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

[Christophe Leroy]

Le 09/08/2019 à 14:36, Michael Ellerman a écrit :
Chuhong Yuan <hsleste...@gmail.com> 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 <hsleste...@gmail.com>

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