From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt1-f197.google.com (mail-qt1-f197.google.com [209.85.160.197]) by kanga.kvack.org (Postfix) with ESMTP id CDBA68E00E5 for ; Wed, 12 Dec 2018 00:15:50 -0500 (EST) Received: by mail-qt1-f197.google.com with SMTP id 41so16751155qto.17 for ; Tue, 11 Dec 2018 21:15:50 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id k15si5449169qke.86.2018.12.11.21.15.49 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 11 Dec 2018 21:15:49 -0800 (PST) Date: Wed, 12 Dec 2018 13:15:40 +0800 From: Peter Xu Subject: Re: [PATCH v2] mm: thp: fix flags for pmd migration when split Message-ID: <20181212051540.GA8970@xz-x1> References: <20181211051254.16633-1-peterx@redhat.com> <1fc103f7-3164-007d-bcfd-7ad7c60bb6ec@yandex-team.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1fc103f7-3164-007d-bcfd-7ad7c60bb6ec@yandex-team.ru> Sender: owner-linux-mm@kvack.org List-ID: To: Konstantin Khlebnikov Cc: linux-kernel@vger.kernel.org, Andrea Arcangeli , Andrew Morton , "Kirill A. Shutemov" , Matthew Wilcox , Michal Hocko , Dave Jiang , "Aneesh Kumar K.V" , Souptick Joarder , linux-mm@kvack.org On Tue, Dec 11, 2018 at 11:21:44AM +0300, Konstantin Khlebnikov wrote: > 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. > Oops yes, I missed the write bit. Thanks for pointing it out. Should I repost with your authorship and your sign-off? Or even I'll consider to directly drop the CONFIG_ARCH_ENABLE_THP_MIGRATION if with that since I don't see much gain to keep it: diff --git a/mm/huge_memory.c b/mm/huge_memory.c index f2d19e4fe854..aebade83cec9 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2145,23 +2145,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, */ old_pmd = pmdp_invalidate(vma, haddr, pmd); -#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION pmd_migration = is_pmd_migration_entry(old_pmd); - if (pmd_migration) { + if (unlikely(pmd_migration)) { swp_entry_t entry; entry = pmd_to_swp_entry(old_pmd); page = pfn_to_page(swp_offset(entry)); - } else -#endif + write = is_write_migration_entry(entry); + young = false; + soft_dirty = pmd_swp_soft_dirty(old_pmd); + } else { 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. Thanks, -- Peter Xu