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 <david@xxxxxxxxxx> >> Signed-off-by: Chengming Zhou <chengming.zhou@xxxxxxxxx> >> --- >> 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 >>