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SubjectRe: [PATCH] mm: Reuse only-pte-mapped KSM page in do_wp_page()
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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>

>
> Kirill

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