linux-kernel - Re: [PATCH] mm/vmalloc: fix numa spreading for large hash tables

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Date:   Tue, 28 Sep 2021 15:33:02 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Chen Wandun <chenwandun@...wei.com>
Cc:     <npiggin@...il.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <edumazet@...gle.com>,
        <wangkefeng.wang@...wei.com>, <guohanjun@...wei.com>
Subject: Re: [PATCH] mm/vmalloc: fix numa spreading for large hash tables

On Tue, 28 Sep 2021 20:10:40 +0800 Chen Wandun <chenwandun@...wei.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@...gle.com>
> Signed-off-by: Chen Wandun <chenwandun@...wei.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 ;)