From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f200.google.com (mail-qk0-f200.google.com [209.85.220.200]) by kanga.kvack.org (Postfix) with ESMTP id 610386B0003 for ; Mon, 13 Aug 2018 19:21:55 -0400 (EDT) Received: by mail-qk0-f200.google.com with SMTP id u19-v6so18714525qkl.13 for ; Mon, 13 Aug 2018 16:21:55 -0700 (PDT) Received: from aserp2120.oracle.com (aserp2120.oracle.com. [141.146.126.78]) by mx.google.com with ESMTPS id k9-v6si4694588qvd.123.2018.08.13.16.21.53 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 13 Aug 2018 16:21:54 -0700 (PDT) Subject: Re: [PATCH] mm: migration: fix migration of huge PMD shared pages References: <20180813034108.27269-1-mike.kravetz@oracle.com> <20180813105821.j4tg6iyrdxgwyr3y@kshutemo-mobl1> From: Mike Kravetz Message-ID: Date: Mon, 13 Aug 2018 16:21:41 -0700 MIME-Version: 1.0 In-Reply-To: <20180813105821.j4tg6iyrdxgwyr3y@kshutemo-mobl1> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A . Shutemov" , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Vlastimil Babka , Naoya Horiguchi , Davidlohr Bueso , Michal Hocko , Andrew Morton On 08/13/2018 03:58 AM, Kirill A. Shutemov wrote: > On Sun, Aug 12, 2018 at 08:41:08PM -0700, Mike Kravetz wrote: >> The page migration code employs try_to_unmap() to try and unmap the >> source page. This is accomplished by using rmap_walk to find all >> vmas where the page is mapped. This search stops when page mapcount >> is zero. For shared PMD huge pages, the page map count is always 1 >> not matter the number of mappings. Shared mappings are tracked via >> the reference count of the PMD page. Therefore, try_to_unmap stops >> prematurely and does not completely unmap all mappings of the source >> page. >> >> This problem can result is data corruption as writes to the original >> source page can happen after contents of the page are copied to the >> target page. Hence, data is lost. >> >> This problem was originally seen as DB corruption of shared global >> areas after a huge page was soft offlined. DB developers noticed >> they could reproduce the issue by (hotplug) offlining memory used >> to back huge pages. A simple testcase can reproduce the problem by >> creating a shared PMD mapping (note that this must be at least >> PUD_SIZE in size and PUD_SIZE aligned (1GB on x86)), and using >> migrate_pages() to migrate process pages between nodes. >> >> To fix, have the try_to_unmap_one routine check for huge PMD sharing >> by calling huge_pmd_unshare for hugetlbfs huge pages. If it is a >> shared mapping it will be 'unshared' which removes the page table >> entry and drops reference on PMD page. After this, flush caches and >> TLB. >> >> Signed-off-by: Mike Kravetz >> --- >> I am not %100 sure on the required flushing, so suggestions would be >> appreciated. This also should go to stable. It has been around for >> a long time so still looking for an appropriate 'fixes:'. > > I believe we need flushing. And huge_pmd_unshare() usage in > __unmap_hugepage_range() looks suspicious: I don't see how we flush TLB in > that case. Thanks Kirill, __unmap_hugepage_range() has two callers: 1) unmap_hugepage_range, which wraps the call with tlb_gather_mmu and tlb_finish_mmu on the range. IIUC, this should cause an appropriate TLB flush. 2) __unmap_hugepage_range_final via unmap_single_vma. unmap_single_vma has three callers: - unmap_vmas which assumes the caller will flush the whole range after return. - zap_page_range wraps the call with tlb_gather_mmu/tlb_finish_mmu - zap_page_range_single wraps the call with tlb_gather_mmu/tlb_finish_mmu So, it appears we are covered. But, I could be missing something. My primary reason for asking the question was with respect to the code added to try_to_unmap_one. In my testing, the changes I added appeared to be required. Just wanted to make sure. I need to fix a build issue and will send another version. -- Mike Kravetz