On Mon, Sep 22, 2025 at 07:59:00PM +0200, David Hildenbrand wrote:
On 22.09.25 19:24, Catalin Marinas wrote:
On Mon, Sep 22, 2025 at 10:14:58AM +0800, Lance Yang wrote:
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 32e0ec2dde36..28d4b02a1aa5 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -4104,29 +4104,20 @@ static unsigned long deferred_split_count(struct shrinker *shrink, static bool thp_underused(struct folio *folio) { int num_zero_pages = 0, num_filled_pages = 0;
- void *kaddr; int i; for (i = 0; i < folio_nr_pages(folio); i++) {
kaddr = kmap_local_folio(folio, i * PAGE_SIZE);if (!memchr_inv(kaddr, 0, PAGE_SIZE)) {num_zero_pages++;if (num_zero_pages > khugepaged_max_ptes_none) {kunmap_local(kaddr);
if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) {if (++num_zero_pages > khugepaged_max_ptes_none) return true;I wonder what the overhead of doing a memcmp() vs memchr_inv() is. The former will need to read from two places. If it's noticeable, it would affect architectures that don't have an MTE equivalent.
Alternatively we could introduce something like folio_has_metadata() which on arm64 simply checks PG_mte_tagged.
We discussed something similar in the other thread (I suggested page_is_mergable()). I'd prefer to use pages_identical() for now, so we have the same logic here and in ksm code.
(this patch here almost looks like a cleanup :) )
If this becomes a problem, what we could do is in pages_identical() would be simply doing the memchr_inv() in case is_zero_pfn(). KSM might benefit from that as well when merging with the shared zeropage through try_to_merge_with_zero_page().
Yes, we can always optimise it later.
I just realised that on arm64 with MTE we won't get any merging with the zero page even if the user page isn't mapped with PROT_MTE. In cpu_enable_mte() we zero the tags in the zero page and set PG_mte_tagged. The reason is that we want to use the zero page with PROT_MTE mappings (until tag setting causes CoW). Hmm, the arm64 memcmp_pages() messed up KSM merging with the zero page even before this patch.
The MTE tag setting evolved a bit over time with some locking using PG_* flags to avoid a set_pte_at() race trying to initialise the tags on the same page. We also moved the swap restoring to arch_swap_restore() rather than the set_pte_at() path. So it is safe now to merge with the zero page if the other page isn't tagged. A subsequent set_pte_at() attempting to clear the tags would notice that the zero page is already tagged.
We could go a step further and add tag comparison (I had some code around) but I think the quick fix is to just not treat the zero page as tagged. Not fully tested yet:
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c index e5e773844889..72a1dfc54659 100644 --- a/arch/arm64/kernel/mte.c +++ b/arch/arm64/kernel/mte.c @@ -73,6 +73,8 @@ int memcmp_pages(struct page *page1, struct page *page2) { char *addr1, *addr2; int ret; + bool page1_tagged = page_mte_tagged(page1) && !is_zero_page(page1); + bool page2_tagged = page_mte_tagged(page2) && !is_zero_page(page2);
addr1 = page_address(page1); addr2 = page_address(page2); @@ -83,11 +85,10 @@ int memcmp_pages(struct page *page1, struct page *page2)
/* * If the page content is identical but at least one of the pages is - * tagged, return non-zero to avoid KSM merging. If only one of the - * pages is tagged, __set_ptes() may zero or change the tags of the - * other page via mte_sync_tags(). + * tagged, return non-zero to avoid KSM merging. Ignore the zero page + * since it is always tagged with the tags cleared. */ - if (page_mte_tagged(page1) || page_mte_tagged(page2)) + if (page1_tagged || page2_tagged) return addr1 != addr2;
return ret;