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([2a09:80c0:192:0:20af:34be:985b:b6c8]) by smtp.gmail.com with ESMTPSA id e12-20020a5d6d0c000000b001a65e479d20sm2327670wrq.83.2022.03.24.01.54.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 24 Mar 2022 01:54:55 -0700 (PDT) Message-ID: <7f74c6f3-3830-a508-7357-39361ba8111f@redhat.com> Date: Thu, 24 Mar 2022 09:54:53 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH v2] mm/vmstat: add events for ksm cow Content-Language: en-US To: CGEL Cc: akpm@linux-foundation.org, yang.yang29@zte.com.cn, dave.hansen@linux.intel.com, ran.xiaokai@zte.com.cn, yang.shi@linux.alibaba.com, saravanand@fb.com, minchan@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, xu xin References: <20220323075714.2345743-1-yang.yang29@zte.com.cn> <614b33de-cdf0-73d2-08e3-196363d816d2@redhat.com> <623bcb0a.1c69fb81.ae484.a006@mx.google.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <623bcb0a.1c69fb81.ae484.a006@mx.google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 24.03.22 02:36, CGEL wrote: > On Wed, Mar 23, 2022 at 07:43:04PM +0100, David Hildenbrand wrote: >> On 23.03.22 08:57, cgel.zte@gmail.com wrote: >>> From: Yang Yang >>> >>> Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want >>> to save memory, it's a tradeoff by suffering delay on ksm cow. Users can >>> get to know how much memory ksm saved by reading >>> /sys/kernel/mm/ksm/pages_sharing, but they don't know what's the costs >>> of ksm cow, and this is important of some delay sensitive tasks. >>> >>> So add ksm cow events to help users evaluate whether or how to use ksm. >>> >>> Signed-off-by: Yang Yang >>> Reviewed-by: xu xin >>> Reviewed-by: Ran Xiaokai >>> --- >>> v2: >>> - fix compile error when CONFIG_KSM is not set >>> --- >>> include/linux/vm_event_item.h | 2 ++ >>> mm/memory.c | 20 +++++++++++++++++--- >>> mm/vmstat.c | 2 ++ >>> 3 files changed, 21 insertions(+), 3 deletions(-) >>> >>> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h >>> index 16a0a4fd000b..6f32be04212f 100644 >>> --- a/include/linux/vm_event_item.h >>> +++ b/include/linux/vm_event_item.h >>> @@ -131,6 +131,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT, >>> SWAP_RA_HIT, >>> #ifdef CONFIG_KSM >>> KSM_SWPIN_COPY, >>> + KSM_COW_SUCCESS, >>> + KSM_COW_FAIL, >>> #endif >>> #endif >>> #ifdef CONFIG_X86 >>> diff --git a/mm/memory.c b/mm/memory.c >>> index 4111f97c91a0..c24d5f04fab5 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -3257,6 +3257,8 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) >>> __releases(vmf->ptl) >>> { >>> struct vm_area_struct *vma = vmf->vma; >>> + vm_fault_t ret = 0; >>> + bool ksm = 0; >>> >>> if (userfaultfd_pte_wp(vma, *vmf->pte)) { >>> pte_unmap_unlock(vmf->pte, vmf->ptl); >>> @@ -3294,6 +3296,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) >>> */ >>> if (PageAnon(vmf->page)) { >>> struct page *page = vmf->page; >>> + ksm = PageKsm(page); >>> >>> /* >>> * We have to verify under page lock: these early checks are >>> @@ -3302,7 +3305,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) >>> * >>> * PageKsm() doesn't necessarily raise the page refcount. >>> */ >>> - if (PageKsm(page) || page_count(page) > 3) >>> + if (ksm || page_count(page) > 3) >>> goto copy; >>> if (!PageLRU(page)) >>> /* >>> @@ -3316,7 +3319,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) >>> goto copy; >>> if (PageSwapCache(page)) >>> try_to_free_swap(page); >>> - if (PageKsm(page) || page_count(page) != 1) { >>> + if (ksm || page_count(page) != 1) { >> >> I think we really want to recheck here, after locking the page. >> Consequently, just do a PageKsm() check below. >> >>> unlock_page(page); >>> goto copy; >>> } >>> @@ -3339,7 +3342,18 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) >>> get_page(vmf->page); >>> >>> pte_unmap_unlock(vmf->pte, vmf->ptl); >>> - return wp_page_copy(vmf); >>> + ret = wp_page_copy(vmf); >>> + >>> +#ifdef CONFIG_KSM >>> + if (ksm) { >>> + if (unlikely(ret & VM_FAULT_ERROR)) >>> + count_vm_event(KSM_COW_FAIL); >>> + else >>> + count_vm_event(KSM_COW_SUCCESS); >>> + } >>> +#endif >> >> Do we really care if we failed or not? I mean, the failure case will >> usually make your app crash either way ... due to OOM. >> > I think we need failed count. Because ksm cow oom is a little different > from general OOM. User may wonder I am not allocing new memory, why it > cause OOM? And OOM may have big impact for some kind of tasks, so we > better let user know that, do we? The log will be flooded by messages from the OOM handler, how will one counter regarding KSM help? I really don't think this is of any use. -- Thanks, David / dhildenb