Re: [PATCH resend] ksm: add merge_only_zero_pages to merge only empty anonymous pages

["Xu Xin (vivo)" <xu.xin@linux.dev>]

On Sat, Nov 07, 2026 at 04:13:40PM +0800, Jinjiang Tu wrote:
> 
> 在 2026/11/6 11:48, xu.xin@linux.dev 写道:
> > From: "Xu Xin (vivo)" <xu.xin@linux.dev>
> > 
> >    KSM side-channel exposure is desired. Many handset vendors may prefer
> >    this knob, which can be used as a default fallback policy.
> > 
> > - Not suitable for server and virtualization workloads containing many
> >    non-zero duplicate pages, where vanilla KSM provides better memory
> >    savings.
> 
> It’s a coincidence that I’m also working on similar optimizations.
> In our scenario, we have a MySQL instance with over 700 GB of anonymous
> memory. Most of this memory belongs to the buffer pool, which caches
> data from storage; these pages cannot be merged. Of the remaining memory,
> around 15 GB is consumed by hashtables, which contain many zero-filled
> pages that are eligible for merging to reduce memory footprint.
> 
> When we enabled KSM for the entire MySQL instance, ksmd created rmap_item
> metadata for each candidate page.The massive metadata overhead wiped out
> any gains from page merging, and even resulted in a negative benefit.
> 
> For this reason, I’m trying to make ksmd merge only zero pages. The goal
> is to have ksmd skip scanning non-zero pages entirely, thereby avoiding
> the rmap_item metadata cost introduced by scanning non-mergeable pages.

Skip generating ksm_rmap_item for non-zero pages ? That dose make sense to me.

> 
> 1) Before get_next_rmap_item(), check whether the page is all zeros.
> If not, skip it directly to avoid allocating an rmap_item.
> 
> 2) When walking page tables to find candidate pages in ksm_next_page_pmd_entry(),
> check whether the first and last contents are zero as a quick filter.
> This is intended to speed up page-table traversal.
> 
> Would you mind to collaborate? For example, by including my optimization
> as the second patch in your patch series?

Happy to do it.

> 
> Here is the diff. Basic functionality has been tested.
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index dbfb8f98b189..8a02976b0aba 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -296,6 +296,9 @@ static unsigned int zero_checksum __read_mostly;
>  /* Whether to merge empty (zeroed) pages with actual zero pages */
>  static bool ksm_use_zero_pages __read_mostly;
> +/* Whether to merge empty (zeroed) pages only */
> +static bool ksm_only_merge_zero_pages __read_mostly;
> +
>  /* Skip pages that couldn't be de-duplicated previously */
>  /* Default to true at least temporarily, for testing */
>  static bool ksm_smart_scan = true;
> @@ -2524,8 +2527,42 @@ struct ksm_next_page_arg {
>  	struct folio *folio;
>  	struct page *page;
>  	unsigned long addr;
> +	unsigned int nr_to_skip;
>  };
> +static inline bool page_maybe_zero(struct page *page)
> +{
> +	unsigned long *p;
> +	bool ret = true;
> +	unsigned int last_pos = PAGE_SIZE / sizeof(*p) - 1;
> +
> +	p = kmap_local_page(page);
> +	if (p[0] || p[last_pos])
> +		ret = false;
> +	kunmap_local(p);
> +	return ret;
> +}
> +
> +static struct page *ksm_skip_nonzero_pages(struct page *page,
> +					   unsigned long *addr,
> +					   unsigned long nr,
> +					   unsigned int *nr_to_skip)
> +{
> +	unsigned long i;
> +
> +	for (i = 0; i < nr; i++) {
> +		if (!*nr_to_skip)
> +			break;
> +		(*nr_to_skip)--;
> +		if (page_maybe_zero(page + i))
> +			break;
> +	}
> +	if (i == nr)
> +		return NULL;
> +	*addr += i << PAGE_SHIFT;
> +	return page + i;
> +}
> +
>  static int ksm_next_page_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long end,
>  		struct mm_walk *walk)
>  {
> @@ -2563,6 +2600,13 @@ static int ksm_next_page_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned lon
>  				goto not_found_unlock;
>  			page += ((addr & (PMD_SIZE - 1)) >> PAGE_SHIFT);
> +			if (ksm_only_merge_zero_pages) {
> +				page = ksm_skip_nonzero_pages(page, &addr,
> +					(end - addr) >> PAGE_SHIFT,
> +					&private->nr_to_skip);
> +				if (!page)
> +					goto not_found_unlock;
> +			}
>  			goto found_unlock;
>  		}
>  		spin_unlock(ptl);
> @@ -2585,6 +2629,14 @@ static int ksm_next_page_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned lon
>  		if (folio_is_zone_device(folio) || !folio_test_anon(folio))
>  			continue;
> +
> +		if (ksm_only_merge_zero_pages && !folio_test_ksm(folio)) {
> +			page = ksm_skip_nonzero_pages(page, &addr, 1,
> +						      &private->nr_to_skip);
> +			if (!page)
> +				continue;
> +		}
> +
>  		goto found_unlock;
>  	}
> @@ -2609,6 +2661,33 @@ static struct mm_walk_ops ksm_next_page_ops = {
>  	.walk_lock = PGWALK_RDLOCK,
>  };
> +static bool should_skip_page(struct ksm_rmap_item **rmap_list,
> +				  unsigned long addr, struct folio *folio,
> +				  struct page *page)
> +{
> +	struct ksm_rmap_item *rmap_item;
> +
> +	if (!ksm_only_merge_zero_pages)
> +		return false;
> +
> +	if (folio_test_ksm(folio))
> +		return false;
> +
> +	if (pages_identical(page, ZERO_PAGE(0)))
> +		return false;
> +
> +	while (*rmap_list) {
> +		rmap_item = *rmap_list;
> +		if ((rmap_item->address & PAGE_MASK) > addr)
> +			break;
> +		*rmap_list = rmap_item->rmap_list;
> +		remove_rmap_item_from_tree(rmap_item);
> +		free_rmap_item(rmap_item);
> +	}
> +	return true;
> +}
> +
> +
>  static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>  {
>  	struct mm_struct *mm;
> @@ -2705,6 +2784,7 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>  			int found;
> +			ksm_next_page_arg.nr_to_skip = 1024;
>  			found = walk_page_range_vma(vma, ksm_scan.address,
>  						    vma->vm_end,
>  						    &ksm_next_page_ops,
> @@ -2722,6 +2802,14 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
>  			if (tmp_page) {
>  				flush_anon_page(vma, tmp_page, ksm_scan.address);
>  				flush_dcache_page(tmp_page);
> +
> +				if (should_skip_page(ksm_scan.rmap_list, ksm_scan.address, folio, tmp_page)) {
> +					folio_put(folio);
> +					ksm_scan.address += PAGE_SIZE;
> +					mmap_read_unlock(mm);
> +					return ERR_PTR(-EAGAIN);
> +				}
> +
>  				rmap_item = get_next_rmap_item(mm_slot,
>  					ksm_scan.rmap_list, ksm_scan.address);
>  				if (rmap_item) {
> @@ -2824,6 +2912,10 @@ static void ksm_do_scan(unsigned int scan_npages)
>  		rmap_item = scan_get_next_rmap_item(&page);
>  		if (!rmap_item)
>  			return;
> +		if (rmap_item == ERR_PTR(-EAGAIN)) {
> +			ksm_pages_scanned++;
> +			continue;
> +		}
>  		cmp_and_merge_page(page, rmap_item);
>  		put_page(page);
>  		ksm_pages_scanned++;
> @@ -3674,6 +3766,28 @@ static ssize_t use_zero_pages_store(struct kobject *kobj,
>  }
>  KSM_ATTR(use_zero_pages);
>