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Shutemov" To: Ralph Campbell Cc: linux-mm@kvack.org, nouveau@lists.freedesktop.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, Jerome Glisse , John Hubbard , Alistair Popple , Christoph Hellwig , Jason Gunthorpe , Bharata B Rao , Ben Skeggs , Shuah Khan , Andrew Morton Subject: Re: [PATCH v2 1/7] mm/thp: fix __split_huge_pmd_locked() for migration PMD Message-ID: <20200903125442.4no5dq7mzcdllery@box> References: <20200902165830.5367-1-rcampbell@nvidia.com> <20200902165830.5367-2-rcampbell@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200902165830.5367-2-rcampbell@nvidia.com> X-Rspamd-Queue-Id: CA1271A4A0 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 02, 2020 at 09:58:24AM -0700, Ralph Campbell wrote: > A migrating transparent huge page has to already be unmapped. Otherwise, > the page could be modified while it is being copied to a new page and > data could be lost. The function __split_huge_pmd() checks for a PMD > migration entry before calling __split_huge_pmd_locked() leading one to > think that __split_huge_pmd_locked() can handle splitting a migrating PMD. > However, the code always increments the page->_mapcount and adjusts the > memory control group accounting assuming the page is mapped. > Also, if the PMD entry is a migration PMD entry, the call to > is_huge_zero_pmd(*pmd) is incorrect because it calls pmd_pfn(pmd) instead > of migration_entry_to_pfn(pmd_to_swp_entry(pmd)). > Fix these problems by checking for a PMD migration entry. > > Signed-off-by: Ralph Campbell Hm. Could you remind me what codepath splits migration PMD? Maybe it should wait until migration is complete? We could avoid a lot of complexity this way. -- Kirill A. Shutemov