On 2025/9/24 00:14, Catalin Marinas wrote:
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():
I can confirm that this alternative patch also works correctly; my tests for MTE all pass ;)
This looks like a better fix since it solves the boot hang issue too.
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;
set_page_mte_tagged(ZERO_PAGE(0));kasan_init_hw_tags_cpu();