From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lj1-f200.google.com (mail-lj1-f200.google.com [209.85.208.200]) by kanga.kvack.org (Postfix) with ESMTP id 6D5228E004D for ; Tue, 11 Dec 2018 03:21:49 -0500 (EST) Received: by mail-lj1-f200.google.com with SMTP id v74-v6so3573326lje.6 for ; Tue, 11 Dec 2018 00:21:49 -0800 (PST) Received: from forwardcorp1j.cmail.yandex.net (forwardcorp1j.cmail.yandex.net. [2a02:6b8:0:1630::190]) by mx.google.com with ESMTPS id y193si9413979lfa.15.2018.12.11.00.21.47 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 11 Dec 2018 00:21:47 -0800 (PST) Subject: Re: [PATCH v2] mm: thp: fix flags for pmd migration when split References: <20181211051254.16633-1-peterx@redhat.com> From: Konstantin Khlebnikov Message-ID: <1fc103f7-3164-007d-bcfd-7ad7c60bb6ec@yandex-team.ru> Date: Tue, 11 Dec 2018 11:21:44 +0300 MIME-Version: 1.0 In-Reply-To: <20181211051254.16633-1-peterx@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-CA Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Peter Xu , linux-kernel@vger.kernel.org Cc: Andrea Arcangeli , Andrew Morton , "Kirill A. Shutemov" , Matthew Wilcox , Michal Hocko , Dave Jiang , "Aneesh Kumar K.V" , Souptick Joarder , linux-mm@kvack.org On 11.12.2018 8:12, Peter Xu wrote: > When splitting a huge migrating PMD, we'll transfer all the existing > PMD bits and apply them again onto the small PTEs. However we are > fetching the bits unconditionally via pmd_soft_dirty(), pmd_write() > or pmd_yound() while actually they don't make sense at all when it's > a migration entry. Fix them up by make it conditional. > > Note that if my understanding is correct about the problem then if > without the patch there is chance to lose some of the dirty bits in > the migrating pmd pages (on x86_64 we're fetching bit 11 which is part > of swap offset instead of bit 2) and it could potentially corrupt the > memory of an userspace program which depends on the dirty bit. > > CC: Andrea Arcangeli > CC: Andrew Morton > CC: "Kirill A. Shutemov" > CC: Matthew Wilcox > CC: Michal Hocko > CC: Dave Jiang > CC: "Aneesh Kumar K.V" > CC: Souptick Joarder > CC: Konstantin Khlebnikov > CC: linux-mm@kvack.org > CC: linux-kernel@vger.kernel.org > Signed-off-by: Peter Xu > --- > v2: > - fix it up for young/write/dirty bits too [Konstantin] > --- > mm/huge_memory.c | 15 ++++++++++----- > 1 file changed, 10 insertions(+), 5 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index f2d19e4fe854..b00941b3d342 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2157,11 +2157,16 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > page = pmd_page(old_pmd); > VM_BUG_ON_PAGE(!page_count(page), page); > page_ref_add(page, HPAGE_PMD_NR - 1); > - if (pmd_dirty(old_pmd)) > - SetPageDirty(page); > - write = pmd_write(old_pmd); > - young = pmd_young(old_pmd); > - soft_dirty = pmd_soft_dirty(old_pmd); > + if (unlikely(pmd_migration)) { > + soft_dirty = pmd_swp_soft_dirty(old_pmd); > + young = write = false; > + } else { > + if (pmd_dirty(old_pmd)) > + SetPageDirty(page); > + write = pmd_write(old_pmd); > + young = pmd_young(old_pmd); > + soft_dirty = pmd_soft_dirty(old_pmd); > + } Write/read-only is encoded into migration entry. I suppose there should be something like this: --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2151,16 +2151,21 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, entry = pmd_to_swp_entry(old_pmd); page = pfn_to_page(swp_offset(entry)); + write = is_write_migration_entry(entry); + young = false; + soft_dirty = pmd_swp_soft_dirty(old_pmd); } else #endif + { page = pmd_page(old_pmd); + if (pmd_dirty(old_pmd)) + SetPageDirty(page); + write = pmd_write(old_pmd); + young = pmd_young(old_pmd); + soft_dirty = pmd_soft_dirty(old_pmd); + } VM_BUG_ON_PAGE(!page_count(page), page); page_ref_add(page, HPAGE_PMD_NR - 1); - if (pmd_dirty(old_pmd)) - SetPageDirty(page); - write = pmd_write(old_pmd); - young = pmd_young(old_pmd); - soft_dirty = pmd_soft_dirty(old_pmd); /* * Withdraw the table only after we mark the pmd entry invalid. > > /* > * Withdraw the table only after we mark the pmd entry invalid. >