~~~~~~~~~~~~~~~
./include/linux/pgtable.h:1371:20: warning: conflicting types for 
‘ptep_modify_prot_commit’; have ‘void(struct vm_area_struct *, long 
unsigned int,  pte_t *, pte_t,  pte_t)’
  1371 | static inline void ptep_modify_prot_commit(struct 
vm_area_struct *vma,
       |                    ^~~~~~~~~~~~~~~~~~~~~~~
./include/linux/pgtable.h:1371:20: error: static declaration of 
‘ptep_modify_prot_commit’ follows non-static declaration
./include/linux/pgtable.h:925:17: note: previous implicit declaration of 
‘ptep_modify_prot_commit’ with type ‘void(struct vm_area_struct *, long 
unsigned int,  pte_t *, pte_t,  pte_t)’
   925 |                 ptep_modify_prot_commit(vma, addr, ptep, 
old_pte, pte);
       |                 ^~~~~~~~~~~~~~~~~~~~~~~
   CC 
/home/runner/work/mm-test-robot/mm-test-robot/linux/tools/objtool/libstring.o
   CC 
/home/runner/work/mm-test-robot/mm-test-robot/linux/tools/objtool/libctype.o
   CC 
/home/runner/work/mm-test-robot/mm-test-robot/linux/tools/objtool/str_error_r.o
   CC 
/home/runner/work/mm-test-robot/mm-test-robot/linux/tools/objtool/librbtree.o
cc1: some warnings being treated as errors
make[2]: *** [scripts/Makefile.build:98: arch/x86/kernel/asm-offsets.s] 
Error 1
make[1]: *** 
[/home/runner/work/mm-test-robot/mm-test-robot/linux/Makefile:1280: 
prepare0] Error 2
make[1]: *** Waiting for unfinished jobs....
   LD 
/home/runner/work/mm-test-robot/mm-test-robot/linux/tools/objtool/objtool-in.o
   LINK 
/home/runner/work/mm-test-robot/mm-test-robot/linux/tools/objtool/objtool
make: *** [Makefile:248: __sub-make] Error 2

Well, modify_prot_start_ptes() calls ptep_modify_prot_start(), but x86
does not define __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION. To avoid
implicit declaration errors, the architecture-independent
ptep_modify_prot_start() must be defined before modify_prot_start_ptes().

With the changes below, things work correctly now ;)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 10cdb87ccecf..d9d6c49bb914 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -895,44 +895,6 @@ static inline void wrprotect_ptes(struct mm_struct 
*mm, unsigned long addr,
  }
  #endif

-/* See the comment for ptep_modify_prot_start */
-#ifndef modify_prot_start_ptes
-static inline pte_t modify_prot_start_ptes(struct vm_area_struct *vma,
-		unsigned long addr, pte_t *ptep, unsigned int nr)
-{
-	pte_t pte, tmp_pte;
-
-	pte = ptep_modify_prot_start(vma, addr, ptep);
-	while (--nr) {
-		ptep++;
-		addr += PAGE_SIZE;
-		tmp_pte = ptep_modify_prot_start(vma, addr, ptep);
-		if (pte_dirty(tmp_pte))
-			pte = pte_mkdirty(pte);
-		if (pte_young(tmp_pte))
-			pte = pte_mkyoung(pte);
-	}
-	return pte;
-}
-#endif
-
-/* See the comment for ptep_modify_prot_commit */
-#ifndef modify_prot_commit_ptes
-static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, 
unsigned long addr,
-		pte_t *ptep, pte_t old_pte, pte_t pte, unsigned int nr)
-{
-	for (;;) {
-		ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte);
-		if (--nr == 0)
-			break;
-		ptep++;
-		addr += PAGE_SIZE;
-		old_pte = pte_next_pfn(old_pte);
-		pte = pte_next_pfn(pte);
-	}
-}
-#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
@@ -1375,6 +1337,45 @@ static inline void ptep_modify_prot_commit(struct 
vm_area_struct *vma,
  	__ptep_modify_prot_commit(vma, addr, ptep, pte);
  }
  #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
+
+/* See the comment for ptep_modify_prot_start */
+#ifndef modify_prot_start_ptes
+static inline pte_t modify_prot_start_ptes(struct vm_area_struct *vma,
+		unsigned long addr, pte_t *ptep, unsigned int nr)
+{
+	pte_t pte, tmp_pte;
+
+	pte = ptep_modify_prot_start(vma, addr, ptep);
+	while (--nr) {
+		ptep++;
+		addr += PAGE_SIZE;
+		tmp_pte = ptep_modify_prot_start(vma, addr, ptep);
+		if (pte_dirty(tmp_pte))
+			pte = pte_mkdirty(pte);
+		if (pte_young(tmp_pte))
+			pte = pte_mkyoung(pte);
+	}
+	return pte;
+}
+#endif
+
+/* See the comment for ptep_modify_prot_commit */
+#ifndef modify_prot_commit_ptes
+static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, 
unsigned long addr,
+		pte_t *ptep, pte_t old_pte, pte_t pte, unsigned int nr)
+{
+	for (;;) {
+		ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte);
+		if (--nr == 0)
+			break;
+		ptep++;
+		addr += PAGE_SIZE;
+		old_pte = pte_next_pfn(old_pte);
+		pte = pte_next_pfn(pte);
+	}
+}
+#endif
+
  #endif /* CONFIG_MMU */

  /*
--

Thanks,
Lance

On 2025/4/29 13:23, Dev Jain wrote:
> This patchset optimizes the mprotect() system call for large folios
> by PTE-batching.
> 
> We use the following test cases to measure performance, mprotect()'ing
> the mapped memory to read-only then read-write 40 times:
> 
> Test case 1: Mapping 1G of memory, touching it to get PMD-THPs, then
> pte-mapping those THPs
> Test case 2: Mapping 1G of memory with 64K mTHPs
> Test case 3: Mapping 1G of memory with 4K pages
> 
> Average execution time on arm64, Apple M3:
> Before the patchset:
> T1: 7.9 seconds   T2: 7.9 seconds   T3: 4.2 seconds
> 
> After the patchset:
> T1: 2.1 seconds   T2: 2.2 seconds   T3: 4.2 seconds
> 
> Observing T1/T2 and T3 before the patchset, we also remove the regression
> introduced by ptep_get() on a contpte block. And, for large folios we get
> an almost 74% performance improvement.
> 
> v1->v2:
>   - Rebase onto mm-unstable (6ebffe676fcf: util_macros.h: make the header more resilient)
>   - Abridge the anon-exclusive condition (Lance Yang)
> 
> Dev Jain (7):
>    mm: Refactor code in mprotect
>    mm: Optimize mprotect() by batch-skipping PTEs
>    mm: Add batched versions of ptep_modify_prot_start/commit
>    arm64: Add batched version of ptep_modify_prot_start
>    arm64: Add batched version of ptep_modify_prot_commit
>    mm: Batch around can_change_pte_writable()
>    mm: Optimize mprotect() through PTE-batching
> 
>   arch/arm64/include/asm/pgtable.h |  10 ++
>   arch/arm64/mm/mmu.c              |  21 +++-
>   include/linux/mm.h               |   4 +-
>   include/linux/pgtable.h          |  42 ++++++++
>   mm/gup.c                         |   2 +-
>   mm/huge_memory.c                 |   4 +-
>   mm/memory.c                      |   6 +-
>   mm/mprotect.c                    | 165 ++++++++++++++++++++-----------
>   mm/pgtable-generic.c             |  16 ++-
>   9 files changed, 198 insertions(+), 72 deletions(-)
>