Re: [PATCH] powerpc/mm: Use refcount_t for refcount

From: Christophe Leroy
Date: Fri Nov 26 2021 - 12:57:55 EST



Le 09/08/2019 à 14:36, Michael Ellerman a écrit :
Chuhong Yuan <hslester96@xxxxxxxxx> writes:
Reference counters are preferred to use refcount_t instead of
atomic_t.
This is because the implementation of refcount_t can prevent
overflows and detect possible use-after-free.
So convert atomic_t ref counters to refcount_t.

Signed-off-by: Chuhong Yuan <hslester96@xxxxxxxxx>

Thanks.

We don't have a fast implementation of refcount_t, so I'm worried this
could cause a measurable performance regression.

Fast implementations have been removed by commit https://github.com/linuxppc/linux/commit/fb041bb7c0a918b95c6889fc965cdc4a75b4c0ca

It's now considered that the generic implementation is good enough for everybody.

However, this series doesn't apply anymore and needs rebase:

Applying: powerpc/mm: Use refcount_t for refcount
Using index info to reconstruct a base tree...
M arch/powerpc/mm/book3s64/mmu_context.c
M arch/powerpc/mm/book3s64/pgtable.c
M arch/powerpc/mm/pgtable-frag.c
M include/linux/mm_types.h
Falling back to patching base and 3-way merge...
Auto-merging include/linux/mm_types.h
CONFLICT (content): Merge conflict in include/linux/mm_types.h
Auto-merging arch/powerpc/mm/pgtable-frag.c
CONFLICT (content): Merge conflict in arch/powerpc/mm/pgtable-frag.c
Auto-merging arch/powerpc/mm/book3s64/pgtable.c
CONFLICT (content): Merge conflict in arch/powerpc/mm/book3s64/pgtable.c
Auto-merging arch/powerpc/mm/book3s64/mmu_context.c
Patch failed at 0001 powerpc/mm: Use refcount_t for refcount
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

Thanks
Christophe



Did you benchmark it at all?

cheers

diff --git a/arch/powerpc/mm/book3s64/mmu_context.c b/arch/powerpc/mm/book3s64/mmu_context.c
index 2d0cb5ba9a47..f836fd5a6abc 100644
--- a/arch/powerpc/mm/book3s64/mmu_context.c
+++ b/arch/powerpc/mm/book3s64/mmu_context.c
@@ -231,7 +231,7 @@ static void pmd_frag_destroy(void *pmd_frag)
/* drop all the pending references */
count = ((unsigned long)pmd_frag & ~PAGE_MASK) >> PMD_FRAG_SIZE_SHIFT;
/* We allow PTE_FRAG_NR fragments from a PTE page */
- if (atomic_sub_and_test(PMD_FRAG_NR - count, &page->pt_frag_refcount)) {
+ if (refcount_sub_and_test(PMD_FRAG_NR - count, &page->pt_frag_refcount)) {
pgtable_pmd_page_dtor(page);
__free_page(page);
}
diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index 7d0e0d0d22c4..40056896ce4e 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c
@@ -277,7 +277,7 @@ static pmd_t *__alloc_for_pmdcache(struct mm_struct *mm)
return NULL;
}
- atomic_set(&page->pt_frag_refcount, 1);
+ refcount_set(&page->pt_frag_refcount, 1);
ret = page_address(page);
/*
@@ -294,7 +294,7 @@ static pmd_t *__alloc_for_pmdcache(struct mm_struct *mm)
* count.
*/
if (likely(!mm->context.pmd_frag)) {
- atomic_set(&page->pt_frag_refcount, PMD_FRAG_NR);
+ refcount_set(&page->pt_frag_refcount, PMD_FRAG_NR);
mm->context.pmd_frag = ret + PMD_FRAG_SIZE;
}
spin_unlock(&mm->page_table_lock);
@@ -317,8 +317,7 @@ void pmd_fragment_free(unsigned long *pmd)
{
struct page *page = virt_to_page(pmd);
- BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
- if (atomic_dec_and_test(&page->pt_frag_refcount)) {
+ if (refcount_dec_and_test(&page->pt_frag_refcount)) {
pgtable_pmd_page_dtor(page);
__free_page(page);
}
diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
index a7b05214760c..4ef8231b677f 100644
--- a/arch/powerpc/mm/pgtable-frag.c
+++ b/arch/powerpc/mm/pgtable-frag.c
@@ -24,7 +24,7 @@ void pte_frag_destroy(void *pte_frag)
/* drop all the pending references */
count = ((unsigned long)pte_frag & ~PAGE_MASK) >> PTE_FRAG_SIZE_SHIFT;
/* We allow PTE_FRAG_NR fragments from a PTE page */
- if (atomic_sub_and_test(PTE_FRAG_NR - count, &page->pt_frag_refcount)) {
+ if (refcount_sub_and_test(PTE_FRAG_NR - count, &page->pt_frag_refcount)) {
pgtable_page_dtor(page);
__free_page(page);
}
@@ -71,7 +71,7 @@ static pte_t *__alloc_for_ptecache(struct mm_struct *mm, int kernel)
return NULL;
}
- atomic_set(&page->pt_frag_refcount, 1);
+ refcount_set(&page->pt_frag_refcount, 1);
ret = page_address(page);
/*
@@ -87,7 +87,7 @@ static pte_t *__alloc_for_ptecache(struct mm_struct *mm, int kernel)
* count.
*/
if (likely(!pte_frag_get(&mm->context))) {
- atomic_set(&page->pt_frag_refcount, PTE_FRAG_NR);
+ refcount_set(&page->pt_frag_refcount, PTE_FRAG_NR);
pte_frag_set(&mm->context, ret + PTE_FRAG_SIZE);
}
spin_unlock(&mm->page_table_lock);
@@ -110,8 +110,7 @@ void pte_fragment_free(unsigned long *table, int kernel)
{
struct page *page = virt_to_page(table);
- BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
- if (atomic_dec_and_test(&page->pt_frag_refcount)) {
+ if (refcount_dec_and_test(&page->pt_frag_refcount)) {
if (!kernel)
pgtable_page_dtor(page);
__free_page(page);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3a37a89eb7a7..7fe23a3faf95 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -14,6 +14,7 @@
#include <linux/uprobes.h>
#include <linux/page-flags-layout.h>
#include <linux/workqueue.h>
+#include <linux/refcount.h>
#include <asm/mmu.h>
@@ -147,7 +148,7 @@ struct page {
unsigned long _pt_pad_2; /* mapping */
union {
struct mm_struct *pt_mm; /* x86 pgds only */
- atomic_t pt_frag_refcount; /* powerpc */
+ refcount_t pt_frag_refcount; /* powerpc */
};
#if ALLOC_SPLIT_PTLOCKS
spinlock_t *ptl;
--
2.20.1