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    SubjectRe: [PATCH] mm/vmalloc: fix numa spreading for large hash tables
    On Tue, 28 Sep 2021 20:10:40 +0800 Chen Wandun <chenwandun@huawei.com> wrote:

    > Eric Dumazet reported a strange numa spreading info in [1], and found
    > commit 121e6f3258fe ("mm/vmalloc: hugepage vmalloc mappings") introduced
    > this issue [2].
    >
    > Dig into the difference before and after this patch, page allocation has
    > some difference:
    >
    > before:
    > alloc_large_system_hash
    > __vmalloc
    > __vmalloc_node(..., NUMA_NO_NODE, ...)
    > __vmalloc_node_range
    > __vmalloc_area_node
    > alloc_page /* because NUMA_NO_NODE, so choose alloc_page branch */
    > alloc_pages_current
    > alloc_page_interleave /* can be proved by print policy mode */
    >
    > after:
    > alloc_large_system_hash
    > __vmalloc
    > __vmalloc_node(..., NUMA_NO_NODE, ...)
    > __vmalloc_node_range
    > __vmalloc_area_node
    > alloc_pages_node /* choose nid by nuam_mem_id() */
    > __alloc_pages_node(nid, ....)
    >
    > So after commit 121e6f3258fe ("mm/vmalloc: hugepage vmalloc mappings"),
    > it will allocate memory in current node instead of interleaving allocate
    > memory.
    >
    > [1]
    > https://lore.kernel.org/linux-mm/CANn89iL6AAyWhfxdHO+jaT075iOa3XcYn9k6JJc7JR2XYn6k_Q@mail.gmail.com/
    >
    > [2]
    > https://lore.kernel.org/linux-mm/CANn89iLofTR=AK-QOZY87RdUZENCZUT4O6a0hvhu3_EwRMerOg@mail.gmail.com/
    >
    > Fixes: 121e6f3258fe ("mm/vmalloc: hugepage vmalloc mappings")
    > Reported-by: Eric Dumazet <edumazet@google.com>
    > Signed-off-by: Chen Wandun <chenwandun@huawei.com>

    This seems like it could cause significant performance regressions in
    some situations?

    If "yes" then wouldn't a cc:stable be appropriate? And some (perhaps
    handwavy) quantification of the slowdown would help people understand
    why we're recommending a backport.

    If "no" then why the heck do we have that feature in there anyway ;)

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