From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.migadu.com (out-168.mta1.migadu.com [95.215.58.168]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 28FD348C3E5 for ; Mon, 5 Oct 2026 13:46:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.168 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791208013; cv=none; b=FfXZeRIi8fMThYcDgPAWoLasdo0yrrlT8jPNGQ9LlUzx+3JsTcETpPwAhAL49q5ugI+ucNCcQkynJ/MUNZWQ4oMZnOKzbsmBcHLJeCLjQswW67B3iw3cyzS5zDUoZENYcYfP94TQHvptSFF22gf8V58g01geZ9eh4O4Q0qJuZQs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791208013; c=relaxed/simple; bh=UPW5rxLXXCUQyxl4itU7Omf05ILDh+oFB7ALgw7XWYk=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=QEHm4ysRzIQ2g+BR95OhYioYt1+MAx0qldZ4d6KdX/xHv5HwSpKR7JBEsK2Ca5OPUUjcd0NH5jkzI6xLpgagYSZ1vrvTN03cBR/VuvcV8TRjFmIiJbSsN4KLQVtUuumas3J8znZ1wRa4hVDiHTKb/PfP94fLRgscSSysgRXip5I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=tx0L2iTv; arc=none smtp.client-ip=95.215.58.168 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="tx0L2iTv" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=UPW5rxLXXCUQyxl4itU7Omf05ILDh+oFB7ALgw7XWYk=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1791208009; v=1; x=1791812809; b=tx0L2iTvL1t4Gc/ZauTkCdpHFTdtsUKl8VPxSamBBRMTVwXZ5qqXopDuGeJG4ImvGBpQ9j5S +vGxW1iw07Lnb0rcg90dbiqvRzPH+Dp/22EmpQXX0xvY4VM4aGH8mpzWGd6EZYzx/hk5JlnatTg yJtBUGrVEm2BY2B+LAGWpIto= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 2868cb32e0b71532; Mon, 05 Oct 2026 13:46:48 +0000 X-Mizu-Trace-ID: 2868cb32e0b71532 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org Cc: lance.yang@linux.dev, pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, 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 Subject: Re: [PATCH 1/1] riscv/mm: fix soft-dirty migration PMDs being treated as present Date: Mon, 5 Oct 2026 21:46:41 +0800 Message-ID: <20261005134641.5801-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Mon, Oct 05, 2026 at 12:37:20PM +0200, David Hildenbrand (Arm) wrote: >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? Yeah, that surprised me too :) Still wrapping my head around the details ... >Is this a sw problem? Who needs that? IIUC, it's for software during a PMD split. __split_huge_pmd_locked() invalidates the huge PMD and flushes the TLB before installing the PTE table. Software still needs pmd_present() and pmd_trans_huge() to recognize the THP in between. RISC-V clears V but keeps the R/W/X bits for that, so the entry is invalid to hardware but still identifiable as a THP by software. Hopefully I didn't miss something. >> >> 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. YES, looks a bit off ... >> >> 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? We don't need all 52 bits of the swap offset. RV64 PFNs only need 44 bits for migration entries, and actual swap is already limited to about 16 TiB per area with 4 KiB pages by last_page (__u32) and swap_info_struct.max (unsigned int). So there's room to reserve a bit without reducing the supported swap size or PFN range. CONFIG_MEM_SOFT_DIRTY is only available on RV64, so RV32 keeps its 20-bit offset. [...] Cheers, Lance