On Tue, 28 Sep 2021 20:10:40 +0800 Chen Wandun [off-list ref] 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/ (local)

[2]
https://lore.kernel.org/linux-mm/CANn89iLofTR=AK-QOZY87RdUZENCZUT4O6a0hvhu3_EwRMerOg@mail.gmail.com/ (local)

Fixes: 121e6f3258fe ("mm/vmalloc: hugepage vmalloc mappings")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Chen Wandun <redacted>

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 ;)