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Tue, 05 Jan 2021 00:57:50 -0800 (PST) MIME-Version: 1.0 References: <20210104142624.GI13207@dhcp22.suse.cz> <23a4eea2-9fdb-fd1d-ee92-9cd8ac6e8f41@redhat.com> <20210104151005.GK13207@dhcp22.suse.cz> <26db2c3e-10c7-c6e3-23f7-21eb5101b31a@redhat.com> <20210104153300.GL13207@dhcp22.suse.cz> <6106ca7f-3247-0916-3e1e-ad6af17272ea@redhat.com> <20210105080057.GT13207@dhcp22.suse.cz> <20210105081654.GU13207@dhcp22.suse.cz> <20210105084224.GV13207@dhcp22.suse.cz> In-Reply-To: <20210105084224.GV13207@dhcp22.suse.cz> From: Dan Williams Date: Tue, 5 Jan 2021 00:57:43 -0800 Message-ID: Subject: Re: uninitialized pmem struct pages To: Michal Hocko Cc: David Hildenbrand , Linux MM , LKML , Oscar Salvador 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 Tue, Jan 5, 2021 at 12:42 AM Michal Hocko wrote: > > On Tue 05-01-21 00:27:34, Dan Williams wrote: > > On Tue, Jan 5, 2021 at 12:17 AM Michal Hocko wrote: > > > > > > On Tue 05-01-21 09:01:00, Michal Hocko wrote: > > > > On Mon 04-01-21 16:44:52, David Hildenbrand wrote: > > > > > On 04.01.21 16:43, David Hildenbrand wrote: > > > > > > On 04.01.21 16:33, Michal Hocko wrote: > > > > > >> On Mon 04-01-21 16:15:23, David Hildenbrand wrote: > > > > > >>> On 04.01.21 16:10, Michal Hocko wrote: > > > > > >> [...] > > > > > >>> Do the physical addresses you see fall into the same section as boot > > > > > >>> memory? Or what's around these addresses? > > > > > >> > > > > > >> Yes I am getting a garbage for the first struct page belonging to the > > > > > >> pmem section [1] > > > > > >> [ 0.020161] ACPI: SRAT: Node 0 PXM 0 [mem 0x100000000-0x603fffffff] > > > > > >> [ 0.020163] ACPI: SRAT: Node 4 PXM 4 [mem 0x6060000000-0x11d5fffffff] non-volatile > > > > > >> > > > > > >> The pfn without the initialized struct page is 0x6060000. This is a > > > > > >> first pfn in a section. > > > > > > > > > > > > Okay, so we're not dealing with the "early section" mess I described, > > > > > > different story. > > > > > > > > > > > > Due to [1], is_mem_section_removable() called > > > > > > pfn_to_page(PHYS_PFN(0x6060000)). page_zone(page) made it crash, as not > > > > > > initialized. > > > > > > > > > > > > Let's assume this is indeed a reserved pfn in the altmap. What's the > > > > > > actual address of the memmap? > > > > > > > > > > > > I do wonder what hosts pfn_to_page(PHYS_PFN(0x6060000)) - is it actually > > > > > > part of the actual altmap (i.e. > 0x6060000) or maybe even self-hosted? > > > > > > > > > > > > If it's not self-hosted, initializing the relevant memmaps should work > > > > > > just fine I guess. Otherwise things get more complicated. > > > > > > > > > > Oh, I forgot: pfn_to_online_page() should at least in your example make > > > > > sure other pfn walkers are safe. It was just an issue of > > > > > is_mem_section_removable(). > > > > > > > > Hmm, I suspect you are right. I haven't put this together, thanks! The memory > > > > section is indeed marked offline so pfn_to_online_page would indeed bail > > > > out: > > > > crash> p (0x6060000>>15) > > > > $3 = 3084 > > > > crash> p mem_section[3084/128][3084 & 127] > > > > $4 = { > > > > section_mem_map = 18446736128020054019, > > > > usage = 0xffff902dcf956680, > > > > page_ext = 0x0, > > > > pad = 0 > > > > } > > > > crash> p 18446736128020054019 & (1UL<<2) > > > > $5 = 0 > > > > > > > > That makes it considerably less of a problem than I thought! > > > > > > Forgot to add that those who are running kernels without 53cdc1cb29e8 > > > ("drivers/base/memory.c: indicate all memory blocks as removable") for > > > some reason can fix the crash by the following simple patch. > > > > > > Index: linux-5.3-users_mhocko_SLE15-SP2_for-next/drivers/base/memory.c > > > =================================================================== > > > --- linux-5.3-users_mhocko_SLE15-SP2_for-next.orig/drivers/base/memory.c > > > +++ linux-5.3-users_mhocko_SLE15-SP2_for-next/drivers/base/memory.c > > > @@ -152,9 +152,14 @@ static ssize_t removable_show(struct dev > > > goto out; > > > > > > for (i = 0; i < sections_per_block; i++) { > > > - if (!present_section_nr(mem->start_section_nr + i)) > > > + unsigned long nr = mem->start_section_nr + i; > > > + if (!present_section_nr(nr)) > > > continue; > > > - pfn = section_nr_to_pfn(mem->start_section_nr + i); > > > + if (!online_section_nr()) { > > > > I assume that's onlince_section_nr(nr) in the version that compiles? > > Yup. > > > This makes sense because the memory block size is larger than the > > section size. I suspect you have 1GB memory block size on this system, > > but since the System RAM and PMEM collide at a 512MB alignment in a > > memory block you end up walking the back end of the last 512MB of the > > System RAM memory block and run into the offline PMEM section. > > Sections are 128MB and memory blocks are 2GB on this system. > > > So, I don't think it's pfn_to_online_page that necessarily needs to > > know how to disambiguate each page, it's things that walk sections and > > memory blocks and expects them to be consistent over the span. > > Well, memory hotplug code is hard wired to sparse memory model so in > this particular case asking about the section is ok. But pfn walkers > shouldn't really care and only rely on pfn_to_online_page. But that will > do the right thing here. So we are good as long as the section is marked > properly. But this would become a problem as soon as the uninitialized > pages where sharing the same memory section as David pointed out. > pfn_to_online_page would then return something containing garbage. So we > should still think of a way to either initialize all those pages or make > sure pfn_to_online_page recognizes them. The former is preferred IMHO. The former would not have saved the crash in this case because pfn_to_online_page() is not used in v5.3:removable_show() that I can see, nor some of the other paths that might walk pfns and the wrong thing with ZONE_DEVICE. However, I do think pfn_to_online_page() should be reliable, and I prefer to just brute force add a section flag to indicate whether the section might be ZONE_DEVICE polluted and fallback to the get_dev_pagemap() slow-path in that case. ...but it would still require hunting to find the places where pfn_to_online_page() is missing for assumptions like this crash which assumed memblock-online + section-present == section-online.