Re: [PATCH v3 2/5] ksm: support unsharing zero pages placed by KSM

[David Hildenbrand <david@redhat.com>]

On 21.10.22 14:54, David Hildenbrand wrote:
> On 21.10.22 12:17, David Hildenbrand wrote:
>> On 11.10.22 04:22, xu.xin.sc@gmail.com wrote:
>>> From: xu xin <xu.xin16@zte.com.cn>
>>>
>>> use_zero_pages may be very useful, not just because of cache colouring
>>> as described in doc, but also because use_zero_pages can accelerate
>>> merging empty pages when there are plenty of empty pages (full of zeros)
>>> as the time of page-by-page comparisons (unstable_tree_search_insert) is
>>> saved.
>>>
>>> But when enabling use_zero_pages, madvise(addr, len, MADV_UNMERGEABLE) and
>>> other ways (like write 2 to /sys/kernel/mm/ksm/run) to trigger unsharing
>>> will *not* unshare the shared zeropage as placed by KSM (which may be
>>> against the MADV_UNMERGEABLE documentation at least).
>>>
>>> To not blindly unshare all shared zero_pages in applicable VMAs, the patch
>>> introduces a dedicated flag ZERO_PAGE_FLAG to mark the rmap_items of those
>>> shared zero_pages. and guarantee that these rmap_items will be not freed
>>> during the time of zero_pages not being writing, so we can only unshare
>>> the *KSM-placed* zero_pages.
>>>
>>> The patch will not degrade the performance of use_zero_pages as it doesn't
>>> change the way of merging empty pages in use_zero_pages's feature.
>>>
>>> Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
>>> Reported-by: David Hildenbrand <david@redhat.com>
>>> Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
>>> Cc: Xuexin Jiang <jiang.xuexin@zte.com.cn>
>>> Signed-off-by: xu xin <xu.xin16@zte.com.cn>
>>> Co-developed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn>
>>> Signed-off-by: Xiaokai Ran <ran.xiaokai@zte.com.cn>
>>> Co-developed-by: Yang Yang <yang.yang29@zte.com.cn>
>>> Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
>>> Signed-off-by: xu xin <xu.xin16@zte.com.cn>
>>> ---
>>>     mm/ksm.c | 136 ++++++++++++++++++++++++++++++++++++++++++-------------
>>>     1 file changed, 105 insertions(+), 31 deletions(-)
>>>
>>> diff --git a/mm/ksm.c b/mm/ksm.c
>>> index 13c60f1071d8..e351d7b6d15e 100644
>>> --- a/mm/ksm.c
>>> +++ b/mm/ksm.c
>>> @@ -213,6 +213,7 @@ struct ksm_rmap_item {
>>>     #define SEQNR_MASK	0x0ff	/* low bits of unstable tree seqnr */
>>>     #define UNSTABLE_FLAG	0x100	/* is a node of the unstable tree */
>>>     #define STABLE_FLAG	0x200	/* is listed from the stable tree */
>>> +#define ZERO_PAGE_FLAG 0x400 /* is zero page placed by KSM */
>>>     
>>>     /* The stable and unstable tree heads */
>>>     static struct rb_root one_stable_tree[1] = { RB_ROOT };
>>> @@ -381,14 +382,6 @@ static inline struct ksm_rmap_item *alloc_rmap_item(void)
>>>     	return rmap_item;
>>>     }
>>>     
>>> -static inline void free_rmap_item(struct ksm_rmap_item *rmap_item)
>>> -{
>>> -	ksm_rmap_items--;
>>> -	rmap_item->mm->ksm_rmap_items--;
>>> -	rmap_item->mm = NULL;	/* debug safety */
>>> -	kmem_cache_free(rmap_item_cache, rmap_item);
>>> -}
>>> -
>>>     static inline struct ksm_stable_node *alloc_stable_node(void)
>>>     {
>>>     	/*
>>> @@ -420,7 +413,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>>>     }
>>>     
>>>     /*
>>> - * We use break_ksm to break COW on a ksm page: it's a stripped down
>>> + * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only
>>> + * happen when enabling use_zero_pages): it's a stripped down
>>>      *
>>>      *	if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1)
>>>      *		put_page(page);
>>> @@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>>>      * of the process that owns 'vma'.  We also do not want to enforce
>>>      * protection keys here anyway.
>>>      */
>>> -static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>>> +static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
>>> +				     bool ksm_check_bypass)
>>>     {
>>>     	struct page *page;
>>>     	vm_fault_t ret = 0;
>>> @@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>>>     			ret = handle_mm_fault(vma, addr,
>>>     					      FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>>>     					      NULL);
>>> +		else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) {
>>> +			/*
>>> +			 * Although it's not ksm page, it's zero page as placed by
>>> +			 * KSM use_zero_page, so we should unshare it when
>>> +			 * ksm_check_bypass is true.
>>> +			 */
>>> +			ret = handle_mm_fault(vma, addr,
>>> +						  FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>>> +						  NULL);
>>> +		}
>>
>> Please don't duplicate that page fault triggering code.
>>
>> Also, please be aware that this collides with
>>
>> https://lkml.kernel.org/r/20221021101141.84170-1-david@redhat.com
>>
>> Adjustments should be comparatively easy.
> 
> ... except that I'm still working on FAULT_FLAG_UNSHARE support for the
> shared zeropage. That will be posted soonish (within next 2 weeks).
> 

Posted: https://lkml.kernel.org/r/20221107161740.144456-1-david@redhat.com

With that, we can use FAULT_FLAG_UNSHARE also to break COW on the shared 
zeropage.

-- 
Thanks,

David / dhildenb