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SubjectRe: [PATCH] mm: Reuse only-pte-mapped KSM page in do_wp_page()
Date
On 14.12.2018 21:06, Yang Shi wrote:
>
>
> On 12/14/18 1:26 AM, Kirill Tkhai wrote:
>> On 13.12.2018 22:15, Yang Shi wrote:
>>>
>>> On 12/13/18 7:29 AM, Kirill Tkhai wrote:
>>>> This patch adds an optimization for KSM pages almost
>>>> in the same way, that we have for ordinary anonymous
>>>> pages. If there is a write fault in a page, which is
>>>> mapped to an only pte, and it is not related to swap
>>>> cache; the page may be reused without copying its
>>>> content.
>>>>
>>>> [Note, that we do not consider PageSwapCache() pages
>>>>    at least for now, since we don't want to complicate
>>>>    __get_ksm_page(), which has nice optimization based
>>>>    on this (for the migration case). Currenly it is
>>>>    spinning on PageSwapCache() pages, waiting for when
>>>>    they have unfreezed counters (i.e., for the migration
>>>>    finish). But we don't want to make it also spinning
>>>>    on swap cache pages, which we try to reuse, since
>>>>    there is not a very high probability to reuse them.
>>>>    So, for now we do not consider PageSwapCache() pages
>>>>    at all.]
>>>>
>>>> So, in reuse_ksm_page() we check for 1)PageSwapCache()
>>>> and 2)page_stable_node(), to skip a page, which KSM
>>>> is currently trying to link to stable tree. Then we
>>>> do page_ref_freeze() to prohibit KSM to merge one more
>>>> page into the page, we are reusing. After that, nobody
>>>> can refer to the reusing page: KSM skips !PageSwapCache()
>>>> pages with zero refcount; and the protection against
>>>> of all other participants is the same as for reused
>>>> ordinary anon pages pte lock, page lock and mmap_sem.
>>>>
>>>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>>>> ---
>>>>    include/linux/ksm.h |    7 +++++++
>>>>    mm/ksm.c            |   25 +++++++++++++++++++++++--
>>>>    mm/memory.c         |   16 ++++++++++++++--
>>>>    3 files changed, 44 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
>>>> index 161e8164abcf..e48b1e453ff5 100644
>>>> --- a/include/linux/ksm.h
>>>> +++ b/include/linux/ksm.h
>>>> @@ -53,6 +53,8 @@ struct page *ksm_might_need_to_copy(struct page *page,
>>>>      void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc);
>>>>    void ksm_migrate_page(struct page *newpage, struct page *oldpage);
>>>> +bool reuse_ksm_page(struct page *page,
>>>> +            struct vm_area_struct *vma, unsigned long address);
>>>>      #else  /* !CONFIG_KSM */
>>>>    @@ -86,6 +88,11 @@ static inline void rmap_walk_ksm(struct page *page,
>>>>    static inline void ksm_migrate_page(struct page *newpage, struct page *oldpage)
>>>>    {
>>>>    }
>>>> +static inline bool reuse_ksm_page(struct page *page,
>>>> +            struct vm_area_struct *vma, unsigned long address)
>>>> +{
>>>> +    return false;
>>>> +}
>>>>    #endif /* CONFIG_MMU */
>>>>    #endif /* !CONFIG_KSM */
>>>>    diff --git a/mm/ksm.c b/mm/ksm.c
>>>> index 383f961e577a..fbd14264d784 100644
>>>> --- a/mm/ksm.c
>>>> +++ b/mm/ksm.c
>>>> @@ -707,8 +707,9 @@ static struct page *__get_ksm_page(struct stable_node *stable_node,
>>>>         * case this node is no longer referenced, and should be freed;
>>>>         * however, it might mean that the page is under page_ref_freeze().
>>>>         * The __remove_mapping() case is easy, again the node is now stale;
>>>> -     * but if page is swapcache in migrate_page_move_mapping(), it might
>>>> -     * still be our page, in which case it's essential to keep the node.
>>>> +     * the same is in reuse_ksm_page() case; but if page is swapcache
>>>> +     * in migrate_page_move_mapping(), it might still be our page,
>>>> +     * in which case it's essential to keep the node.
>>>>         */
>>>>        while (!get_page_unless_zero(page)) {
>>>>            /*
>>>> @@ -2666,6 +2667,26 @@ void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc)
>>>>            goto again;
>>>>    }
>>>>    +bool reuse_ksm_page(struct page *page,
>>>> +            struct vm_area_struct *vma,
>>>> +            unsigned long address)
>>>> +{
>>>> +    VM_BUG_ON_PAGE(is_zero_pfn(page_to_pfn(page)), page);
>>>> +    VM_BUG_ON_PAGE(!page_mapped(page), page);
>>>> +    VM_BUG_ON_PAGE(!PageLocked(page), page);
>>>> +
>>>> +    if (PageSwapCache(page) || !page_stable_node(page))
>>>> +        return false;
>>>> +    /* Prohibit parallel get_ksm_page() */
>>>> +    if (!page_ref_freeze(page, 1))
>>>> +        return false;
>>>> +
>>>> +    page_move_anon_rmap(page, vma);
>>> Once the mapping is changed, it is not KSM mapping anymore. It looks later get_ksm_page() would always fail on this page. Is this expected?
>> Yes, this is the thing that the patch makes. Let's look at the actions,
>> we have without the patch, when there is a writing to an only-pte-mapped
>> KSM page.
>>
>> We enter to do_wp_page() with page_count() == 1, since KSM page is mapped
>> in only pte (and we do not get extra reference to a page, when we add it
>> to KSM stable tree). Then:
>>
>>    do_wp_page()
>>      get_page(vmf->page) <- page_count() is 2
>>      wp_page_copy()
>>        ..
>>        cow_user_page() /* Copy user page to a new one */
>>        ..
>>        put_page(vmf->page) <- page_count() is 1
>>        put_page(vmf->page) <- page_count() is 0
>>
>> Second put_page() frees the page (and also zeroes page->mapping),
>> and since that it's not a PageKsm() page anymore. Further
>> __get_ksm_page() calls will fail on this page (since the mapping
>> was zeroed), and its node will be unlinked from ksm stable tree:
>>
>> __get_ksm_page()
>> {
>>     /* page->mapping == NULL, expected_mapping != NULL */
>>     if (READ_ONCE(page->mapping) != expected_mapping)
>>         goto stale;
>>     .......
>> stale:
>>     remove_node_from_stable_tree(stable_node);
>> }
>>
>>
>> The patch optimizes do_wp_page(), and makes it to avoid the copying
>> (like we have for ordinary anon pages). Since KSM page is freed anyway,
>> after we dropped the last reference to it; we reuse it instead of this.
>> So, the thing will now work in this way:
>>
>> do_wp_page()
>>    lock_page(vmf->page)
>>    reuse_ksm_page()
>>      check PageSwapCache() and page_stable_node()
>>      page_ref_freeze(page, 1) <- Freeze the page to make parallel
>>                                  __get_ksm_page() (if any) waiting
>>      page_move_anon_rmap()    <- Write new mapping, so __get_ksm_page()
>>                                  sees this is not a KSM page anymore,
>>                                  and it removes stable node.
>>
>> So, the result is the same, but after the patch we achieve it faster :)
>>
>> Also, note, that in the most probably case, do_wp_page() does not cross
>> with __get_ksm_page() (the race window is very small; __get_ksm_page()
>> is spinning, only when reuse_ksm_page() is between page_ref_freeze()
>> and page_move_anon_rmap(), which are on neighboring lines).
>>
>> So, this is the idea. Please, let me know in case of something is unclear
>> for you.
>
> Thanks for elaborating this. It sounds reasonable. You can add Reviewed-by: Yang Shi <yang.shi@linux.alibaba.com>

Thanks!

Kirill
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