Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934952AbcCJDE4 (ORCPT ); Wed, 9 Mar 2016 22:04:56 -0500 Received: from mail-ig0-f179.google.com ([209.85.213.179]:38890 "EHLO mail-ig0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933731AbcCJDEr (ORCPT ); Wed, 9 Mar 2016 22:04:47 -0500 MIME-Version: 1.0 In-Reply-To: References: <1457499768-31176-1-git-send-email-gkulkarni@caviumnetworks.com> <20160309100605.GJ6192@e104818-lin.cambridge.arm.com> <20160309160352.GM6192@e104818-lin.cambridge.arm.com> Date: Thu, 10 Mar 2016 08:34:46 +0530 Message-ID: Subject: Re: [PATCH] arm64: Fix the ptep_set_wrprotect() to set PTE_DIRTY if (PTE_DBM && !PTE_RDONLY) From: Ganapatrao Kulkarni To: Catalin Marinas Cc: Dann Frazier , Ganapatrao Kulkarni , Will Deacon , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4871 Lines: 108 On Wed, Mar 9, 2016 at 11:13 PM, Ganapatrao Kulkarni wrote: > On Wed, Mar 9, 2016 at 9:33 PM, Catalin Marinas wrote: >> On Wed, Mar 09, 2016 at 05:17:39PM +0530, Ganapatrao Kulkarni wrote: >>> On Wed, Mar 9, 2016 at 3:36 PM, Catalin Marinas wrote: >>> > On Wed, Mar 09, 2016 at 10:32:48AM +0530, Ganapatrao Kulkarni wrote: >>> >> Commit 2f4b829c625e ("arm64: Add support for hardware updates of the >>> >> access and dirty pte bits") introduced support for handling hardware >>> >> updates of the access flag and dirty status. >>> >> >>> >> ptep_set_wrprotect is setting PTR_DIRTY if !PTE_RDONLY, >>> >> however by design it suppose to set PTE_DIRTY >>> >> only if (PTE_DBM && !PTE_RDONLY). This patch addes code to >>> >> test and set accordingly. >>> > >>> > The reasoning behind the original code is that if !PTE_RDONLY, you have >>> > no way to tell whether the page was written or not since it is already >>> > writable, independent of the DBM. So by clearing the DBM bit (making the >>> > page read-only), we need to ensure that a potential dirty state is >>> > transferred to the software PTE_DIRTY bit. >>> > >>> > By checking PTE_DBM && !PTE_RDONLY, you kind of imply that you can have >>> > a page with !PTE_DBM && !PTE_RDONLY. Given that PTE_DBM is actually >>> > PTE_WRITE, PTE_RDONLY must always be set when !PTE_DBM. The bug may be >>> > elsewhere not setting these bits correctly. >>> >>> but i do see this macro, >>> #define pte_hw_dirty(pte) (pte_write(pte) && !(pte_val(pte) & PTE_RDONLY)) >> >> This was added in commit b847415ce96e ("arm64: Fix the pte_hw_dirty() >> check when AF/DBM is enabled") for the pte_modify() case which is not >> called on the actual PTE but a local variable. A pte passed to this >> function as !PTE_DBM && !PTE_RDONLY should not be assumed dirty since >> PTE_RDONLY will be set later by set_pte_at() when the actual page table >> write occurs. >> >> ptep_set_wrprotect() is run directly on the actual PTE, so here a >> !PTE_RDONLY only means potentially dirty, independent of the PTE_DBM >> bit. I consider the additional PTE_DBM check superfluous in this case >> but we need to understand when we would actually get a pte with both >> PTE_DBM and PTE_RDONLY cleared. >> >> The only way I see this happening is if the pte doesn't have PTE_VALID >> set, IOW it probably has PTE_PROT_NONE set which is used by the NUMA >> balancing. So calling set_pte_at() on a !PTE_VALID && !PTE_DBM pte does >> not currently set PTE_RDONLY and ptep_set_wrprotect() wrongly assumes it >> is dirty. >> >>> i dont see this issue, if i comment out arm64 implementation of >>> ptep_set_wrprotect() >> >> Because the default implementation discards any existing hw dirty >> information by clearing the PTE_DBM bit and setting PTE_RDONLY via the >> set_pte_at (of course, apart from the atomicity issues). >> >>> >> This patch fixes BUG, >>> >> kernel BUG at /build/linux-StrpB2/linux-4.4.0/fs/ext4/inode.c:2394! >>> >> Internal error: Oops - BUG: 0 [#1] SMP >>> > >>> > Which bug is this? It's a PageWriteback() check in the for-next/core >>> > branch. What kernel version are you using? >>> >>> i am using 4.4.0 >> >> I guess with additional NUMA patches since it only fails when you enable >> the NUMA_BALANCING configuration. >> >>> > BTW, in 4.5-rc2 we pushed commit ac15bd63bbb2 ("arm64: Honour !PTE_WRITE >>> > in set_pte_at() for kernel mappings"), though not sure that's what you >>> > are hitting. >>> >>> i have tried this patch, but issue still exist. crash log below >>> >>> root@ubuntu:/home/ganapat/test# [ 733.853009] kernel BUG at >>> fs/ext4/inode.c:2394! >> >> Is this the BUG_ON in page_buffers(!PagePrivate(page))? I can see in the >> code above this that wrongly marking a page as dirty could have some >> side effects. >> >> Can you give this patch a try, on top of commit ac15bd63bbb2? > > thanks, this fixes the issue, i have tried making pte_valid same as pte_present > however, i have overlooked that set_pte_at is using pte_valid_user(in 4.4) > > >> >> -------------8<---------------------- >> >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h >> index 7c73b365fcfa..b409a983f870 100644 >> --- a/arch/arm64/include/asm/pgtable.h >> +++ b/arch/arm64/include/asm/pgtable.h >> @@ -201,7 +201,7 @@ extern void __sync_icache_dcache(pte_t pteval, unsigned long addr); >> static inline void set_pte_at(struct mm_struct *mm, unsigned long addr, >> pte_t *ptep, pte_t pte) >> { >> - if (pte_valid(pte)) { >> + if (pte_present(pte)) { >> if (pte_sw_dirty(pte) && pte_write(pte)) >> pte_val(pte) &= ~PTE_RDONLY; >> else > this diff works for me. Tested-by: Ganapatrao Kulkarni > Ganapat