Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933591AbcCIRnb (ORCPT ); Wed, 9 Mar 2016 12:43:31 -0500 Received: from mail-io0-f173.google.com ([209.85.223.173]:36354 "EHLO mail-io0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752107AbcCIRnI (ORCPT ); Wed, 9 Mar 2016 12:43:08 -0500 MIME-Version: 1.0 In-Reply-To: <20160309160352.GM6192@e104818-lin.cambridge.arm.com> 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: Wed, 9 Mar 2016 23:13:07 +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: 4602 Lines: 102 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 Ganapat