linux-kernel - Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers

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Date:   Thu, 9 Sep 2021 15:09:59 -0700
From:   Peter Oskolkov <posk@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Peter Oskolkov <posk@...k.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Andrei Vagin <avagin@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers

On Thu, Sep 9, 2021 at 2:21 PM Jann Horn <jannh@...gle.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.

[...]