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

[David Hildenbrand]

On 21.10.22 12:17, David Hildenbrand wrote:
> On 11.10.22 04:22, xu.xin.sc@xxxxxxxxx wrote:
> > From: xu xin <xu.xin16@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> > Cc: Xuexin Jiang <jiang.xuexin@xxxxxxxxxx>
> > Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
> > Co-developed-by: Xiaokai Ran <ran.xiaokai@xxxxxxxxxx>
> > Signed-off-by: Xiaokai Ran <ran.xiaokai@xxxxxxxxxx>
> > Co-developed-by: Yang Yang <yang.yang29@xxxxxxxxxx>
> > Signed-off-by: Yang Yang <yang.yang29@xxxxxxxxxx>
> > Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
> > ---
> >    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@xxxxxxxxxx
>
> 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).

--
Thanks,

David / dhildenb