From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id A9283284662; Thu, 15 May 2025 10:07:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747303629; cv=none; b=Kw/Lg4CtmMr7lIdbhubwzCQwl1fC7/UUcgr0PB0wh1Rod+qHWoSmA7gpXrOlQj5t1mv3sIjCOgYJ8C2x9mdZbdbnrsWYe1o1p7BHwmsSt6+sOywwF6WIGhTR175LW583/CF6WvMzlTserwslh0GFOcC/xxXKxG/HP5K31fPSPcI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747303629; c=relaxed/simple; bh=IG4wFVTijQlszqMFKwdjV9K2k4N/Z+mxGNA1k/olemk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=P6V3G+g7rcGqi/+byPfiq5h3xBtHD6mzG8v/F133pLHBdlP1UOUSump73NDkIZfn5qkfWITttDxbFC39vHtFPnvPX+0On5GRTQ/AORUhEYubXjG9zpJi/NjSiDObUM/ejH0hjrrwpLvTycFR4Q/c/cckIeP8vI1W1/4NxSNIT70= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 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: [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 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); > >