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Mon, 15 Feb 2021 04:45:33 -0800 (PST) MIME-Version: 1.0 References: <20210208085013.89436-1-songmuchun@bytedance.com> <20210208085013.89436-5-songmuchun@bytedance.com> In-Reply-To: From: Muchun Song Date: Mon, 15 Feb 2021 20:44:57 +0800 Message-ID: Subject: Re: [External] Re: [PATCH v15 4/8] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page To: Michal Hocko Cc: Jonathan Corbet , Mike Kravetz , Thomas Gleixner , mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org, Peter Zijlstra , viro@zeniv.linux.org.uk, Andrew Morton , paulmck@kernel.org, mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com, Randy Dunlap , oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de, Mina Almasry , David Rientjes , Matthew Wilcox , Oscar Salvador , "Song Bao Hua (Barry Song)" , David Hildenbrand , =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= , Joao Martins , Xiongchun duan , linux-doc@vger.kernel.org, LKML , Linux Memory Management List , linux-fsdevel Content-Type: text/plain; charset="UTF-8" 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 Mon, Feb 15, 2021 at 8:18 PM Michal Hocko wrote: > > On Mon 15-02-21 20:00:07, Muchun Song wrote: > > On Mon, Feb 15, 2021 at 7:51 PM Muchun Song wrote: > > > > > > On Mon, Feb 15, 2021 at 6:33 PM Michal Hocko wrote: > > > > > > > > On Mon 15-02-21 18:05:06, Muchun Song wrote: > > > > > On Fri, Feb 12, 2021 at 11:32 PM Michal Hocko wrote: > > > > [...] > > > > > > > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head) > > > > > > > +{ > > > > > > > + int ret; > > > > > > > + unsigned long vmemmap_addr = (unsigned long)head; > > > > > > > + unsigned long vmemmap_end, vmemmap_reuse; > > > > > > > + > > > > > > > + if (!free_vmemmap_pages_per_hpage(h)) > > > > > > > + return 0; > > > > > > > + > > > > > > > + vmemmap_addr += RESERVE_VMEMMAP_SIZE; > > > > > > > + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h); > > > > > > > + vmemmap_reuse = vmemmap_addr - PAGE_SIZE; > > > > > > > + > > > > > > > + /* > > > > > > > + * The pages which the vmemmap virtual address range [@vmemmap_addr, > > > > > > > + * @vmemmap_end) are mapped to are freed to the buddy allocator, and > > > > > > > + * the range is mapped to the page which @vmemmap_reuse is mapped to. > > > > > > > + * When a HugeTLB page is freed to the buddy allocator, previously > > > > > > > + * discarded vmemmap pages must be allocated and remapping. > > > > > > > + */ > > > > > > > + ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse, > > > > > > > + GFP_ATOMIC | __GFP_NOWARN | __GFP_THISNODE); > > > > > > > > > > > > I do not think that this is a good allocation mode. GFP_ATOMIC is a non > > > > > > sleeping allocation and a medium memory pressure might cause it to > > > > > > fail prematurely. I do not think this is really an atomic context which > > > > > > couldn't afford memory reclaim. I also do not think we want to grant > > > > > > > > > > Because alloc_huge_page_vmemmap is called under hugetlb_lock > > > > > now. So using GFP_ATOMIC indeed makes the code more simpler. > > > > > > > > You can have a preallocated list of pages prior taking the lock. > > > > > > A discussion about this can refer to here: > > > > > > https://patchwork.kernel.org/project/linux-mm/patch/20210117151053.24600-5-songmuchun@bytedance.com/ > > > > > > > Moreover do we want to manipulate vmemmaps from under spinlock in > > > > general. I have to say I have missed that detail when reviewing. Need to > > > > think more. > > > > > > > > > From the document of the kernel, I learned that __GFP_NOMEMALLOC > > > > > can be used to explicitly forbid access to emergency reserves. So if > > > > > we do not want to use the reserve memory. How about replacing it to > > > > > > > > > > GFP_ATOMIC | __GFP_NOMEMALLOC | __GFP_NOWARN | __GFP_THISNODE > > > > > > > > The whole point of GFP_ATOMIC is to grant access to memory reserves so > > > > the above is quite dubious. If you do not want access to memory reserves > > > > > > Look at the code of gfp_to_alloc_flags(). > > > > > > static inline unsigned int gfp_to_alloc_flags(gfp_t gfp_mask) > > > { > > > [...] > > > if (gfp_mask & __GFP_ATOMIC) { > > > /* > > > * Not worth trying to allocate harder for __GFP_NOMEMALLOC even > > > * if it can't schedule. > > > */ > > > if (!(gfp_mask & __GFP_NOMEMALLOC)) > > > alloc_flags |= ALLOC_HARDER; > > > [...] > > > } > > > > > > Seems to allow this operation (GFP_ATOMIC | __GFP_NOMEMALLOC). > > Please read my response again more carefully. I am not claiming that > combination is not allowed. I have said it doesn't make any sense in > this context. I see you are worried that using GFP_ATOMIC will use reverse memory unlimited. So I think that __GFP_NOMEMALLOC may be suitable for us. Sorry, I may not understand the point you said. What I missed? > > -- > Michal Hocko > SUSE Labs