Messages in this thread Patch in this message | | | Date | Tue, 23 May 2023 11:41:24 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v8 1/6] ksm: support unsharing KSM-placed zero pages |
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On 22.05.23 12:49, Yang Yang wrote: > From: xu xin <xu.xin16@zte.com.cn> > > When use_zero_pages of ksm is enabled, madvise(addr, len, MADV_UNMERGEABLE) > and other ways (like write 2 to /sys/kernel/mm/ksm/run) to trigger > unsharing will *not* actually unshare the shared zeropage as placed by KSM > (which is against the MADV_UNMERGEABLE documentation). As these KSM-placed > zero pages are out of the control of KSM, the related counts of ksm pages > don't expose how many zero pages are placed by KSM (these special zero > pages are different from those initially mapped zero pages, because the > zero pages mapped to MADV_UNMERGEABLE areas are expected to be a complete > and unshared page) > > To not blindly unshare all shared zero_pages in applicable VMAs, the patch > use pte_mkdirty (related with architecture) to mark KSM-placed zero pages. > Thus, MADV_UNMERGEABLE will only unshare those 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. >
Maybe add: "We'll reuse this mechanism to reliably identify KSM-placed zeropages to properly account for them (e.g., calculating the KSM profit that includes zeropages) next."
> Signed-off-by: xu xin <xu.xin16@zte.com.cn> > Suggested-by: David Hildenbrand <david@redhat.com> > Cc: Claudio Imbrenda <imbrenda@linux.ibm.com> > Cc: Xuexin Jiang <jiang.xuexin@zte.com.cn> > Reviewed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn> > Reviewed-by: Yang Yang <yang.yang29@zte.com.cn> > --- > include/linux/ksm.h | 6 ++++++ > mm/ksm.c | 5 +++-- > 2 files changed, 9 insertions(+), 2 deletions(-) > > diff --git a/include/linux/ksm.h b/include/linux/ksm.h > index 899a314bc487..7989200cdbb7 100644 > --- a/include/linux/ksm.h > +++ b/include/linux/ksm.h > @@ -26,6 +26,9 @@ int ksm_disable(struct mm_struct *mm); > > int __ksm_enter(struct mm_struct *mm); > void __ksm_exit(struct mm_struct *mm); > +/* use pte_mkdirty to track a KSM-placed zero page */ > +#define set_pte_ksm_zero(pte) pte_mkdirty(pte_mkspecial(pte))
If there is only a single user (which I assume), please inline it instead.
Let's add some more documentation:
/* * To identify zeropages that were mapped by KSM, we reuse the dirty bit * in the PTE. If the PTE is dirty, the zeropage was mapped by KSM when * deduplicating memory. */
> +#define is_ksm_zero_pte(pte) (is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte)) > > static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm) > { > @@ -95,6 +98,9 @@ static inline void ksm_exit(struct mm_struct *mm) > { > } > > +#define set_pte_ksm_zero(pte) pte_mkspecial(pte) > +#define is_ksm_zero_pte(pte) 0 > + > #ifdef CONFIG_MEMORY_FAILURE > static inline void collect_procs_ksm(struct page *page, > struct list_head *to_kill, int force_early) > diff --git a/mm/ksm.c b/mm/ksm.c > index 0156bded3a66..9962f5962afd 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -447,7 +447,8 @@ static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long nex > if (is_migration_entry(entry)) > page = pfn_swap_entry_to_page(entry); > } > - ret = page && PageKsm(page); > + /* return 1 if the page is an normal ksm page or KSM-placed zero page */ > + ret = (page && PageKsm(page)) || is_ksm_zero_pte(*pte); > pte_unmap_unlock(pte, ptl); > return ret; > } > @@ -1220,7 +1221,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page, > page_add_anon_rmap(kpage, vma, addr, RMAP_NONE); > newpte = mk_pte(kpage, vma->vm_page_prot); > } else { > - newpte = pte_mkspecial(pfn_pte(page_to_pfn(kpage), > + newpte = set_pte_ksm_zero(pfn_pte(page_to_pfn(kpage), > vma->vm_page_prot)); > /* > * We're replacing an anonymous page with a zero page, which is
Apart from that LGTM.
-- Thanks,
David / dhildenb
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