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Shutemov" To: Matthew Wilcox Cc: Yang Shi , hughd@google.com, aarcange@redhat.com, kirill.shutemov@linux.intel.com, gavin.dg@linux.alibaba.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [v4 PATCH] mm: thp: handle page cache THP correctly in PageTransCompoundMap Message-ID: <20191025153805.6unvxxjzmq4gsxzs@box> References: <1571865575-42913-1-git-send-email-yang.shi@linux.alibaba.com> <20191024135547.GH2963@bombadil.infradead.org> <20191025153205.GQ2963@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191025153205.GQ2963@bombadil.infradead.org> User-Agent: NeoMutt/20180716 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 Fri, Oct 25, 2019 at 08:32:05AM -0700, Matthew Wilcox wrote: > On Thu, Oct 24, 2019 at 09:33:11AM -0700, Yang Shi wrote: > > On 10/24/19 6:55 AM, Matthew Wilcox wrote: > > > On Thu, Oct 24, 2019 at 05:19:35AM +0800, Yang Shi wrote: > > > > We have usecase to use tmpfs as QEMU memory backend and we would like to > > > > take the advantage of THP as well. But, our test shows the EPT is not > > > > PMD mapped even though the underlying THP are PMD mapped on host. > > > > The number showed by /sys/kernel/debug/kvm/largepage is much less than > > > > the number of PMD mapped shmem pages as the below: > > > > > > > > 7f2778200000-7f2878200000 rw-s 00000000 00:14 262232 /dev/shm/qemu_back_mem.mem.Hz2hSf (deleted) > > > > Size: 4194304 kB > > > > [snip] > > > > AnonHugePages: 0 kB > > > > ShmemPmdMapped: 579584 kB > > > > [snip] > > > > Locked: 0 kB > > > > > > > > cat /sys/kernel/debug/kvm/largepages > > > > 12 > > > > > > > > And some benchmarks do worse than with anonymous THPs. > > > > > > > > By digging into the code we figured out that commit 127393fbe597 ("mm: > > > > thp: kvm: fix memory corruption in KVM with THP enabled") checks if > > > > there is a single PTE mapping on the page for anonymous THP when > > > > setting up EPT map. But, the _mapcount < 0 check doesn't fit to page > > > > cache THP since every subpage of page cache THP would get _mapcount > > > > inc'ed once it is PMD mapped, so PageTransCompoundMap() always returns > > > > false for page cache THP. This would prevent KVM from setting up PMD > > > > mapped EPT entry. > > > > > > > > So we need handle page cache THP correctly. However, when page cache > > > > THP's PMD gets split, kernel just remove the map instead of setting up > > > > PTE map like what anonymous THP does. Before KVM calls get_user_pages() > > > > the subpages may get PTE mapped even though it is still a THP since the > > > > page cache THP may be mapped by other processes at the mean time. > > > > > > > > Checking its _mapcount and whether the THP has PTE mapped or not. > > > > Although this may report some false negative cases (PTE mapped by other > > > > processes), it looks not trivial to make this accurate. > > > I don't understand why you care how it's mapped into userspace. If there > > > is a PMD-sized page in the page cache, then you can use a PMD mapping > > > in the EPT tables to map it. Why would another process having a PTE > > > mapping on the page cause you to not use a PMD mapping? > > > > We don't care if the THP is PTE mapped by other process, but either > > PageDoubleMap flag or _mapcount/compound_mapcount can't tell us if the PTE > > map comes from the current process or other process unless gup could return > > pmd's status. > > But why do you care if the THP is PTE mapped by _this_ process? > This process has a reference to the page; the page is PMD sized and PMD > aligned, so you can use a PMD mapping in the guest, regardless of how > it's mapped by userspace. Maybe this process doesn't even have the page > mapped at all? Consider the case with MAP_PRIVATE. If part of the THP was overritten in the process, you want EPT to reflect the case, not map the page from page cache regardless. -- Kirill A. Shutemov