linux-kernel - Re: [PATCH v2 bpf 1/3] vmalloc: replace VM_NO_HUGE_VMAP with VM_ALLOW_HUGE

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Date:   Thu, 21 Apr 2022 13:35:55 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Christoph Hellwig <hch@...radead.org>, Song Liu <song@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>, imbrenda@...ux.ibm.com,
        open list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH v2 bpf 1/3] vmalloc: replace VM_NO_HUGE_VMAP with
 VM_ALLOW_HUGE_VMAP

Excerpts from Nicholas Piggin's message of April 21, 2022 12:24 pm:
> Excerpts from Song Liu's message of April 12, 2022 4:00 pm:
>> On Mon, Apr 11, 2022 at 9:18 PM Christoph Hellwig <hch@...radead.org> wrote:
>>>
>>> On Mon, Apr 11, 2022 at 04:35:46PM -0700, Song Liu wrote:
>>> > Huge page backed vmalloc memory could benefit performance in many cases.
>>> > Since some users of vmalloc may not be ready to handle huge pages,
>>> > VM_NO_HUGE_VMAP was introduced to allow vmalloc users to opt-out huge
>>> > pages. However, it is not easy to add VM_NO_HUGE_VMAP to all the users
>>> > that may try to allocate >= PMD_SIZE pages, but are not ready to handle
>>> > huge pages properly.
>>>
>>> This is a good place to document what the problems are, and how they are
>>> hard to track down (e.g. because the allocations are passed down I/O
>>> stacks)
>> 
>> Will add it in v3.
>> 
>>>
>>> >
>>> > Replace VM_NO_HUGE_VMAP with an opt-in flag, VM_ALLOW_HUGE_VMAP, so that
>>> > users that benefit from huge pages could ask specificially.
>>> >
>>> > Also, replace vmalloc_no_huge() with opt-in helper vmalloc_huge().
>>>
>>> We still need to find out what the primary users of the large vmalloc
>>> hashes was and convert them.
>> 
>> @ Claudio and Nicholas,
>> 
>> Could you please help identify users of large vmalloc? So far, I found
>> alloc_large_system_hash(), and something like the following seems to
>> work:
> 
> The large system hashes were the main ones I was interested in. IIRC 
> there was a few more in some drivers or tracing things depending on
> config but those are less important (to me at least).

Oh there is also a reverse map array in KVM now I think of it.

Thanks,
Nick