From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4C3D6CA5FF0 for ; Mon, 5 Oct 2026 14:24:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To:Subject: MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=cbtp1AY6Rmu3DEl27J0Ktw6hrBvJP3hQLuad//YG3Lo=; b=H3tnSXnT20Zh9m dVQJYap+ZdlcDlKtWqgsBGtAdIJyLHXU0T1M0vxxgY9mQiw6Bf/JPSWRSbOEkl7qlWOPXbGKD8531 MsAS6/mz9gRod1tUXOrcidjBSGN30IIuZTOLPU93jERZG5SHHG2FgsRagVQYOYtK9u978yhBWsDBA RYfRhK5wqZEK9jijIrwjGTmUiFBXk4CREoCt2TU5Qpx3+pERPUaJ8zJ6Q+8b4Gk/WbQq350suom5y 9RRe3CFJ/oTiy3QQAahHJ/A44Hw2/UNrHsKxFUnDR1fU/3OKpHfCfdYxBkYbl7sTdm5eilLl+4hBj V5R1OtxUKrLmjjQ/amJw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1xDjc7-0000000GcyN-0vvg; Mon, 05 Oct 2026 14:24:24 +0000 Received: from out-238.mta1.migadu.com ([2001:41d0:203:375::ee] helo=mta1.migadu.com) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1xDjc4-0000000Gcv1-0SOz for linux-riscv@lists.infradead.org; Mon, 05 Oct 2026 14:24:21 +0000 X-Envelope-To: linux-riscv@lists.infradead.org DKIM-Signature: a=rsa-sha256; bh=CbENxEWT081LxG9Ca/66oabxXg/6aD0RFXIFh5r/llo=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1791210255; v=1; x=1791815055; b=h6zOtN/qBwuHI80mhLldSlsEthK5OvmsfCxKoEmVv2prrKumZMQj6qMudQmsfYofcg7Jl3EM L/j2MfunxfX0R4jcbjOvxnrn6aynQP4GlaV/XAgKD6WmslFoiCAUaIzX7R7qx7D6UigUlZCz6g8 Ue9KIaQgKfVXdGG8VrRLtjfo= X-Envelope-To: linux-riscv@lists.infradead.org Received: by smtp.migadu.com with ESMTPS id ca78f09690eedc77; Mon, 05 Oct 2026 14:24:15 +0000 X-Mizu-Trace-ID: ca78f09690eedc77 X-Migadu-Flow: FLOW_OUT Message-ID: Date: Mon, 5 Oct 2026 22:23:57 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] riscv/mm: fix soft-dirty migration PMDs being treated as present Content-Language: en-US To: david@kernel.org Cc: 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 References: <20261005134641.5801-1-lance.yang@linux.dev> From: Lance Yang In-Reply-To: <20261005134641.5801-1-lance.yang@linux.dev> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261005_072420_316737_4B3E5AB1 X-CRM114-Status: GOOD ( 20.15 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On 2026/10/5 21:46, Lance Yang wrote: > > 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. Also, x86 keeps _PAGE_PSE to identify the huge PMD. RISC-V doesn't have a separate leaf bit, so it keeps the R/W/X bits to identify the PMD as a leaf entry :) > > 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 _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv