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From: Peter Oskolkov <posk@google.com>
To: Jann Horn <jannh@google.com>
Cc: Peter Oskolkov <posk@posk.io>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
	Andrei Vagin <avagin@google.com>,
	Thierry Delisle <tdelisle@uwaterloo.ca>,
	Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers
Date: Thu, 9 Sep 2021 15:09:59 -0700	[thread overview]
Message-ID: <CAPNVh5eF6x8e4Lp=ZDOspwrbRYNOEyjeNW4WC99jCAZyeKLGww@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez0mgCXpXnqAUsa0TcFBPjrid-74Gj=xG8HZqj2n+OPoKw@mail.gmail.com>

On Thu, Sep 9, 2021 at 2:21 PM Jann Horn <jannh@google.com> wrote:
>

[...]

> >
> > Option 1: as you suggest, pin pages holding struct umcg_task in sys_umcg_ctl;
>
> FWIW, there is a variant on this that might also be an option:
>
> You can create a new memory mapping from kernel code and stuff pages
> into it that were originally allocated as normal kernel pages. This is
> done in a bunch of places, e.g.:
>
> This has the advantage that it avoids pinning random pages that were
> originally allocated from ZONE_MOVABLE blocks. (Or pinning hugepages,
> or something like that.)
> The downsides are that it reduces userspace's freedom to place the
> UAPI structs wherever it wants (so userspace e.g. probably can't
> directly put the struct in thread-local storage, instead it has to
> store a pointer to the struct), and that you need to write a bunch of
> code to create the mapping and allocate slots in these pages for
> userspace threads.

Thanks again, Jann! Why do you think using custom mapping like this is
preferable to doing just kzalloc(size, GFP_USER), or maybe
alloc_page(GFP_USER)?

The documentation here
https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html
says:

"GFP_USER means that the allocated memory is not movable and it must
be directly accessible by the kernel", which sounds exactly what we
need here.

[...]

  reply	other threads:[~2021-09-09 22:10 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-08 18:49 [PATCH 0/4 v0.5] sched/umcg: RFC UMCG patchset Peter Oskolkov
2021-09-08 18:49 ` [PATCH 1/4 v0.5] sched/umcg: add WF_CURRENT_CPU and externise ttwu Peter Oskolkov
2021-09-08 18:49 ` [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers Peter Oskolkov
2021-09-08 23:38   ` Jann Horn
2021-09-09  1:16     ` Jann Horn
2021-09-09 19:06     ` Peter Oskolkov
2021-09-09 21:20       ` Jann Horn
2021-09-09 22:09         ` Peter Oskolkov [this message]
2021-09-09 23:13           ` Jann Horn
2021-09-14 16:52         ` Andy Lutomirski
2021-09-14 18:11           ` Peter Zijlstra
2021-09-14 18:40             ` Andy Lutomirski
2021-09-15 15:42               ` Peter Zijlstra
2021-09-15 16:50                 ` Andy Lutomirski
2021-09-15 19:10                   ` Peter Zijlstra
2021-09-14  8:07       ` Peter Zijlstra
2021-09-14 16:29         ` Peter Oskolkov
2021-09-14 18:04           ` Peter Zijlstra
2021-09-14 18:15             ` Peter Zijlstra
2021-09-14 18:29             ` Peter Oskolkov
2021-09-14 18:48               ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 3/4 v0.5] sched/umcg: RFC: implement UMCG syscalls Peter Oskolkov
2021-09-09  1:39   ` Jann Horn
2021-09-14 16:51     ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 4/4 v0.5] sched/umcg: add Documentation/userspace-api/umcg.rst Peter Oskolkov
2021-09-14 16:35   ` Tao Zhou
2021-09-14 16:57     ` Peter Oskolkov

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