From: Linus Torvalds <torvalds@linux-foundation.org>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: Linux FS Devel <linux-fsdevel@vger.kernel.org>,
hch@lst.de, brauner@kernel.org, david@redhat.com,
tglx@linutronix.de, patches@lists.linux.dev,
linux-modules@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, pmladek@suse.com,
petr.pavlu@suse.com, prarit@redhat.com, lennart@poettering.net,
gregkh@linuxfoundation.org, rafael@kernel.org, song@kernel.org,
lucas.de.marchi@gmail.com, lucas.demarchi@intel.com,
christophe.leroy@csgroup.eu, peterz@infradead.org,
rppt@kernel.org, dave@stgolabs.net, willy@infradead.org,
vbabka@suse.cz, mhocko@suse.com, dave.hansen@linux.intel.com,
colin.i.king@gmail.com, jim.cromie@gmail.com,
catalin.marinas@arm.com, jbaron@akamai.com,
rick.p.edgecombe@intel.com, yujie.liu@intel.com
Subject: Re: [PATCH 1/2] fs/kernel_read_file: add support for duplicate detection
Date: Thu, 25 May 2023 11:50:21 -0700 [thread overview]
Message-ID: <CAHk-=wgWCDw58fZDLGYVqVC2ee-Zec25unewdHFp8syCZFumvg@mail.gmail.com> (raw)
In-Reply-To: <ZG+kDevFH6uE1I/j@bombadil.infradead.org>
On Thu, May 25, 2023 at 11:08 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> Certainly on the track where I wish we could go. Now this goes tested.
> On 255 cores:
>
> Before:
>
> vagrant@kmod ~ $ sudo systemd-analyze
> Startup finished in 41.653s (kernel) + 44.305s (userspace) = 1min 25.958s
> graphical.target reached after 44.178s in userspace.
>
> root@kmod ~ # grep "Virtual mem wasted bytes" /sys/kernel/debug/modules/stats
> Virtual mem wasted bytes 1949006968
>
>
> ; 1949006968/1024/1024/1024
> ~1.81515418738126754761
>
> So ~1.8 GiB... of vmalloc space wasted during boot.
>
> After:
>
> systemd-analyze
> Startup finished in 24.438s (kernel) + 41.278s (userspace) = 1min 5.717s
> graphical.target reached after 41.154s in userspace.
>
> root@kmod ~ # grep "Virtual mem wasted bytes" /sys/kernel/debug/modules/stats
> Virtual mem wasted bytes 354413398
>
> So still 337.99 MiB of vmalloc space wasted during boot due to
> duplicates.
Ok. I think this will count as 'good enough for mitigation purposes'
> The reason is the exclusive_deny_write_access() must be
> kept during the life of the module otherwise as soon as it is done
> others can still race to load
Yes. The exclusion only applies while the file is actively being read.
> So with two other hunks added (2nd and 4th), this now matches parity with
> my patch, not suggesting this is right,
Yeah, we can't do that, because user space may quite validly want to
write the file afterwards.
Or, in fact, unload the module and re-load it.
So the "exclusion" really needs to be purely temporary.
That said, I considered moving the exclusion to module/main.c itself,
rather than the reading part. That wouild get rid of the hacky "id ==
READING_MODULE", and put the exclusion in the place that actually
wants it.
And that would allow us to at least extend that temporary exlusion a
bit - we could keep it until the module has actually been loaded and
inited.
So it would probably improve on those numbers a bit more, but you'd
still have the fundamental race where *serial* duplicates end up
always wasting CPU effort and temporary vmalloc space.
Linus
next prev parent reply other threads:[~2023-05-25 19:22 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-24 21:36 [PATCH 0/2] module: avoid all memory pressure due to duplicates Luis Chamberlain
2023-05-24 21:36 ` [PATCH 1/2] fs/kernel_read_file: add support for duplicate detection Luis Chamberlain
2023-05-24 21:52 ` Linus Torvalds
2023-05-24 21:56 ` Linus Torvalds
2023-05-24 22:07 ` Luis Chamberlain
2023-05-25 4:00 ` Linus Torvalds
2023-05-25 18:08 ` Luis Chamberlain
2023-05-25 18:35 ` Luis Chamberlain
2023-05-25 18:50 ` Linus Torvalds [this message]
2023-05-25 19:32 ` Luis Chamberlain
2023-05-25 7:01 ` Christian Brauner
2023-05-24 21:36 ` [PATCH 2/2] module: add support to avoid duplicates early on load Luis Chamberlain
2023-05-25 11:40 ` Petr Pavlu
2023-05-25 16:07 ` Linus Torvalds
2023-05-25 16:42 ` Greg KH
2023-05-25 18:22 ` Luis Chamberlain
2023-05-25 17:52 ` Linus Torvalds
2023-05-25 18:45 ` Lucas De Marchi
2023-05-25 21:12 ` Linus Torvalds
2023-05-25 22:02 ` Luis Chamberlain
2023-05-26 1:39 ` Linus Torvalds
2023-05-29 8:58 ` Johan Hovold
2023-05-29 11:00 ` Linus Torvalds
2023-05-29 12:44 ` Johan Hovold
2023-05-29 15:18 ` Johan Hovold
2023-05-30 1:55 ` Linus Torvalds
2023-05-30 9:40 ` Johan Hovold
2023-06-05 12:25 ` Johan Hovold
2023-05-30 16:22 ` Luis Chamberlain
2023-05-30 17:16 ` Lucas De Marchi
2023-05-30 19:41 ` Luis Chamberlain
2023-05-30 22:17 ` Linus Torvalds
2023-05-31 5:30 ` Lucas De Marchi
2023-05-31 0:31 ` Luis Chamberlain
2023-05-31 7:51 ` David Hildenbrand
2023-05-31 16:57 ` Luis Chamberlain
2023-06-02 15:19 ` David Hildenbrand
2023-06-02 16:04 ` Luis Chamberlain
2023-06-05 11:26 ` David Hildenbrand
2023-06-05 15:17 ` Luis Chamberlain
2023-06-05 15:28 ` Luis Chamberlain
2023-06-28 18:52 ` Luis Chamberlain
2023-06-28 20:14 ` Linus Torvalds
2023-06-28 22:07 ` Linus Torvalds
2023-06-28 23:17 ` Linus Torvalds
2023-06-29 0:18 ` Luis Chamberlain
2023-06-02 16:06 ` Linus Torvalds
2023-06-02 16:37 ` David Hildenbrand
2023-05-30 22:45 ` Dan Williams
2023-06-04 14:26 ` Rudi Heitbaum
2023-05-29 17:47 ` Linus Torvalds
2023-05-30 10:01 ` Johan Hovold
2023-05-25 16:54 ` Lucas De Marchi
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