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b=UFzxhi2OKhI7lOTnQNuT5HIVGpaxAbS1fgEPpdjeuyurAwRR8AVghIocUlZW6JGwR mnGCuSA4j3JWEpMfC4xPkdvyQAKVzSd/Jkj5tOT+vRbqFF0BcqFAj4HDAiqRyeXpgK rVFOS1s6YKwWKEiM//Z2aU/vsloh6wNr8iUpImltVL/MxI0rmEfGjkQkyLO3tsVRvK npu3l3PXt8mv0PZPj+ZEqOibABFIOqYktx455TvhuC6zXUmzbvs0vEi+sj+Xp5wBKR 9N8QlqbV6+nm2AsncAr4ROh3G1k2HdVRDF68iEod47EmNtXDGQTexWFteH0cOtkckq LQ4dPVDY4yBrQ== Message-ID: Date: Mon, 5 Oct 2026 12:37:20 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] riscv/mm: fix soft-dirty migration PMDs being treated as present To: Lance Yang , pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu Cc: alex@ghiti.fr, akpm@linux-foundation.org, zhangchunyan@iscas.ac.cn, rppt@kernel.org, kas@kernel.org, andrew+kernel@donnellan.id.au, rmclure@linux.ibm.com, debug@rivosinc.com, baolin.wang@linux.alibaba.com, usama.arif@linux.dev, wangruikang@iscas.ac.cn, namcao@linutronix.de, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, alexghiti@rivosinc.com, viro@zeniv.linux.org.uk, ajones@ventanamicro.com, arnd@arndb.de, axelrasmussen@google.com, brauner@kernel.org, conor.dooley@microchip.com, conor@kernel.org, jack@suse.cz, liam@infradead.org, ljs@kernel.org, mhocko@suse.com, paul.walmsley@sifive.com, peterx@redhat.com, robh@kernel.org, surenb@google.com, vbabka@kernel.org, yuanchu@google.com, stable@vger.kernel.org, pasha.tatashin@soleen.com, linux-mm@kvack.org, me@ziyao.cc References: <20261004030312.90163-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: r68jnj7m6mtkda85x4nyhpybihnyhdsu X-Rspamd-Queue-Id: C74BF180003 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1791196655-842306 X-HE-Meta: U2FsdGVkX19rQmMvEw4JUHOb3a4e/rOnTkhiahRpOdbOddv7BAV79s9+rvm320xfLI+h8lCFGGKTSSOHsqqlecdSKLhY1445Jb1q5uMjnRXezqZYo/6v/Tb3DiWaIG+gxS6AqtSf7anu1Uivj5eQwkJDUN0DFN6XWeDdRyNbDxQW79UguQzjJBB3CHpQj4ZwYGQ84/3tj6HM2vzu/FYCtL2Vtfq4Ltx+uJXsZ0AC8Ya35W32ZzGcRKtbVtRsJ7sT+C6gn+p8HSFC7vwgQEZt5Ia/V2kVkRMAY46O7oUsj1v8r6JeZClusstDCQs1P79UK1Yd0eSFsB1tF3fIWnHRWtrbJcw7vzK9IJ3fkRFTIQ1hbj9KvsohB4SJVut9q2OE9IWmfFYk0PYqSXHJwx0FHCKhnIPr1FWGWgfS7l/G2dNMztD39fVAhPPtiTKbX29mKqW7ZJMWtkBU0OYeA3XnyJDsAoPWoMMeqBfTxgtXvLVmoqFeoeuUJQ/SMazsij7s+UmXpWJxnhmM/b+MA50HwnKD2h0NeEXWBUMjqHEea55OjyR58thguHMa5tBpC0vXEOkEsfyfeebXwURH/UfweoGZqZRHU2/jqWR+frjpzAtJ/SlBIL2fGC2qz1OQNqmVDgV/7VQUT2ltrltQJc/yBZ/xmP4pAncWy3iZhFUZwT6lhCy8lpyXj64vSn4SbCwWWN2OQQY8Re6gMBFp/zzzU/pdmMb+c0tjv5mLwJO1iH85K6y1YagTQN5a6ud+30AxZhXsyWXOxXyz41gfRGVauKkiMH/SO37K0rLqJo414tgO+o8RWsItUixvquCdyGBuMsrWJpC1Vx+iWy9rn0d0wOqbJP+NWGdpbGhQt0ro0B+cCz0ISQXaW87SJdTrZUY1u0+5V16mvbqyFEbyhD5Ajwp1DXq78kKg6jpUdLnZBWa1suCn/VONSJDygr7oYKJcgwUFCGeugSAO6OkojuW /4pNUR6r O3EXYs2LISzP0cUcSj5y6mhotDJSQpgt+3tgEWMOh/eVAgF4fVhDlXTWy3mG/HXj+4Hdd37cRGQbrUSVysYwL+nBfip01EnCpx2gCtZGNjuuNQMrqGrI82D9K08EJB0tXlyhAViq5aW5TAbKTSUlx6OKdWssc1FNlBDPkgZc+XvK18ePu8l0hbI3LN5M2Nw4hekDYA6Sm4EKsXb+QsdlhRVE1qJdzCXmkvyT8FDKKeHkD4oeQWrEW+K9CRcP7Jsriob9+x2jqL86yNZSXod1Tl49+4830NeovI5rdfIQxUgQsEotBh8e2Pf/RwIwdSlmCM8qLe7G6qYMPQN19UcK5Z3Mx7smx4IaDknIBSx5KZVbX63kwWVd1TcIhoUCPIz6MLGil9T/pSM7C6Fc3jWu8y2jL5g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/4/26 05:03, Lance Yang wrote: > RISC-V uses _PAGE_EXEC for swap soft-dirty tracking when > CONFIG_MEM_SOFT_DIRTY is enabled and Svrsw60t59b is available. That's > a problem for PMD migration entries, since pmd_present() also checks > _PAGE_LEAF (R/W/X) to recognize THPs with _PAGE_PRESENT temporarily > cleared during splitting. I'm curious: why do we have to set leaf indications for non-present things? The HW sure will ignore it, right? Is this a sw problem? Who needs that? > > When a soft-dirty THP is migrated, set_pmd_migration_entry() preserves > soft-dirty with pmd_swp_mksoft_dirty(), setting the X bit in the > migration PMD. Even with _PAGE_PRESENT clear, we end up treating a > migration PMD as a present THP! The fault handler skips > pmd_migration_entry_wait(), and a write fault can end up in > do_huge_pmd_wp_page(), where pmd_page() decodes the migration entry > as a mapped PFN. That sounds bad. > > Move the swap soft-dirty bit to bit 12 and start the swap offset at > bit 13 when CONFIG_MEM_SOFT_DIRTY is enabled. This keeps R/W/X clear > in migration PMDs and lets us keep the existing pmd_present() check > for invalidated THPs. Leave the offset at bit 12 when soft-dirty > tracking is disabled. That reduces the effective swap size (and PFN we can store). Could that be a problem? > > Fixes: 2a3ebad4db63 ("riscv: mm: add soft-dirty page tracking support") > Cc: stable@vger.kernel.org > Signed-off-by: Lance Yang > --- > I found this while reviewing Usama's PMD-level swap entries series [1] > and comparing the swap flag bits with the PMD presence checks across > architectures. > > [1] https://lore.kernel.org/all/20261002095503.3585565-1-usama.arif@linux.dev/ > > arch/riscv/include/asm/pgtable-bits.h | 6 +++--- > arch/riscv/include/asm/pgtable.h | 10 ++++++---- > 2 files changed, 9 insertions(+), 7 deletions(-) > > diff --git a/arch/riscv/include/asm/pgtable-bits.h b/arch/riscv/include/asm/pgtable-bits.h > index d5a86b4df3ce6..d84b459e010c1 100644 > --- a/arch/riscv/include/asm/pgtable-bits.h > +++ b/arch/riscv/include/asm/pgtable-bits.h > @@ -27,12 +27,12 @@ > ((riscv_has_extension_unlikely(RISCV_ISA_EXT_SVRSW60T59B)) ? \ > (1UL << 59) : 0) > /* > - * Bit 3 is always zero for swap entry computation, so we > - * can borrow it for swap page soft-dirty tracking. > + * Bit 12 is reserved for swap soft-dirty tracking. The swap offset > + * starts at bit 13 when CONFIG_MEM_SOFT_DIRTY is enabled. > */ > #define _PAGE_SWP_SOFT_DIRTY \ > ((riscv_has_extension_unlikely(RISCV_ISA_EXT_SVRSW60T59B)) ? \ > - _PAGE_EXEC : 0) > + (1UL << 12) : 0) > #else > #define _PAGE_SOFT_DIRTY 0 > #define _PAGE_SWP_SOFT_DIRTY 0 > diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h > index b644db16bda94..acaf6d3b54d04 100644 > --- a/arch/riscv/include/asm/pgtable.h > +++ b/arch/riscv/include/asm/pgtable.h > @@ -1178,18 +1178,20 @@ static inline pud_t pud_modify(pud_t pud, pgprot_t newprot) > * > * Format of swap PTE: > * bit 0: _PAGE_PRESENT (zero) > - * bit 1 to 2: (zero) > - * bit 3: _PAGE_SWP_SOFT_DIRTY > + * bit 1 to 3: _PAGE_LEAF (zero) > * bit 4: _PAGE_SWP_UFFD > * bit 5: _PAGE_PROT_NONE (zero) > * bit 6: exclusive marker > * bits 7 to 11: swap type > - * bits 12 to XLEN-1: swap offset > + * bit 12: _PAGE_SWP_SOFT_DIRTY (CONFIG_MEM_SOFT_DIRTY) > + * bits 12/13 to XLEN-1: swap offset (without/with CONFIG_MEM_SOFT_DIRTY) > */ > #define __SWP_TYPE_SHIFT 7 > #define __SWP_TYPE_BITS 5 > #define __SWP_TYPE_MASK ((1UL << __SWP_TYPE_BITS) - 1) > -#define __SWP_OFFSET_SHIFT (__SWP_TYPE_BITS + __SWP_TYPE_SHIFT) > +#define __SWP_OFFSET_SHIFT \ > + (__SWP_TYPE_BITS + __SWP_TYPE_SHIFT + \ > + IS_ENABLED(CONFIG_MEM_SOFT_DIRTY)) > > #define MAX_SWAPFILES_CHECK() \ > BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > __SWP_TYPE_BITS) -- Cheers, David