Commit c2dc78b8 authored by Chengming Zhou's avatar Chengming Zhou Committed by Andrew Morton
Browse files

mm/ksm: fix ksm_zero_pages accounting 

We normally ksm_zero_pages++ in ksmd when page is merged with zero page,
but ksm_zero_pages-- is done from page tables side, where there is no any
accessing protection of ksm_zero_pages.

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.

Link: https://lkml.kernel.org/r/20240528-b4-ksm-counters-v3-2-34bb358fdc13@linux.dev


Fixes: e2942062 ("ksm: count all zero pages placed by KSM")
Fixes: 6080d19f ("ksm: add ksm zero pages for each process")
Signed-off-by: default avatarChengming Zhou <chengming.zhou@linux.dev>
Acked-by: default avatarDavid Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Cc: Stefan Roesch <shr@devkernel.io>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: Yang Yang <yang.yang29@zte.com.cn>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 730cdc2c

fs/proc/base.c

+1 −1
Original line number Diff line number Diff line
@@ -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);

include/linux/ksm.h

+14 −3
Original line number Diff line number Diff line
@@ -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))

include/linux/mm_types.h

+1 −1
Original line number Diff line number Diff line
@@ -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 {

mm/ksm.c

+5 −6
Original line number Diff line number Diff line
@@ -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
@@ -3374,7 +3373,7 @@ 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 */
@@ -3663,7 +3662,7 @@ 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);
@@ -3672,7 +3671,7 @@ 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);