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    SubjectRe: [PATCH v4 1/5] mm,memory_hotplug: Allocate memmap from the added memory range
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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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