linux-kernel - Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag refs in page flags

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAJuCfpE_aSyjokF=xuwXvq9-jpjDfC+OH0etspK=G6PS7SvMFg@mail.gmail.com>
Date: Mon, 21 Oct 2024 08:05:16 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Michal Hocko <mhocko@...e.com>, John Hubbard <jhubbard@...dia.com>, 
	Yosry Ahmed <yosryahmed@...gle.com>, akpm@...ux-foundation.org, 
	kent.overstreet@...ux.dev, corbet@....net, arnd@...db.de, mcgrof@...nel.org, 
	rppt@...nel.org, paulmck@...nel.org, thuth@...hat.com, tglx@...utronix.de, 
	bp@...en8.de, xiongwei.song@...driver.com, ardb@...nel.org, vbabka@...e.cz, 
	hannes@...xchg.org, roman.gushchin@...ux.dev, dave@...olabs.net, 
	willy@...radead.org, liam.howlett@...cle.com, pasha.tatashin@...een.com, 
	souravpanda@...gle.com, keescook@...omium.org, dennis@...nel.org, 
	yuzhao@...gle.com, vvvvvv@...gle.com, rostedt@...dmis.org, 
	iamjoonsoo.kim@....com, rientjes@...gle.com, minchan@...gle.com, 
	kaleshsingh@...gle.com, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, linux-mm@...ck.org, 
	linux-modules@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag
 refs in page flags

On Mon, Oct 21, 2024 at 2:13 AM David Hildenbrand <david@...hat.com> wrote:
>
>
>
> Am 21.10.24 um 09:26 schrieb Michal Hocko:
> > On Fri 18-10-24 14:57:26, Suren Baghdasaryan wrote:
> >> On Fri, Oct 18, 2024 at 10:45 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> >>>
> >>> On Fri, Oct 18, 2024 at 10:08 AM Michal Hocko <mhocko@...e.com> wrote:
> >>>
> >>> Automatic fallback is possible during boot, when we decide whether to
> >>> enable page extensions or not. So, if during boot we decide to disable
> >>> page extensions and use page flags, we can't go back and re-enable
> >>> page extensions after boot is complete. Since there is a possibility
> >>> that we run out of page flags at runtime when we load a new module,
> >>> this leaves this case when we can't reference the module tags and we
> >>> can't fall back to page extensions, so we have to disable memory
> >>> profiling.
> >>> I could keep page extensions always on just in case this happens but
> >>> that's a lot of memory waste to handle a rare case...
> >>
> >> After thinking more about this, I suggest a couple of changes that
> >> IMHO would make configuration simpler:
> >> 1. Change the CONFIG_PGALLOC_TAG_USE_PAGEFLAGS to an early boot
> >> parameter.
> >
> > This makes much more sense!
> >
> >> Today we have a "mem_profiling" parameter to enable/disable
> >> memory profiling. I suggest adding "mem_profiling_use_pgflags" to
> >> switch the current behavior of using page extensions to use page
> >> flags.
> >
> > I do not want to bikeshed about this but to me it would make more sense
> > to have an extension paramater to mem_profiling and call it something
> > like compress or similar so that page flags are not really carved into
> > naming. The docuemntation then can explain that the copression cannot be
> > always guaranteed and it might fail so this is more of a optimistic and
> > potentially failing optimization that might need to be dropped in some
> > usege scenarios.
>
> Maybe we can reuse the existing parameter (e.g., tristate). Only makes sense if
> we don't expect too many other modes though :)

Yeah, I thought about adding new values to "mem_profiling" but it's a
bit complicated. Today it's a tristate:

mem_profiling=0|1|never

0/1 means we disable/enable memory profiling by default but the user
can enable it at runtime using a sysctl. This means that we enable
page_ext at boot even when it's set to 0.
"never" means we do not enable page_ext, memory profiling is disabled
and sysctl to enable it will not be exposed. Used when a distribution
has CONFIG_MEM_ALLOC_PROFILING=y but the user does not use it and does
not want to waste memory on enabling page_ext.

I can add another option like "pgflags" but then it also needs to
specify whether we should enable or disable profiling by default
(similar to 0|1 for page_ext mode). IOW we will need to encode also
the default state we want. Something like this:

mem_profiling=0|1|never|pgflags_on|pgflags_off

Would this be acceptable?


>
> >
> >> We keep the current behavior of using page extensions as
> >> default (mem_profiling_use_pgflags=0) because it always works even
> >> though it has higher overhead.
> >
> > Yes this seems to be a safe default.
>
> Agreed.
>
> >
> >> 2. No auto-fallback. If mem_profiling_use_pgflags=1 and we don't have
> >> enough page flags (at boot time or later when we load a module), we
> >> simply disable memory profiling with a warning.
>
> Sounds reasonable to me.
>
> --
> Cheers,
>
> David / dhildenb
>