linux-kernel - Re: [PATCH v2 2/5] powerpc: Allow 4224 bytes of stack expansion for the signal frame

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Date:   Mon, 27 Jul 2020 10:23:31 +0200
From:   Gabriel Paubert <paubert@...m.es>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
        dja@...ens.net
Subject: Re: [PATCH v2 2/5] powerpc: Allow 4224 bytes of stack expansion for
 the signal frame

On Fri, Jul 24, 2020 at 07:25:25PM +1000, Michael Ellerman wrote:
> We have powerpc specific logic in our page fault handling to decide if
> an access to an unmapped address below the stack pointer should expand
> the stack VMA.
> 
> The code was originally added in 2004 "ported from 2.4". The rough
> logic is that the stack is allowed to grow to 1MB with no extra
> checking. Over 1MB the access must be within 2048 bytes of the stack
> pointer, or be from a user instruction that updates the stack pointer.
> 
> The 2048 byte allowance below the stack pointer is there to cover the
> 288 byte "red zone" as well as the "about 1.5kB" needed by the signal
> delivery code.
> 
> Unfortunately since then the signal frame has expanded, and is now
> 4224 bytes on 64-bit kernels with transactional memory enabled.

Are there really users of transactional memory in the wild? 

Just asking because Power10 removes TM, and Power9 has had some issues
with it AFAICT.

Getting rid of it (if possible) would result in smaller signal frames,
with simpler signal delivery code (probably slightly faster also).

	Gabriel