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SubjectRe: [PATCH 2/4] mm/ksm: fix ksm_zero_pages accounting
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On 08.05.24 11:55, Chengming Zhou 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.
>
> 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.
>
> Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
> ---
> fs/proc/base.c | 2 +-
> include/linux/ksm.h | 22 +++++++++++++++++++---
> include/linux/mm_types.h | 2 +-
> mm/ksm.c | 11 +++++------
> 4 files changed, 26 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..bfc2cf756b0d 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -33,16 +33,32 @@ 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 get_ksm_zero_pages(void)
> +{
> + return atomic_long_read(&ksm_zero_pages);
> +}

I suggest inlining that one. The naming of the function also is a bit
inconsistent staring at the others.

> +
> +static inline long mm_ksm_zero_pages(struct mm_struct *mm)
> +{
> + return atomic_long_read(&mm->ksm_zero_pages);
> +}
> +

Apart from that LGTM

Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers,

David / dhildenb


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