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Date: Tue, 23 May 2023 11:47:55 +0200
From: David Hildenbrand <david@...hat.com>
To: Yang Yang <yang.yang29@....com.cn>, akpm@...ux-foundation.org
Cc: imbrenda@...ux.ibm.com, jiang.xuexin@....com.cn,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
ran.xiaokai@....com.cn, xu.xin.sc@...il.com, xu.xin16@....com.cn
Subject: Re: [PATCH v8 2/6] ksm: count all zero pages placed by KSM
On 22.05.23 12:52, Yang Yang wrote:
> From: xu xin <xu.xin16@....com.cn>
>
> As pages_sharing and pages_shared don't include the number of zero pages
> merged by KSM, we cannot know how many pages are zero pages placed by KSM
> when enabling use_zero_pages, which leads to KSM not being transparent with
> all actual merged pages by KSM. In the early days of use_zero_pages,
> zero-pages was unable to get unshared by the ways like MADV_UNMERGEABLE so
> it's hard to count how many times one of those zeropages was then unmerged.
>
> But now, unsharing KSM-placed zero page accurately has been achieved, so we
> can easily count both how many times a page full of zeroes was merged with
> zero-page and how many times one of those pages was then unmerged. and so,
> it helps to estimate memory demands when each and every shared page could
> get unshared.
>
> So we add ksm_zero_pages under /sys/kernel/mm/ksm/ to show the number
> of all zero pages placed by KSM.
>
> v7->v8:
> Handle the case when khugepaged replaces a shared zeropage by a THP.
>
> Signed-off-by: xu xin <xu.xin16@....com.cn>
> Suggested-by: David Hildenbrand <david@...hat.com>
> Cc: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Cc: Xuexin Jiang <jiang.xuexin@....com.cn>
> Reviewed-by: Xiaokai Ran <ran.xiaokai@....com.cn>
> Reviewed-by: Yang Yang <yang.yang29@....com.cn>
> ---
> include/linux/ksm.h | 17 +++++++++++++++++
> mm/khugepaged.c | 3 +++
> mm/ksm.c | 12 ++++++++++++
> mm/memory.c | 7 ++++++-
> 4 files changed, 38 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
> index 7989200cdbb7..1adcae0205e3 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -29,6 +29,16 @@ void __ksm_exit(struct mm_struct *mm);
> /* use pte_mkdirty to track a KSM-placed zero page */
> #define set_pte_ksm_zero(pte) pte_mkdirty(pte_mkspecial(pte))
> #define is_ksm_zero_pte(pte) (is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte))
> +extern unsigned long ksm_zero_pages;
> +static inline void inc_ksm_zero_pages(void)
> +{
> + ksm_zero_pages++;
> +}
> +
No need to export the inc, just inline this.
> +static inline void dec_ksm_zero_pages(void)
> +{
> + ksm_zero_pages--;
> +}
>
> static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
> {
> @@ -100,6 +110,13 @@ static inline void ksm_exit(struct mm_struct *mm)
>
> #define set_pte_ksm_zero(pte) pte_mkspecial(pte)
> #define is_ksm_zero_pte(pte) 0
> +static inline void inc_ksm_zero_pages(void)
> +{
> +}
> +
> +static inline void dec_ksm_zero_pages(void)
> +{
> +}
>
> #ifdef CONFIG_MEMORY_FAILURE
> static inline void collect_procs_ksm(struct page *page,
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 6b9d39d65b73..ba0d077b6951 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -19,6 +19,7 @@
> #include <linux/page_table_check.h>
> #include <linux/swapops.h>
> #include <linux/shmem_fs.h>
> +#include <linux/ksm.h>
>
> #include <asm/tlb.h>
> #include <asm/pgalloc.h>
> @@ -711,6 +712,8 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> spin_lock(ptl);
> ptep_clear(vma->vm_mm, address, _pte);
> spin_unlock(ptl);
> + if (is_ksm_zero_pte(pteval))
> + dec_ksm_zero_pages();
> }
> } else {
> src_page = pte_page(pteval);
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 9962f5962afd..2ca7e8860faa 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -278,6 +278,9 @@ static unsigned int zero_checksum __read_mostly;
> /* Whether to merge empty (zeroed) pages with actual zero pages */
> static bool ksm_use_zero_pages __read_mostly;
>
> +/* The number of zero pages which is placed by KSM */
> +unsigned long ksm_zero_pages;
> +
> #ifdef CONFIG_NUMA
> /* Zeroed when merging across nodes is not allowed */
> static unsigned int ksm_merge_across_nodes = 1;
> @@ -1223,6 +1226,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
> } else {
> newpte = set_pte_ksm_zero(pfn_pte(page_to_pfn(kpage),
> vma->vm_page_prot));
> + inc_ksm_zero_pages();
> /*
> * We're replacing an anonymous page with a zero page, which is
> * not anonymous. We need to do proper accounting otherwise we
> @@ -3350,6 +3354,13 @@ static ssize_t pages_volatile_show(struct kobject *kobj,
> }
> KSM_ATTR_RO(pages_volatile);
>
> +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);
> +}
> +KSM_ATTR_RO(ksm_zero_pages);
> +
> static ssize_t general_profit_show(struct kobject *kobj,
> struct kobj_attribute *attr, char *buf)
> {
> @@ -3417,6 +3428,7 @@ static struct attribute *ksm_attrs[] = {
> &pages_sharing_attr.attr,
> &pages_unshared_attr.attr,
> &pages_volatile_attr.attr,
> + &ksm_zero_pages_attr.attr,
> &full_scans_attr.attr,
> #ifdef CONFIG_NUMA
> &merge_across_nodes_attr.attr,
> diff --git a/mm/memory.c b/mm/memory.c
> index 8358f3b853f2..058b416adf24 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1415,8 +1415,11 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> tlb_remove_tlb_entry(tlb, pte, addr);
> zap_install_uffd_wp_if_needed(vma, addr, pte, details,
> ptent);
> - if (unlikely(!page))
> + if (unlikely(!page)) {
> + if (is_ksm_zero_pte(ptent))
> + dec_ksm_zero_pages();
> continue;
> + }
>
> delay_rmap = 0;
> if (!PageAnon(page)) {
> @@ -3120,6 +3123,8 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
> inc_mm_counter(mm, MM_ANONPAGES);
> }
> } else {
> + if (is_ksm_zero_pte(vmf->orig_pte))
> + dec_ksm_zero_pages();
> inc_mm_counter(mm, MM_ANONPAGES);
> }
> flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
Can we maybe avoid exporting the dec semantics and rather add a callback
to KSM? Ideally, we'd even distill that down to a single call, and
handle the details in ksm.h. Maybe simply:
ksm_notify_unmap_zero_page(vmf->orig_pte);
and then just have in ksm.h
static inline void ksm_notify_unmap_zero_page(pte_t pte)
{
if (is_ksm_zero_pte(pte))
ksm_zero_pages--;
}
--
Thanks,
David / dhildenb
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