Messages in this thread | | | Subject | Re: [PATCH v4 1/5] mm,memory_hotplug: Allocate memmap from the added memory range | From | David Hildenbrand <> | Date | Wed, 17 Mar 2021 15:35:41 +0100 |
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On 17.03.21 15:08, Oscar Salvador wrote: > On Tue, Mar 16, 2021 at 06:45:17PM +0100, David Hildenbrand wrote: >>> 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. > > I guess you meant sizeof(struct page) != 64 >
Yes!
> But other usecases of using altmap (ZONE_DEVICE stuff) might not care whether > they have sub-populated PMDs when populating sections from altmap?
Just assume you have two ranges
[ ZONE_DEVICE 0 ][ ZONE_DEVICE 1]
If the vmemmap of ZONE_DEVICE 1 could be taken from the altmap of ZONE_DEVICE 0, we could be in trouble, as both parts can be removed/repurposed independently ...
> > Current vmemmap code populates PMD with PMD_SIZE if empty, and with basepages > if there are still holes. > >>> 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 agree with the above, but see below: > >>> 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 >>> */ >>> 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; > > While it might be true that we need to back off from populating with altmap in > case PMDs are not going to be fully populated because of the size of the struct > page (I am not still not sure though as I said above, other usecases might not > care at all), I would go __for now__ with placing vmemmap_size == PMD_SIZE in > the check below as well. > > If the check comes true, we know that we fully populate PMDs when populating > sections, so the feature can be used. > > Then I commit to have a look whether we need to back off in vmemmap-populating > code in case altmap && !NOT_FULLY_POPULATED_PMDS. > > What do you think?
If we check for
IS_ALIGNED(nr_vmemmap_pages, PMD_SIZE), please add a proper TODO comment that this is most probably the wrong place to take care of this.
-- Thanks,
David / dhildenb
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