linux-kernel - Re: [PATCH] ext4: use private version of page_zero_new

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Message-ID: <CAHk-=wigdcg+FtWm5Fds5M2P_7GKSfXxpk-m9jkx0C6FMCJ_Jw@mail.gmail.com>
Date: Sun, 26 Jan 2025 10:48:47 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: "Theodore Ts'o" <tytso@....edu>, Ext4 Developers List <linux-ext4@...r.kernel.org>, 
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>, dave.hansen@...el.com, 
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ext4: use private version of page_zero_new_buffers() for
 data=journal mode

On Sun, 26 Jan 2025 at 09:02, Mateusz Guzik <mjguzik@...il.com> wrote:
>
> Hello there, a blast from the past.
>
> I see this has landed in b90197b655185a11640cce3a0a0bc5d8291b8ad2

Whee. What archeology are you doing to notice this decade-old issue?

> I came here from looking at a pwrite vs will-it-scale and noticing that
> pre-faulting eats CPU (over 5% on my Sapphire Rapids) due to SMAP trips.

Ugh. Yeah, turning SMAP on/off is expensive on most cores (apparently
fixed in AMD Zen 5).

> It used to be that pre-faulting was avoided specifically for that
> reason, but it got temporarily reverted due to bugs in ext4, to quote
> Linus (see 00a3d660cbac05af34cca149cb80fb611e916935):

Yeah, I think we should revert the revert (except we've done other
changes in the last decade - surprise surprise - so it would be a
completely manual revert).

If you send me a tested revert of the revert (aka re-do) of the "don't
pre-fault" patch, I'll apply it.

Note that the ext4 problem could exist in other filesystems, so we
might have to revert (again).  It's not necessarily that ext4 was
_particularly_ buggy, it's quite possible that the problem was
originally found on ext4 just because it was more widely used than
others.

               Linus