On Tue, Nov 17, 2020 at 7:08 PM Song Bao Hua (Barry Song) <song.bao....@hisilicon.com> wrote: > > > > > -----Original Message----- > > From: Muchun Song [mailto:songmuc...@bytedance.com] > > Sent: Tuesday, November 17, 2020 11:50 PM > > To: Song Bao Hua (Barry Song) <song.bao....@hisilicon.com> > > Cc: cor...@lwn.net; mike.krav...@oracle.com; t...@linutronix.de; > > mi...@redhat.com; b...@alien8.de; x...@kernel.org; h...@zytor.com; > > dave.han...@linux.intel.com; l...@kernel.org; pet...@infradead.org; > > v...@zeniv.linux.org.uk; a...@linux-foundation.org; paul...@kernel.org; > > mchehab+hua...@kernel.org; pawan.kumar.gu...@linux.intel.com; > > rdun...@infradead.org; oneu...@suse.com; anshuman.khand...@arm.com; > > jroe...@suse.de; almasrym...@google.com; rient...@google.com; > > wi...@infradead.org; osalva...@suse.de; mho...@suse.com; > > duanxiongc...@bytedance.com; linux-...@vger.kernel.org; > > linux-kernel@vger.kernel.org; linux...@kvack.org; > > linux-fsde...@vger.kernel.org > > Subject: Re: [External] RE: [PATCH v4 00/21] Free some vmemmap pages of > > hugetlb page > > > > On Tue, Nov 17, 2020 at 6:16 PM Song Bao Hua (Barry Song) > > <song.bao....@hisilicon.com> wrote: > > > > > > > > > > > > > -----Original Message----- > > > > From: owner-linux...@kvack.org [mailto:owner-linux...@kvack.org] On > > > > Behalf Of Muchun Song > > > > Sent: Saturday, November 14, 2020 12:00 AM > > > > To: cor...@lwn.net; mike.krav...@oracle.com; t...@linutronix.de; > > > > mi...@redhat.com; b...@alien8.de; x...@kernel.org; h...@zytor.com; > > > > dave.han...@linux.intel.com; l...@kernel.org; pet...@infradead.org; > > > > v...@zeniv.linux.org.uk; a...@linux-foundation.org; paul...@kernel.org; > > > > mchehab+hua...@kernel.org; pawan.kumar.gu...@linux.intel.com; > > > > rdun...@infradead.org; oneu...@suse.com; > > anshuman.khand...@arm.com; > > > > jroe...@suse.de; almasrym...@google.com; rient...@google.com; > > > > wi...@infradead.org; osalva...@suse.de; mho...@suse.com > > > > Cc: duanxiongc...@bytedance.com; linux-...@vger.kernel.org; > > > > linux-kernel@vger.kernel.org; linux...@kvack.org; > > > > linux-fsde...@vger.kernel.org; Muchun Song > > <songmuc...@bytedance.com> > > > > Subject: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page > > > > > > > > Hi all, > > > > > > > > This patch series will free some vmemmap pages(struct page structures) > > > > associated with each hugetlbpage when preallocated to save memory. > > > > > > > > Nowadays we track the status of physical page frames using struct page > > > > structures arranged in one or more arrays. And here exists one-to-one > > > > mapping between the physical page frame and the corresponding struct > > page > > > > structure. > > > > > > > > The HugeTLB support is built on top of multiple page size support that > > > > is provided by most modern architectures. For example, x86 CPUs normally > > > > support 4K and 2M (1G if architecturally supported) page sizes. Every > > > > HugeTLB has more than one struct page structure. The 2M HugeTLB has > > 512 > > > > struct page structure and 1G HugeTLB has 4096 struct page structures. > > > > But > > > > in the core of HugeTLB only uses the first 4 (Use of first 4 struct page > > > > structures comes from HUGETLB_CGROUP_MIN_ORDER.) struct page > > > > structures to > > > > store metadata associated with each HugeTLB. The rest of the struct page > > > > structures are usually read the compound_head field which are all the > > > > same > > > > value. If we can free some struct page memory to buddy system so that we > > > > can save a lot of memory. > > > > > > > > When the system boot up, every 2M HugeTLB has 512 struct page > > structures > > > > which size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE). > > > > > > > > hugetlbpage struct pages(8 pages) page > > > > frame(8 pages) > > > > +-----------+ ---virt_to_page---> +-----------+ mapping to > > > > +-----------+ > > > > | | | 0 | -------------> | > > 0 > > > > | > > > > | | | 1 | -------------> | > > 1 > > > > | > > > > | | | 2 | -------------> | > > 2 > > > > | > > > > | | | 3 | -------------> | > > 3 > > > > | > > > > | | | 4 | -------------> | > > 4 > > > > | > > > > | 2M | | 5 | -------------> | > > > > 5 | > > > > | | | 6 | -------------> | > > 6 > > > > | > > > > | | | 7 | -------------> | > > 7 > > > > | > > > > | | +-----------+ > > > > +-----------+ > > > > | | > > > > | | > > > > +-----------+ > > > > > > > > > > > > When a hugetlbpage is preallocated, we can change the mapping from > > above > > > > to > > > > bellow. > > > > > > > > hugetlbpage struct pages(8 pages) page > > > > frame(8 pages) > > > > +-----------+ ---virt_to_page---> +-----------+ mapping to > > > > +-----------+ > > > > | | | 0 | -------------> | > > 0 > > > > | > > > > | | | 1 | -------------> | > > 1 > > > > | > > > > | | | 2 | -------------> > > > > +-----------+ > > > > | | | 3 | -----------------^ ^ > > ^ ^ > > > > ^ > > > > | | | 4 | -------------------+ > > | | > > > > | > > > > | 2M | | 5 | > > ---------------------+ | > > > > | > > > > | | | 6 | > > -----------------------+ | > > > > | | | 7 | > > -------------------------+ > > > > | | +-----------+ > > > > | | > > > > | | > > > > +-----------+ > > > > > > > > For tail pages, the value of compound_head is the same. So we can reuse > > > > first page of tail page structs. We map the virtual addresses of the > > > > remaining 6 pages of tail page structs to the first tail page struct, > > > > and then free these 6 pages. Therefore, we need to reserve at least 2 > > > > pages as vmemmap areas. > > > > > > > > When a hugetlbpage is freed to the buddy system, we should allocate six > > > > pages for vmemmap pages and restore the previous mapping relationship. > > > > > > > > If we uses the 1G hugetlbpage, we can save 4088 pages(There are 4096 > > pages > > > > for > > > > struct page structures, we reserve 2 pages for vmemmap and 8 pages for > > page > > > > tables. So we can save 4088 pages). This is a very substantial gain. On > > > > our > > > > server, run some SPDK/QEMU applications which will use 1024GB > > hugetlbpage. > > > > With this feature enabled, we can save ~16GB(1G hugepage)/~11GB(2MB > > > > hugepage) > > > > > > Hi Muchun, > > > > > > Do we really save 11GB for 2MB hugepage? > > > How much do we save if we only get one 2MB hugetlb from one 128MB > > mem_section? > > > It seems we need to get at least one page for the PTEs since we are > > > splitting > > PMD of > > > vmemmap into PTE? > > > > There are 524288(1024GB/2MB) 2MB HugeTLB pages. We can save 6 pages for > > each > > 2MB HugeTLB page. So we can save 3145728 pages. But we need to split PMD > > page > > table for every one 128MB mem_section and every section need one page > > as PTE page > > table. So we need 8192(1024GB/128MB) pages as PTE page tables. > > Finally, we can save > > 3137536(3145728-8192) pages which is 11.97GB. > > The worst case I can see is that: > if we get 100 hugetlb with 2MB size, but the 100 hugetlb comes from different > mem_section, we won't save 11.97GB. we only save 5/8 * 16GB=10GB. > > Anyway, it seems 11GB is in the middle of 10GB and 11.97GB, > so sounds sensible :-) > > ideally, we should be able to free PageTail if we change struct page in some > way. > Then we will save much more for 2MB hugetlb. but it seems it is not easy.
Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. But your words woke me up. Maybe we really can free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page structures has beed set PG_head flag. If we can adjust compound_head() slightly and make compound_head() return the real head struct page when the parameter is the tail struct page but with PG_head flag set. I will start an investigation and a test. Thanks. > > Thanks > Barry -- Yours, Muchun
