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

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Message-ID: <CAGudoHGJah8VNm6V1pZo2-C0P-y6aUbjMedp1SeAGwwDLua2OQ@mail.gmail.com>
Date: Sun, 26 Jan 2025 20:49:34 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
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, Jan 26, 2025 at 7:49 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> 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?
>

Me? Archeology? Not even once!

I was curious what's up with this very much *fresh* sucker:
https://lore.kernel.org/oe-lkp/202501261527.c3bf4764-lkp@intel.com/

> > 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).
>

Interesting, I thought it sucks everywhere.

Anyhow it definitely still sucks on Sapphire Rapids which is pretty
high up there as far as Intel goes, so...

> > 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.
>

:( ok

This being your revert I was lowkey hoping you would do the honors.

I'll sort it out if Ted confirms that as far as he knows this is fixed in ext4.

> 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.

Indeed. Or they might have regressed since, which is why I mentioned
-next for testing.

-- 
Mateusz Guzik <mjguzik gmail.com>