Messages in this thread | | | Subject | Re: [PATCH v4 1/5] mm,memory_hotplug: Allocate memmap from the added memory range | From | David Hildenbrand <> | Date | Tue, 16 Mar 2021 18:45:17 +0100 |
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On 16.03.21 17:46, David Hildenbrand wrote: > On 15.03.21 11:22, Oscar Salvador wrote: >> On Thu, Mar 11, 2021 at 08:06:53PM +0100, David Hildenbrand wrote: >>> This looks essentially good to me, except some parts in >>> mhp_supports_memmap_on_memory() >>> >>>> +bool mhp_supports_memmap_on_memory(unsigned long size) >>>> +{ >>>> + unsigned long pageblock_size = PFN_PHYS(pageblock_nr_pages); >>>> + unsigned long remaining_mem = size - PMD_SIZE; >> >> Hi David, thanks for the review! >> >>> This looks weird. I think what you want to test is that >>> >>> >>> a) "nr_vmemmap_pages * sizeof(struct page)" spans complete PMDs (IOW, we >>> won't map too much via the altmap when populating a PMD in the vmemmap) >>> >>> b) "remaining = size - nr_vmemmap_pages * sizeof(struct page)" spans >>> complete pageblock. >> >> We do not know the nr_vmemmap_pages at this point in time, although it is >> easy to pre-compute. >> >> For every section we populate, we use PMD_SIZE. So, PMD_SIZE/PAGE_SIZE lays >> the nr_vmemmap_pages that are used for populating a single section. > > I find that cross reference to vmemmap code a little hard to digest. > I would have assume that we don't have to care about PMDs in this > code here at all. The vmemmap population code should handle that. > > I think I already mentioned that somewhere, I think it should be like this: > > a) vmemmap code should *never* populate more memory than requested for > a single memory section when we are populating from the altmap. > If that cannot be guaranteed for PMDs, then we have to fallback > to populating base pages. Populating PMDs from an altmap with > sizeof(struct page) == 64 is highly dangerous. > > Assume we have sizeof(struct page) == 56. A 128 MiB section > spans 32768 pages - we need 32768 * sizeof(struct page) > space for the vmemmap. > > With 64k pages we *can* use exactly one PMD. With 56k pages > we need 448 individual (full!) pages for the vmemmap. > > IOW, we don't care how vmemmap code will do the mapping. > vmemmap code has to get it right. IMHO, asserting it in > this code is wrong. > > > b) In this code, we really should only care about what > memory onlining/offlining code can or can't do. > We really only care that > > 1) size == memory_block_size_bytes() > 2) remaining_size > 3) IS_ALIGNED(remaining_size, pageblock_size); > > > I think a) is a logical consequence of b) for x86-64, > whereby the pageblock size corresponds to PMD, so we > might not have to care about a) right now. > > See below for my take. > > >> >> But let me explain the reasoning I use in the current code: >> >> I will enumarate the assumptions that must hold true in order to support the >> feature together with their check: >> >> - We span a single memory block >> >> size == memory_block_size_bytes() >> >> - The vmemmap pages span a complete PMD and no more than a PMD. >> >> !(PMD_SIZE % sizeof(struct page)) >> >> - The vmemmap pages and the pages exposed to the buddy have to cover full >> pageblocks >> >> remaining_mem = size - PMD_SIZE; >> IS_ALIGNED(remaining_mem, pageblock_size) >> >> Although this check only covers the range without the vmemmap pages, one could >> argue that since we use only a PMD_SIZE at a time, we know that PMD_SIZE is >> pageblock aligned, so the vmemmap range is PMD_SIZE as well. >> >> Now, I see how this might be confusing and rather incomplete. >> So I guess a better and more clear way to write it would be: >> >> bool mhp_supports_memmap_on_memory(unsigned long size) >> { >> unsigned long nr_vmemmap_pages = PMD_SIZE / PAGE_SIZE; >> unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page); >> unsigned long remaining_size = size - vmemmap_size; >> >> return memmap_on_memory && >> IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) && >> size == memory_block_size_bytes() && >> !(PMD_SIZE % vmemmap_size) && >> IS_ALIGNED(vmemmap_size, pageblock_size) && >> remaining_size && >> IS_ALIGNED(remaining_size, pageblock_size); >> } >> >> Note that above check is only for a single section, but if assumptions hold true >> for a single section, it will for many as well. > > Okay, please document the statement about single sections, that's > important to understand what's happening. > > My take would be > > bool mhp_supports_memmap_on_memory(unsigned long size) > { > /* > * Note: We calculate for a single memory section. The calculation > * implicitly covers memory blocks that span multiple sections. > */ > unsigned long nr_vmemmap_pages = SECTION_SIZE / PAGE_SIZE; > unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page); > unsigned long remaining_size = size - vmemmap_size; > > /* > * Note: vmemmap code has to make sure to not populate more memory > * via the altmap than absolutely necessary for a single section. > * This always holds when we allocate pageblock-sized chunks from > * the altmap, as we require pageblock alignment here. > * > * TODO, other doc > */ > return memmap_on_memory && > IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) && > size == memory_block_size_bytes() && > remaining_size && > IS_ALIGNED(remaining_size, pageblock_size);
Oh, I forgot, we can also drop the check for "remaining_size" in that code. If we ever have PAGE_SIZE == sizeof(struct page) we'll be in bigger trouble :D
-- Thanks,
David / dhildenb
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