linux-kernel - Re: [PATCH 2/2] module: add support to avoid duplicates early on load

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Date:   Thu, 25 May 2023 13:40:32 +0200
From:   Petr Pavlu <petr.pavlu@...e.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     gregkh@...uxfoundation.org, rafael@...nel.org, song@...nel.org,
        lucas.de.marchi@...il.com, lucas.demarchi@...el.com,
        christophe.leroy@...roup.eu, peterz@...radead.org, rppt@...nel.org,
        dave@...olabs.net, willy@...radead.org, vbabka@...e.cz,
        mhocko@...e.com, dave.hansen@...ux.intel.com,
        colin.i.king@...il.com, jim.cromie@...il.com,
        catalin.marinas@....com, jbaron@...mai.com,
        rick.p.edgecombe@...el.com, yujie.liu@...el.com, david@...hat.com,
        tglx@...utronix.de, hch@....de, patches@...ts.linux.dev,
        linux-modules@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, pmladek@...e.com, prarit@...hat.com,
        torvalds@...ux-foundation.org, lennart@...ttering.net
Subject: Re: [PATCH 2/2] module: add support to avoid duplicates early on load

On 5/24/23 23:36, Luis Chamberlain wrote:
> Add support to use the new kread_uniq_fd() to avoid duplicate kernel
> reads on modules. At the cost of about ~945 bytes to your kernel size,
> enabling this on a 255 CPU x86_64 qemu guest this saves about ~1.8 GiB
> of memory during boot which would otherwise be free'd, and reduces boot
> time by about ~11 seconds.
> 
> Userspace loads modules through finit_module(), this in turn will
> use vmalloc space up to 3 times:
> 
>   a) The kernel_read_file() call
>   b) Optional module decompression
>   c) Our final copy of the module
> 
> Commit 064f4536d139 ("module: avoid allocation if module is already
> present and ready") shows a graph of the amount of vmalloc space
> observed allocated but freed for duplicate module request which end
> up in the trash bin. Since there is a linear relationship with the
> number of CPUs eventually this will bite us and you end up not being
> able to boot. That commit put a stop gap for c) but to avoid the
> vmalloc() space wasted on a) and b) we need to detect duplicates
> earlier.
> 
> We could just have userspace fix this, but as reviewed at LSFMM 2023
> this year in Vancouver, fixing this in userspace can be complex and we
> also can't know when userpace is fixed. Fixing this in kernel turned
> out to be easy with the inode and with a simple kconfig option we can
> let users / distros decide if this full stop gap is worthy to enable.

kmod normally uses finit_module() only if a module is not compressed,
otherwise it decompresses it first and then invokes init_module().

Looking at Fedora and openSUSE Tumbleweed, they compress kernel modules
with xz and zstd, respectively. They also have their kernels built
without any CONFIG_MODULE_COMPRESS_{GZIP,XZ,ZSTD} options.

It means that these and similarly organized distributions end up using
init_module(), and adding complexity to optimize finit_module() wouldn't
actually help in their case.

-- Petr