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From: Lance Yang In-Reply-To: <8bf8302a-6aba-4f7e-8356-a933bcf9e4a1@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: DAF7C40015 X-Rspamd-Server: rspam05 X-Stat-Signature: 3nsnaq5xju8kfd3n8h3e6hyoskjha6ma X-Rspam-User: X-HE-Tag: 1758648031-29104 X-HE-Meta: U2FsdGVkX1+4hKrL/awA8z/yA1VfTmRLWoxnZAn0LKi//vcOI21ohI5iq7A1+XWnAEsYRu1KuocCrajSavaQiX8/WLyJo36seLv3deqqbwcWkbGUwCJ/2YiIQ+2bwuaqDo2URnuTvSmkVQh83scWUQy1NSauSHfqNaJ6ZGW8knm9lUAUQH3VCkT/WzFuZJ4KSIBpBOSuKxFxtXCTCPE6wcezLiMLD8MpLxilUJPZ7EtbwxrfQLFWIdpkQVdU1TODwxlMsIRBZxQFZkiH590wddkVOwTZDABhl1bMpfteKja3Na3pHsO2vgJVQhd0HUU5RCMg/hFCt8VT7PcebJ8MM6epXIT1rFvis2AsTxv28ZGwT997KNiqoDj+MqwLzIGB87Iypv2wkZCAzURcF1kcXiw5Rj6kOlxCk2sH0poEAzY/wjvQYUWraALfMQ+Ft6Aa+x9fbT5MnolbGGg67i14zJZp9zxK+kXJr+8Z6aEPCcA2bcV5XpwKSFwtJl8baLrAZXdAaEatf8tOPrpYoelb/ZH8BhdtqNYxvD1ciwjo7o8mJCiggMfCmVXjXwOxy1NCsgSS6BHGwGCjN901VoqXur9RIq8UQWVCDiSNQE1uzzWSoLr06V47dqdLwbbpzy6oD0zvPWjK6SwhkW6TGhOhZP1GZRMXODuIohuD6VNmL3NBP41PnId7Kpt8zMTnzww6rusyQ2ruX3gNOlvWKz5lrPGijlnY8QHXUnwRD1YYcDWpvtRDJoxy0HcExqmygj1XeqVM7aOQkPpjVmIsERPfnSCl6eUT1rbqYyV7U1Isw94Q4luHf9kLI+DqWA1MgRkwZul+xiRXLHEzH72doVAbJ7DTqSZlDLfgde7q061P4ldXnvJmiaAHfD0VOYgC+u+JF2Llc3giVWEednkEue+S2IdwZyjrRb8U5PO+iIpTBGU5kvt3720B1B7nMCDgCRRes8CzR/YWwMzvIz9Dh7x COS0Nd25 Upa5ZTldc0IGPdspJISkrWj9W9Hh/O0YGSdQ0BfpXpPPjfYN0BKKYNf3fOO6E/cXiLJgQh6ODwxve4RvaEqBQ+08Y7guWwMAjYq9beeT08Rkbbs4xbYqxARMjGann3dQH6Rtmi/E0/D6lwnvSVmz76daqVUQs3zsapKmsr6LeAmpccrSQsW1fdy12MsnZ73c5zvBBa+XDOVHUZFzPLT9Fm7I/Jh+JTfV2XXtDM9eLZ16eZ+m5tW8dXH2LDsqWGbjK1ZlvDOwybNCB3MTjiaYNZkxEd0XafCUr32BEw+fLJTl8PEJFELJFz/O2gA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2025/9/23 20:00, David Hildenbrand wrote: > On 23.09.25 13:52, Catalin Marinas wrote: >> 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. > > I assume any tag changes would result in CoW. Right, I did a test and confirmed that any attempt to change tags on the shared zero page results in a Copy-on-Write, as expected ;) > > It would be interesting to know if there are use cases with VMs or other > workloads where that could be beneficial with KSM. > >> 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; > > That looks reasonable to me. > > @Lance as you had a test setup, could you give this a try as well with > KSM shared zeropage deduplication enabled whether it now works as > expected as well? This works as expected. Both KSM (with use_zero_pages enabled) and the THP shrinker are now able to successfully merge zero-filled pages with the shared zero page, as long as those pages are not mapped with PROT_MTE. > > Then, this should likely be an independent fix. @Catalin could you send a real patch?