Messages in this thread Patch in this message |  | | Date | Tue, 23 Sep 2025 17:14:51 +0100 | | From | Catalin Marinas <> | | Subject | Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages |
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On Tue, Sep 23, 2025 at 12:52:06PM +0100, Catalin Marinas wrote: > 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. [...] > 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;
Unrelated to this discussion, I got an internal report that Linux hangs during boot with CONFIG_DEFERRED_STRUCT_PAGE_INIT because try_page_mte_tagging() locks up on uninitialised page flags.
Since we (always?) map the zero page as pte_special(), set_pte_at() won't check if the tags have to be initialised, so we can skip the PG_mte_tagged altogether. We actually had this code for some time until we introduced the pte_special() check in set_pte_at().
So alternative patch that also fixes the deferred struct page init (on the assumptions that the zero page is always mapped as pte_special():
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c index 7b78c95a9017..e325ba34f45c 100644 --- a/arch/arm64/kernel/cpufeature.c +++ b/arch/arm64/kernel/cpufeature.c @@ -2419,17 +2419,21 @@ static void bti_enable(const struct arm64_cpu_capabilities *__unused) #ifdef CONFIG_ARM64_MTE static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap) { + static bool cleared_zero_page = false; + sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_ATA | SCTLR_EL1_ATA0); mte_cpu_setup(); /* * Clear the tags in the zero page. This needs to be done via the - * linear map which has the Tagged attribute. + * linear map which has the Tagged attribute. Since this page is + * always mapped as pte_special(), set_pte_at() will not attempt to + * clear the tags or set PG_mte_tagged. */ - if (try_page_mte_tagging(ZERO_PAGE(0))) { + if (!cleared_zero_page) { + cleared_zero_page = true; mte_clear_page_tags(lm_alias(empty_zero_page)); - set_page_mte_tagged(ZERO_PAGE(0)); } kasan_init_hw_tags_cpu(); -- Catalin
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