From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-1.mta0.migadu.com [91.218.175.1]) (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 57B13544D42 for ; Tue, 22 Sep 2026 13:08:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.1 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790082502; cv=none; b=tJA1moKBcsdLI7yQLgvz0UuVlbm7LryJNrX8BJk0yNOKYEVvS5fHeAN3mp13ndzHtzpv2BJlPqvNWfx+5ERITO7OOTRURcFPVkfIvuVmFnJrHr7apoAd8MaJ7yFAHtYjpK7rhMPvWP4qRMJRtxW00h55R1ZLd59soP5B0WFtcpk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790082502; c=relaxed/simple; bh=0Alsyxotn/BT24NUFO3uaJEACOJcA+aK/asZoGhrk7Q=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WZJiMyP4R0uP7psp0xndKnoTxLSmvyFG/Bqqi+awXONVEVftL8qWoyXrVaryawASMo6DtpUkUxpsVcpXQ2CNYTuxQsS/zJi3UOjR1Z70jhe7DgatPI6bN6aujaX9V+M9BOG0jPc1rWA6+QWE7o+9ioozVROmaR1phWc8kxk3Dqw= 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=k4Ly927A; arc=none smtp.client-ip=91.218.175.1 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="k4Ly927A" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=0Alsyxotn/BT24NUFO3uaJEACOJcA+aK/asZoGhrk7Q=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1790082498; v=1; x=1790687298; b=k4Ly927AxwTDx+IqTVAQvR41odXWaapkByxEgIXFyjeDAr+M5DXLHOkZwNzJGWeB3qExjh1U 1VWZ3ac9kRstHDVAfgMjEjWuRRC6NdxAA6n2U8uXvLLwYgTFCPKB4bFpyiSmCeMoIIWD23leVwD OTz5WSDADRE92f0Vpc3ojt3Q= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 659ba7dc6cd41b5b; Tue, 22 Sep 2026 13:08:17 +0000 X-Mizu-Trace-ID: 659ba7dc6cd41b5b X-Migadu-Flow: FLOW_OUT Message-ID: <47ff450a-c2df-4520-8589-63dbb1746f80@linux.dev> Date: Tue, 22 Sep 2026 14:08:13 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RESEND v7 08/29] mm: recognize PMD swap entries in the softleaf layer To: "David Hildenbrand (Arm)" , Andrew Morton , chrisl@kernel.org, kasong@tencent.com, ljs@kernel.org, ziy@nvidia.com, linux-mm@kvack.org Cc: ying.huang@linux.alibaba.com, Baoquan He , willy@infradead.org, youngjun.park@lge.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, alex@ghiti.fr, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, Nico Pache , "Liam R. Howlett" , ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, yosry@kernel.org, qi.zheng@linux.dev, luizcap@redhat.com, kernel-team@meta.com References: <20260914122950.3283997-1-usama.arif@linux.dev> <20260914122950.3283997-9-usama.arif@linux.dev> Content-Language: en-US From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 18/09/2026 22:56, David Hildenbrand (Arm) wrote: > On 9/14/26 14:27, Usama Arif wrote: >> Reclaim splits a PMD-mapped anonymous THP into PTE-level swap entries >> before unmapping it, so an ordinary swap entry has never had to appear in a >> PMD. Later patches install one there instead, and the softleaf layer is > > "Prepare for ..." > >> where every consumer decodes non-present PMDs. >> >> Accept swap entries as valid PMD softleaves and add pmd_is_swap_entry(). >> A swap entry carries no PFN, so make pmd_softleaf_to_folio() warn and >> return NULL rather than interpret a swap offset as a page frame number. >> >> Unlike migration and device-private entries, a PMD swap entry can also >> carry the swap-exclusive marker, which softleaf_from_pmd() has to strip >> before decoding. Strip all three overlays unconditionally while we are >> here: each clear is a plain bit clear, so testing first only buys a branch. >> >> Signed-off-by: Usama Arif >> --- > > > [...] > >> +/** >> + * pmd_is_swap_entry() - Does this PMD entry encode an actual swap entry? >> + * @pmd: PMD entry. >> + * >> + * Returns: true if the PMD encodes a swap entry, otherwise false. >> + */ >> +static inline bool pmd_is_swap_entry(pmd_t pmd) >> +{ >> + return softleaf_is_swap(softleaf_from_pmd(pmd)); >> } > > We don't have a pte_is_swap_entry(), I guess because we always want to handle > different softleaf types through softleaf_from_pte() first. > >> >> /** >> * pmd_is_valid_softleaf() - Is this PMD entry a valid softleaf entry? >> * @pmd: PMD entry. >> * >> - * PMD leaf entries are valid only if they are device private or migration >> - * entries. This function asserts that a PMD leaf entry is valid in this >> - * respect. >> + * PMD leaf entries are valid only if they are device private, migration, >> + * or swap entries. This function asserts that a PMD leaf entry is valid >> + * in this respect. >> * >> * Returns: true if the PMD entry is a valid leaf entry, otherwise false. >> */ >> @@ -660,10 +670,12 @@ static inline bool pmd_is_valid_softleaf(pmd_t pmd) >> * pmd_softleaf_to_folio() - Convert the PMD softleaf entry to a folio. >> * @pmd: PMD entry. >> * >> - * The PMD entry is expected to be a valid PMD softleaf entry. >> + * The PMD entry is expected to be a valid PMD softleaf entry that references a >> + * PFN, that is a migration or device private entry. A PMD swap entry is a valid >> + * softleaf entry but encodes swap slots rather than a PFN, so it has no folio. >> * >> - * Returns: the folio the softleaf entry references if this is a valid softleaf >> - * entry, otherwise NULL. >> + * Returns: the folio the softleaf entry references, or NULL if the entry is not >> + * a valid PMD softleaf entry or does not reference a PFN. >> */ >> static inline struct folio *pmd_softleaf_to_folio(pmd_t pmd) >> { >> @@ -673,6 +685,10 @@ static inline struct folio *pmd_softleaf_to_folio(pmd_t pmd) >> VM_WARN_ON_ONCE(true); >> return NULL; >> } >> + if (!softleaf_has_pfn(entry)) { >> + VM_WARN_ON_ONCE(true); >> + return NULL; > softleaf_to_folio() does a > > VM_WARN_ON_ONCE(!softleaf_has_pfn(entry)); > > Just curious: why is that not sufficient? Which could would we expect to call > into this fuction with nonesense? (same applies to the existing runtime check) > Because softleaf_to_folio() warns after the damage: static inline struct folio *softleaf_to_folio(softleaf_t entry) { struct folio *folio = pfn_folio(softleaf_to_pfn(entry)); VM_WARN_ON_ONCE(!softleaf_has_pfn(entry)); pfn_folio() is page_folio(pfn_to_page(pfn)), and page_folio() is _compound_head(), i.e. READ_ONCE(page->compound_info). So a swap offset interpreted as a PFN is already dereferenced by the time the warning runs. As for who would call it with nonsense: nobody today. zap_huge_pmd() is the only path that reaches it with a non-present PMD and it takes the pmd_is_swap_entry() branch first. > Acked-by: David Hildenbrand (Arm) >