linux-kernel - Re: [PATCH] regset: use vmalloc() for regset_get

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Date: Fri, 2 Feb 2024 08:24:17 -0800
From: Doug Anderson <dianders@...omium.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>, Eric Biederman <ebiederm@...ssion.com>, Jan Kara <jack@...e.cz>, 
	Kees Cook <keescook@...omium.org>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Oleg Nesterov <oleg@...hat.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] regset: use vmalloc() for regset_get_alloc()

Hi,

On Thu, Feb 1, 2024 at 8:05 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Fri, Feb 02, 2024 at 03:49:25AM +0000, Al Viro wrote:
> > On Thu, Feb 01, 2024 at 07:15:48PM -0800, Doug Anderson wrote:
> > > >
> > > > Well, the next step would be to see which regset it is - if you
> > > > see that kind of allocation, print regset->n, regset->size and
> > > > regset->core_note_type.
> > >
> > > Of course! Here are the big ones:
> > >
> > > [   45.875574] DOUG: Allocating 279584 bytes, n=17474, size=16,
> > > core_note_type=1029
> >
> > 0x405, NT_ARM_SVE
> >         [REGSET_SVE] = { /* Scalable Vector Extension */
> >                 .core_note_type = NT_ARM_SVE,
> >                 .n = DIV_ROUND_UP(SVE_PT_SIZE(SVE_VQ_MAX, SVE_PT_REGS_SVE),
> >                                   SVE_VQ_BYTES),
> >                 .size = SVE_VQ_BYTES,
> >
> > IDGI.  Wasn't SVE up to 32 * 2Kbit, i.e. 8Kbyte max?  Any ARM folks around?
> > Sure, I understand that it's variable-sized and we want to allocate enough
> > for the worst case, but can we really get about 280Kb there?  Context switches
> > would be really unpleasant on such boxen...
>
> FWIW, this apparently intends to be "variable, up to SVE_PT_SIZE(...) bytes";
> no idea if SVE_PT_SIZE is the right thing to use here.

+folks from `./scripts/get_maintainer.pl -f arch/arm64/kernel/ptrace.c`

Trying to follow the macros to see where "n" comes from is a maze of
twisty little passages, all alike. Hopefully someone from the ARM
world can help tell if the value of 17474 for n here is correct or if
something is wonky.