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@xxxxxxxxx wrote:
>>> From: Yang Yang <yang.yang29@xxxxxxxxxx>
>>>
>>> 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 <yang.yang29@xxxxxxxxxx>
>>> Reviewed-by: xu xin <xu.xin16@xxxxxxxxxx>
>>> Reviewed-by: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
>>> ---
>>> 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
