Changes in v2:
- Add Fixes and Acked-by tags from David Hildenbrand, thanks!
- Inline atomic_long_read(&ksm_zero_pages), per David Hildenbrand.
- Don't include the last two optimization patches to keep this fix
simple.
- Link to v1: https://lore.kernel.org/r/[email protected]
We encounter some abnormal ksm_pages_scanned and ksm_zero_pages during
some random tests.
1. ksm_pages_scanned unchanged even ksmd scanning has progress.
2. ksm_zero_pages maybe -1 in some rare cases.
Thanks for review and comments!
Signed-off-by: Chengming Zhou <[email protected]>
---
Chengming Zhou (2):
mm/ksm: fix ksm_pages_scanned accounting
mm/ksm: fix ksm_zero_pages accounting
fs/proc/base.c | 2 +-
include/linux/ksm.h | 17 ++++++++++++++---
include/linux/mm_types.h | 2 +-
mm/ksm.c | 17 +++++++----------
4 files changed, 23 insertions(+), 15 deletions(-)
---
base-commit: fb0f40125feec3de7ef4524600ac83946207117e
change-id: 20240508-b4-ksm-counters-04817b40d3ee
Best regards,
--
Chengming Zhou <[email protected]>
During testing, I found ksm_pages_scanned is unchanged although the
scan_get_next_rmap_item() did return valid rmap_item that is not NULL.
The reason is the scan_get_next_rmap_item() will return NULL after
a full scan, so ksm_do_scan() just return without accounting of the
ksm_pages_scanned.
Fix it by just putting ksm_pages_scanned accounting in that loop,
and it will be accounted more timely if that loop would last for
a long time.
Fixes: b348b5fe2b5f ("mm/ksm: add pages scanned metric")
Acked-by: David Hildenbrand <[email protected]>
Signed-off-by: Chengming Zhou <[email protected]>
---
mm/ksm.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index e1034bf1c937..0f9c491552ff 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2753,18 +2753,16 @@ static void ksm_do_scan(unsigned int scan_npages)
{
struct ksm_rmap_item *rmap_item;
struct page *page;
- unsigned int npages = scan_npages;
- while (npages-- && likely(!freezing(current))) {
+ while (scan_npages-- && likely(!freezing(current))) {
cond_resched();
rmap_item = scan_get_next_rmap_item(&page);
if (!rmap_item)
return;
cmp_and_merge_page(page, rmap_item);
put_page(page);
+ ksm_pages_scanned++;
}
-
- ksm_pages_scanned += scan_npages - npages;
}
static int ksmd_should_run(void)
--
2.45.0
We normally ksm_zero_pages++ in ksmd when page is merged with zero page,
but ksm_zero_pages-- is done from page tables side, which can't protected
by the ksmd mutex.
So we can read very exceptional value of ksm_zero_pages in rare cases,
such as -1, which is very confusing to users.
Fix it by changing to use atomic_long_t, and the same case with the
mm->ksm_zero_pages.
Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
Fixes: 6080d19f0704 ("ksm: add ksm zero pages for each process")
Acked-by: David Hildenbrand <[email protected]>
Signed-off-by: Chengming Zhou <[email protected]>
---
fs/proc/base.c | 2 +-
include/linux/ksm.h | 17 ++++++++++++++---
include/linux/mm_types.h | 2 +-
mm/ksm.c | 11 +++++------
4 files changed, 21 insertions(+), 11 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 18550c071d71..72a1acd03675 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3214,7 +3214,7 @@ static int proc_pid_ksm_stat(struct seq_file *m, struct pid_namespace *ns,
mm = get_task_mm(task);
if (mm) {
seq_printf(m, "ksm_rmap_items %lu\n", mm->ksm_rmap_items);
- seq_printf(m, "ksm_zero_pages %lu\n", mm->ksm_zero_pages);
+ seq_printf(m, "ksm_zero_pages %ld\n", mm_ksm_zero_pages(mm));
seq_printf(m, "ksm_merging_pages %lu\n", mm->ksm_merging_pages);
seq_printf(m, "ksm_process_profit %ld\n", ksm_process_profit(mm));
mmput(mm);
diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 52c63a9c5a9c..11690dacd986 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -33,16 +33,27 @@ void __ksm_exit(struct mm_struct *mm);
*/
#define is_ksm_zero_pte(pte) (is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte))
-extern unsigned long ksm_zero_pages;
+extern atomic_long_t ksm_zero_pages;
+
+static inline void ksm_map_zero_page(struct mm_struct *mm)
+{
+ atomic_long_inc(&ksm_zero_pages);
+ atomic_long_inc(&mm->ksm_zero_pages);
+}
static inline void ksm_might_unmap_zero_page(struct mm_struct *mm, pte_t pte)
{
if (is_ksm_zero_pte(pte)) {
- ksm_zero_pages--;
- mm->ksm_zero_pages--;
+ atomic_long_dec(&ksm_zero_pages);
+ atomic_long_dec(&mm->ksm_zero_pages);
}
}
+static inline long mm_ksm_zero_pages(struct mm_struct *mm)
+{
+ return atomic_long_read(&mm->ksm_zero_pages);
+}
+
static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
{
if (test_bit(MMF_VM_MERGEABLE, &oldmm->flags))
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 24323c7d0bd4..af3a0256fa93 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -985,7 +985,7 @@ struct mm_struct {
* Represent how many empty pages are merged with kernel zero
* pages when enabling KSM use_zero_pages.
*/
- unsigned long ksm_zero_pages;
+ atomic_long_t ksm_zero_pages;
#endif /* CONFIG_KSM */
#ifdef CONFIG_LRU_GEN_WALKS_MMU
struct {
diff --git a/mm/ksm.c b/mm/ksm.c
index 0f9c491552ff..6f461411d070 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -296,7 +296,7 @@ static bool ksm_use_zero_pages __read_mostly;
static bool ksm_smart_scan = true;
/* The number of zero pages which is placed by KSM */
-unsigned long ksm_zero_pages;
+atomic_long_t ksm_zero_pages = ATOMIC_LONG_INIT(0);
/* The number of pages that have been skipped due to "smart scanning" */
static unsigned long ksm_pages_skipped;
@@ -1429,8 +1429,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
* the dirty bit in zero page's PTE is set.
*/
newpte = pte_mkdirty(pte_mkspecial(pfn_pte(page_to_pfn(kpage), vma->vm_page_prot)));
- ksm_zero_pages++;
- mm->ksm_zero_pages++;
+ ksm_map_zero_page(mm);
/*
* We're replacing an anonymous page with a zero page, which is
* not anonymous. We need to do proper accounting otherwise we
@@ -3373,7 +3372,7 @@ static void wait_while_offlining(void)
#ifdef CONFIG_PROC_FS
long ksm_process_profit(struct mm_struct *mm)
{
- return (long)(mm->ksm_merging_pages + mm->ksm_zero_pages) * PAGE_SIZE -
+ return (long)(mm->ksm_merging_pages + mm_ksm_zero_pages(mm)) * PAGE_SIZE -
mm->ksm_rmap_items * sizeof(struct ksm_rmap_item);
}
#endif /* CONFIG_PROC_FS */
@@ -3662,7 +3661,7 @@ KSM_ATTR_RO(pages_skipped);
static ssize_t ksm_zero_pages_show(struct kobject *kobj,
struct kobj_attribute *attr, char *buf)
{
- return sysfs_emit(buf, "%ld\n", ksm_zero_pages);
+ return sysfs_emit(buf, "%ld\n", atomic_long_read(&ksm_zero_pages));
}
KSM_ATTR_RO(ksm_zero_pages);
@@ -3671,7 +3670,7 @@ static ssize_t general_profit_show(struct kobject *kobj,
{
long general_profit;
- general_profit = (ksm_pages_sharing + ksm_zero_pages) * PAGE_SIZE -
+ general_profit = (ksm_pages_sharing + atomic_long_read(&ksm_zero_pages)) * PAGE_SIZE -
ksm_rmap_items * sizeof(struct ksm_rmap_item);
return sysfs_emit(buf, "%ld\n", general_profit);
--
2.45.0
>Fixes: b348b5fe2b5f ("mm/ksm: add pages scanned metric")
>Acked-by: David Hildenbrand <[email protected]>
>Signed-off-by: Chengming Zhou <[email protected]>
>---
> mm/ksm.c | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
>diff --git a/mm/ksm.c b/mm/ksm.c
>index e1034bf1c937..0f9c491552ff 100644
>--- a/mm/ksm.c
>+++ b/mm/ksm.c
>@@ -2753,18 +2753,16 @@ static void ksm_do_scan(unsigned int scan_npages)
> {
> struct ksm_rmap_item *rmap_item;
> struct page *page;
>- unsigned int npages = scan_npages;
>
>- while (npages-- && likely(!freezing(current))) {
>+ while (scan_npages-- && likely(!freezing(current))) {
> cond_resched();
> rmap_item = scan_get_next_rmap_item(&page);
> if (!rmap_item)
> return;
> cmp_and_merge_page(page, rmap_item);
> put_page(page);
>+ ksm_pages_scanned++;
> }
>-
>- ksm_pages_scanned += scan_npages - npages;
> }
>
> static int ksmd_should_run(void)
Looks good to me.
Reviewed-by: xu xin <[email protected]>
> We normally ksm_zero_pages++ in ksmd when page is merged with zero page,
> but ksm_zero_pages-- is done from page tables side, which can't protected
> by the ksmd mutex.
"cant protected" -> "can't be protected".
But It's better to say "where there is no any accessing protection of
ksm_zero_pages" because ksmd mutex is to protect the flag of ksm_run, not to
protect the counters of KSM.
Anyway, The following code looks OK to me.
>
> So we can read very exceptional value of ksm_zero_pages in rare cases,
> such as -1, which is very confusing to users.
>
> Fix it by changing to use atomic_long_t, and the same case with the
> mm->ksm_zero_pages.
>
> Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
> Fixes: 6080d19f0704 ("ksm: add ksm zero pages for each process")
> Acked-by: David Hildenbrand <[email protected]>
> Signed-off-by: Chengming Zhou <[email protected]>
> ---
> fs/proc/base.c | 2 +-
> include/linux/ksm.h | 17 ++++++++++++++---
> include/linux/mm_types.h | 2 +-
> mm/ksm.c | 11 +++++------
> 4 files changed, 21 insertions(+), 11 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 18550c071d71..72a1acd03675 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3214,7 +3214,7 @@ static int proc_pid_ksm_stat(struct seq_file *m, struct pid_namespace *ns,
> mm = get_task_mm(task);
> if (mm) {
> seq_printf(m, "ksm_rmap_items %lu\n", mm->ksm_rmap_items);
> - seq_printf(m, "ksm_zero_pages %lu\n", mm->ksm_zero_pages);
> + seq_printf(m, "ksm_zero_pages %ld\n", mm_ksm_zero_pages(mm));
> seq_printf(m, "ksm_merging_pages %lu\n", mm->ksm_merging_pages);
> seq_printf(m, "ksm_process_profit %ld\n", ksm_process_profit(mm));
> mmput(mm);
> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
> index 52c63a9c5a9c..11690dacd986 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -33,16 +33,27 @@ void __ksm_exit(struct mm_struct *mm);
> */
> #define is_ksm_zero_pte(pte) (is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte))
>
> -extern unsigned long ksm_zero_pages;
> +extern atomic_long_t ksm_zero_pages;
> +
> +static inline void ksm_map_zero_page(struct mm_struct *mm)
> +{
> + atomic_long_inc(&ksm_zero_pages);
> + atomic_long_inc(&mm->ksm_zero_pages);
> +}
>
> static inline void ksm_might_unmap_zero_page(struct mm_struct *mm, pte_t pte)
> {
> if (is_ksm_zero_pte(pte)) {
> - ksm_zero_pages--;
> - mm->ksm_zero_pages--;
> + atomic_long_dec(&ksm_zero_pages);
> + atomic_long_dec(&mm->ksm_zero_pages);
> }
> }
>
> +static inline long mm_ksm_zero_pages(struct mm_struct *mm)
> +{
> + return atomic_long_read(&mm->ksm_zero_pages);
> +}
> +
> static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
> {
> if (test_bit(MMF_VM_MERGEABLE, &oldmm->flags))
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 24323c7d0bd4..af3a0256fa93 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -985,7 +985,7 @@ struct mm_struct {
> * Represent how many empty pages are merged with kernel zero
> * pages when enabling KSM use_zero_pages.
> */
> - unsigned long ksm_zero_pages;
> + atomic_long_t ksm_zero_pages;
> #endif /* CONFIG_KSM */
> #ifdef CONFIG_LRU_GEN_WALKS_MMU
> struct {
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 0f9c491552ff..6f461411d070 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -296,7 +296,7 @@ static bool ksm_use_zero_pages __read_mostly;
> static bool ksm_smart_scan = true;
>
> /* The number of zero pages which is placed by KSM */
> -unsigned long ksm_zero_pages;
> +atomic_long_t ksm_zero_pages = ATOMIC_LONG_INIT(0);
>
> /* The number of pages that have been skipped due to "smart scanning" */
> static unsigned long ksm_pages_skipped;
> @@ -1429,8 +1429,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
> * the dirty bit in zero page's PTE is set.
> */
> newpte = pte_mkdirty(pte_mkspecial(pfn_pte(page_to_pfn(kpage), vma->vm_page_prot)));
> - ksm_zero_pages++;
> - mm->ksm_zero_pages++;
> + ksm_map_zero_page(mm);
> /*
> * We're replacing an anonymous page with a zero page, which is
> * not anonymous. We need to do proper accounting otherwise we
> @@ -3373,7 +3372,7 @@ static void wait_while_offlining(void)
> #ifdef CONFIG_PROC_FS
> long ksm_process_profit(struct mm_struct *mm)
> {
> - return (long)(mm->ksm_merging_pages + mm->ksm_zero_pages) * PAGE_SIZE -
> + return (long)(mm->ksm_merging_pages + mm_ksm_zero_pages(mm)) * PAGE_SIZE -
> mm->ksm_rmap_items * sizeof(struct ksm_rmap_item);
> }
> #endif /* CONFIG_PROC_FS */
> @@ -3662,7 +3661,7 @@ KSM_ATTR_RO(pages_skipped);
> static ssize_t ksm_zero_pages_show(struct kobject *kobj,
> struct kobj_attribute *attr, char *buf)
> {
> - return sysfs_emit(buf, "%ld\n", ksm_zero_pages);
> + return sysfs_emit(buf, "%ld\n", atomic_long_read(&ksm_zero_pages));
> }
> KSM_ATTR_RO(ksm_zero_pages);
>
> @@ -3671,7 +3670,7 @@ static ssize_t general_profit_show(struct kobject *kobj,
> {
> long general_profit;
>
> - general_profit = (ksm_pages_sharing + ksm_zero_pages) * PAGE_SIZE -
> + general_profit = (ksm_pages_sharing + atomic_long_read(&ksm_zero_pages)) * PAGE_SIZE -
> ksm_rmap_items * sizeof(struct ksm_rmap_item);
>
> return sysfs_emit(buf, "%ld\n", general_profit);
>
> --
> 2.45.0
>
On 2024/5/13 14:00, xu xin wrote:
>> We normally ksm_zero_pages++ in ksmd when page is merged with zero page,
>> but ksm_zero_pages-- is done from page tables side, which can't protected
>> by the ksmd mutex.
>
> "cant protected" -> "can't be protected".
Right, will fix.
>
> But It's better to say "where there is no any accessing protection of
> ksm_zero_pages" because ksmd mutex is to protect the flag of ksm_run, not to
> protect the counters of KSM.
Ah, I thought the introduced ksm_zero_pages counters were protected by ksmd mutex
like all other ksm counters, no? But the difference with those ksm counters is
that ksm_zero_pages could be changed by the page table side operations, which
can't take ksmd mutex, this is the reason why we need to use atomic variables.
Thanks.
>
>
> Anyway, The following code looks OK to me.
>>
>> So we can read very exceptional value of ksm_zero_pages in rare cases,
>> such as -1, which is very confusing to users.
>>
>> Fix it by changing to use atomic_long_t, and the same case with the
>> mm->ksm_zero_pages.
>>
>> Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM")
>> Fixes: 6080d19f0704 ("ksm: add ksm zero pages for each process")
>> Acked-by: David Hildenbrand <[email protected]>
>> Signed-off-by: Chengming Zhou <[email protected]>
>> ---
>> fs/proc/base.c | 2 +-
>> include/linux/ksm.h | 17 ++++++++++++++---
>> include/linux/mm_types.h | 2 +-
>> mm/ksm.c | 11 +++++------
>> 4 files changed, 21 insertions(+), 11 deletions(-)
>>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index 18550c071d71..72a1acd03675 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -3214,7 +3214,7 @@ static int proc_pid_ksm_stat(struct seq_file *m, struct pid_namespace *ns,
>> mm = get_task_mm(task);
>> if (mm) {
>> seq_printf(m, "ksm_rmap_items %lu\n", mm->ksm_rmap_items);
>> - seq_printf(m, "ksm_zero_pages %lu\n", mm->ksm_zero_pages);
>> + seq_printf(m, "ksm_zero_pages %ld\n", mm_ksm_zero_pages(mm));
>> seq_printf(m, "ksm_merging_pages %lu\n", mm->ksm_merging_pages);
>> seq_printf(m, "ksm_process_profit %ld\n", ksm_process_profit(mm));
>> mmput(mm);
>> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
>> index 52c63a9c5a9c..11690dacd986 100644
>> --- a/include/linux/ksm.h
>> +++ b/include/linux/ksm.h
>> @@ -33,16 +33,27 @@ void __ksm_exit(struct mm_struct *mm);
>> */
>> #define is_ksm_zero_pte(pte) (is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte))
>>
>> -extern unsigned long ksm_zero_pages;
>> +extern atomic_long_t ksm_zero_pages;
>> +
>> +static inline void ksm_map_zero_page(struct mm_struct *mm)
>> +{
>> + atomic_long_inc(&ksm_zero_pages);
>> + atomic_long_inc(&mm->ksm_zero_pages);
>> +}
>>
>> static inline void ksm_might_unmap_zero_page(struct mm_struct *mm, pte_t pte)
>> {
>> if (is_ksm_zero_pte(pte)) {
>> - ksm_zero_pages--;
>> - mm->ksm_zero_pages--;
>> + atomic_long_dec(&ksm_zero_pages);
>> + atomic_long_dec(&mm->ksm_zero_pages);
>> }
>> }
>>
>> +static inline long mm_ksm_zero_pages(struct mm_struct *mm)
>> +{
>> + return atomic_long_read(&mm->ksm_zero_pages);
>> +}
>> +
>> static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
>> {
>> if (test_bit(MMF_VM_MERGEABLE, &oldmm->flags))
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 24323c7d0bd4..af3a0256fa93 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -985,7 +985,7 @@ struct mm_struct {
>> * Represent how many empty pages are merged with kernel zero
>> * pages when enabling KSM use_zero_pages.
>> */
>> - unsigned long ksm_zero_pages;
>> + atomic_long_t ksm_zero_pages;
>> #endif /* CONFIG_KSM */
>> #ifdef CONFIG_LRU_GEN_WALKS_MMU
>> struct {
>> diff --git a/mm/ksm.c b/mm/ksm.c
>> index 0f9c491552ff..6f461411d070 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -296,7 +296,7 @@ static bool ksm_use_zero_pages __read_mostly;
>> static bool ksm_smart_scan = true;
>>
>> /* The number of zero pages which is placed by KSM */
>> -unsigned long ksm_zero_pages;
>> +atomic_long_t ksm_zero_pages = ATOMIC_LONG_INIT(0);
>>
>> /* The number of pages that have been skipped due to "smart scanning" */
>> static unsigned long ksm_pages_skipped;
>> @@ -1429,8 +1429,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
>> * the dirty bit in zero page's PTE is set.
>> */
>> newpte = pte_mkdirty(pte_mkspecial(pfn_pte(page_to_pfn(kpage), vma->vm_page_prot)));
>> - ksm_zero_pages++;
>> - mm->ksm_zero_pages++;
>> + ksm_map_zero_page(mm);
>> /*
>> * We're replacing an anonymous page with a zero page, which is
>> * not anonymous. We need to do proper accounting otherwise we
>> @@ -3373,7 +3372,7 @@ static void wait_while_offlining(void)
>> #ifdef CONFIG_PROC_FS
>> long ksm_process_profit(struct mm_struct *mm)
>> {
>> - return (long)(mm->ksm_merging_pages + mm->ksm_zero_pages) * PAGE_SIZE -
>> + return (long)(mm->ksm_merging_pages + mm_ksm_zero_pages(mm)) * PAGE_SIZE -
>> mm->ksm_rmap_items * sizeof(struct ksm_rmap_item);
>> }
>> #endif /* CONFIG_PROC_FS */
>> @@ -3662,7 +3661,7 @@ KSM_ATTR_RO(pages_skipped);
>> static ssize_t ksm_zero_pages_show(struct kobject *kobj,
>> struct kobj_attribute *attr, char *buf)
>> {
>> - return sysfs_emit(buf, "%ld\n", ksm_zero_pages);
>> + return sysfs_emit(buf, "%ld\n", atomic_long_read(&ksm_zero_pages));
>> }
>> KSM_ATTR_RO(ksm_zero_pages);
>>
>> @@ -3671,7 +3670,7 @@ static ssize_t general_profit_show(struct kobject *kobj,
>> {
>> long general_profit;
>>
>> - general_profit = (ksm_pages_sharing + ksm_zero_pages) * PAGE_SIZE -
>> + general_profit = (ksm_pages_sharing + atomic_long_read(&ksm_zero_pages)) * PAGE_SIZE -
>> ksm_rmap_items * sizeof(struct ksm_rmap_item);
>>
>> return sysfs_emit(buf, "%ld\n", general_profit);
>>
>> --
>> 2.45.0
>>