From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8FC032222CC for ; Fri, 4 Jul 2025 11:22:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751628131; cv=none; b=CA+4gMd2G7E2/RGxxBBk4Z61ivKD5zmyQDqvsmpab3OyFXeyLmew3AbdINkpy4FipjAuNW+hNtZZwxZT9qyoa65Uyf5eyRpQS0QkL7EqHvktLHd1YDltWap7e0lFZgsgIypzxxFi8NR+QaVtObQEYDTuBsap62Qq3RqERSo/0K4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751628131; c=relaxed/simple; bh=kw1Ts3/LTsZ4+wKtri5gY1iYq+HjlpSCJi/WkIwgMnk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=NtYoOasDzbnWRs2apBZ/qeRMXbedtk4Y8A3LlhmQFQXKoJkr80nSEAZm2+2LjOInvxVbk9EUlixgj1xi4qpRp5+tFPdlte5A+AyE+wZdbPiMip9MaaVXEMAscy1lV0NDTN4vsVqTc66halfWN6LiUZNp0PVu6HgrqyVKhrjn1Zo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id DA469C4CEEF; Fri, 4 Jul 2025 11:22:08 +0000 (UTC) Date: Fri, 4 Jul 2025 12:22:06 +0100 From: Catalin Marinas To: Dev Jain Cc: will@kernel.org, anshuman.khandual@arm.com, quic_zhenhuah@quicinc.com, ryan.roberts@arm.com, kevin.brodsky@arm.com, yangyicong@hisilicon.com, joey.gouly@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, david@redhat.com Subject: Re: [PATCH v4] arm64: Enable vmalloc-huge with ptdump Message-ID: References: <20250626052524.332-1-dev.jain@arm.com> 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=us-ascii Content-Disposition: inline In-Reply-To: <20250626052524.332-1-dev.jain@arm.com> On Thu, Jun 26, 2025 at 10:55:24AM +0530, Dev Jain wrote: > @@ -1301,16 +1314,39 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr) > } > > table = pmd_offset(pudp, addr); > + /* > + * Isolate the PMD table; in case of race with ptdump, this helps > + * us to avoid taking the lock in __pmd_free_pte_page(). > + * > + * Static key logic: > + * > + * Case 1: If ptdump does static_branch_enable(), and after that we > + * execute the if block, then this patches in the read lock, ptdump has > + * the write lock patched in, therefore ptdump will never read from > + * a potentially freed PMD table. > + * > + * Case 2: If the if block starts executing before ptdump's > + * static_branch_enable(), then no locking synchronization > + * will be done. However, pud_clear() + the dsb() in > + * __flush_tlb_kernel_pgtable will ensure that ptdump observes an Statements like "observes" really need to be relative, not absolute. Like in "observes an empty PUD before/after ...". > + * empty PUD. Thus, it will never walk over a potentially freed > + * PMD table. > + */ > + pud_clear(pudp); > + __flush_tlb_kernel_pgtable(addr); > + if (static_branch_unlikely(&ptdump_lock_key)) { > + mmap_read_lock(&init_mm); > + mmap_read_unlock(&init_mm); > + } This needs a formal model ;). static_branch_enable() is called before the mmap_write_lock(), so even if the above observes the new branch, it may do the read_unlock() before the ptdump_walk_pgd() attempted the write lock. So your case 1 description is not entirely correct. I don't get case 2. You want to ensure pud_clear() is observed by the ptdump code before the pmd_free(). The DSB in the TLB flushing code ensures some ordering between the pud_clear() and presumably something that the ptdump code can observe as well. Would that be the mmap semaphore? However, the read_lock would only be attempted if this code is seeing the static branch update, which is not guaranteed. I don't think it even matters since the lock may be released anyway before the write_lock in ptdump. For example, you do a pud_clear() above, skip the whole static branch. The ptdump comes along on another CPU but does not observe the pud_clear() since there's no synchronisation. It goes ahead and dereferences it while this CPU does a pmd_free(). And I can't get my head around memory ordering, it doesn't look sound. static_branch_enable() I don't think has acquire semantics, at least not in relation to the actual branch update. The static_branch_unlikely() test, again, not sure what guarantees it has (I don't think it has any in relation to memory loads). Maybe you have worked it all out and is fine but it needs a better explanation and ideally some simple formal model. Show it's correct with a global variable first and then we can optimise with static branches. -- Catalin