linux-kernel - Re: [PATCH] mm: Remove stub for non_swap

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Message-ID: <Ylgt9aLU5T+e+Qju@xz-m1.local>
Date:   Thu, 14 Apr 2022 10:21:41 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: Remove stub for non_swap_entry()

On Thu, Apr 14, 2022 at 03:48:33PM +1000, Alistair Popple wrote:
> Peter Xu <peterx@...hat.com> writes:
> 
> > The stub for non_swap_entry() may not help much, because MAX_SWAPFILES has
> > already contained all the information to decide whether a swap entry is
> > real swap entry of pesudo ones (migrations, ...).
> >
> > There can be some performance influences on non_swap_entry() with below
> > conditions all met:
> >
> >   !CONFIG_MIGRATION && !CONFIG_MEMORY_FAILURE && !CONFIG_DEVICE_PRIVATE
> >
> > But that's definitely not the major config most machines will use, at the
> > meantime it's already in a slow path of swap entry (being parsed from a
> > swap pte), so IMHO it shouldn't be a major issue.  Also according to the
> > analysis from Alistair, somehow the stub didn't do the job right [1].
> 
> I wasn't so much concerned about execution speed given it's on the slow path
> anyway but overall code size, which is one reason all those config options might
> be disabled. However in practice it made little to no difference as those config
> options already remove most of the extra code so I agree we can drop the stub.

I see, yeah that's a good point.

I'd wildly guess a minumum set of Linux build could still like that, but
not strongly, as I'd first think about not having CONFIG_SWAP at all if so.

> 
> Reviewed-by: Alistair Popple <apopple@...dia.com>

Thanks!

-- 
Peter Xu