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 A418DC3ABC9 for ; Thu, 15 May 2025 10:34:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type: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=gb1sQLiOdkTwXGQI8snYFomx3ioTPL04QJ+FR44XH0Q=; b=ijFeLXArJ9P3jTHIeNPMeuCx5a qfmgluep9JVo97lw2M+J0CZOKHj27y0oe6Vx1UeA51gKfzNQn+oMo+m3ISYMLcxMISUIw4/cGz5HG nac21gOpTSfnND0aahJe24d3iIB8P66Hdv5DK3khgJD5sud5Np757o/CYmLJWndUYCIe/CT8EZ5ul U+kIYajwd81mY0y5ehOixF2O8fXBhbmKdy7iuxAP770G/pLVZHBYdgmp26Vbkd+sk+3D3sbztA7mB 6pd565cOWKRm5EFeRyEFT+cZNWK2CcDIg0Jm8uQCCrBeK1rro2afHbHYwvqbwTEhFuZlZQCqyNDE9 8212PuDg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1uFVvC-00000000JCZ-2c89; Thu, 15 May 2025 10:34:38 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1uFVUb-00000000Fzt-1TXd for linux-arm-kernel@lists.infradead.org; Thu, 15 May 2025 10:07:10 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5097114BF; Thu, 15 May 2025 03:06:55 -0700 (PDT) Received: from [10.1.32.187] (XHFQ2J9959.cambridge.arm.com [10.1.32.187]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 73AA93F673; Thu, 15 May 2025 03:07:05 -0700 (PDT) Message-ID: Date: Thu, 15 May 2025 11:07:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] arm64: Check pxd_leaf() instead of !pxd_table() while tearing down page tables Content-Language: en-GB To: David Hildenbrand , Dev Jain , catalin.marinas@arm.com, will@kernel.org Cc: anshuman.khandual@arm.com, mark.rutland@arm.com, yang@os.amperecomputing.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, stable@vger.kernel.org References: <20250515063450.86629-1-dev.jain@arm.com> <332ecda7-14c4-4dc3-aeff-26801b74ca04@redhat.com> <4904d02f-6595-4230-a321-23327596e085@arm.com> <6fe7848c-485e-4639-b65c-200ed6abe119@redhat.com> <35ef7691-7eac-4efa-838d-c504c88c042b@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250515_030709_476136_5A5E5FAF X-CRM114-Status: GOOD ( 24.83 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 15/05/2025 09:53, David Hildenbrand wrote: > On 15.05.25 10:47, Dev Jain wrote: >> >> >> On 15/05/25 2:06 pm, David Hildenbrand wrote: >>> On 15.05.25 10:22, Dev Jain wrote: >>>> >>>> >>>> On 15/05/25 1:43 pm, David Hildenbrand wrote: >>>>> On 15.05.25 08:34, Dev Jain wrote: >>>>>> Commit 9c006972c3fe removes the pxd_present() checks because the caller >>>>>> checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller >>>>>> only >>>>>> checks pud_present(); pud_free_pmd_page() recurses on each pmd through >>>>>> pmd_free_pte_page(), wherein the pmd may be none. >>>>> The commit states: "The core code already has a check for pXd_none()", >>>>> so I assume that assumption was not true in all cases? >>>>> >>>>> Should that one problematic caller then check for pmd_none() instead? >>>> >>>>    From what I could gather of Will's commit message, my interpretation is >>>> that the concerned callers are vmap_try_huge_pud and vmap_try_huge_pmd. >>>> These individually check for pxd_present(): >>>> >>>> if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr)) >>>>      return 0; >>>> >>>> The problem is that vmap_try_huge_pud will also iterate on pte entries. >>>> So if the pud is present, then pud_free_pmd_page -> pmd_free_pte_page >>>> may encounter a none pmd and trigger a WARN. >>> >>> Yeah, pud_free_pmd_page()->pmd_free_pte_page() looks shaky. >>> >>> I assume we should either have an explicit pmd_none() check in >>> pud_free_pmd_page() before calling pmd_free_pte_page(), or one in >>> pmd_free_pte_page(). >>> >>> With your patch, we'd be calling pte_free_kernel() on a NULL pointer, >>> which sounds wrong -- unless I am missing something important. >> >> Ah thanks, you seem to be right. We will be extracting table from a none >> pmd. Perhaps we should still bail out for !pxd_present() but without the >> warning, which the fix commit used to do. > > Right. We just make sure that all callers of pmd_free_pte_page() already check > for it. > > I'd just do something like: I just reviewed the patch and had the same feedback as David. I agree with the patch below, with some small mods... > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 8fcf59ba39db7..e98dd7af147d5 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -1274,10 +1274,8 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) >   >         pmd = READ_ONCE(*pmdp); >   > -       if (!pmd_table(pmd)) { > -               VM_WARN_ON(1); > -               return 1; > -       } > +       VM_WARN_ON(!pmd_present(pmd)); > +       VM_WARN_ON(!pmd_table(pmd)); You don't need both of these warnings; pmd_table() is only true if the pmd is present (well actually only if it's _valid_ which is more strict than present), so the second one is sufficient on its own. >   >         table = pte_offset_kernel(pmdp, addr); >         pmd_clear(pmdp); > @@ -1305,7 +1303,8 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr) Given you are removing the runtime check and early return in pmd_free_pte_page(), I think you should modify this function to use the same style too. >         next = addr; >         end = addr + PUD_SIZE; >         do { > -               pmd_free_pte_page(pmdp, next); > +               if (pmd_present(*pmdp)) question: I wonder if it is better to use !pmd_none() as the condition here? It should either be none or a table at this point, so this allows the warning in pmd_free_pte_page() to catch more error conditions. No strong opinion though. Thanks, Ryan > +                       pmd_free_pte_page(pmdp, next); >         } while (pmdp++, next += PMD_SIZE, next != end); >   >         pud_clear(pudp); > >