linux-kernel - Re: [regression] oops on heavy compilations ("kernel BUG at mm/zswap.c:1005!" and "Oops: invalid opcode: 0000")

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Message-ID: <CAJD7tkZi8Wg+ZCWMy=2YKZ3hLZZ7JYQaCG+w7y=3Tb8wmHC48g@mail.gmail.com>
Date: Tue, 3 Sep 2024 10:49:19 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Piotr Oniszczuk <piotr.oniszczuk@...il.com>
Cc: Pedro Falcato <pedro.falcato@...il.com>, Nhat Pham <nphamcs@...il.com>, 
	Matthew Wilcox <willy@...radead.org>, 
	Linux regressions mailing list <regressions@...ts.linux.dev>, LKML <linux-kernel@...r.kernel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [regression] oops on heavy compilations ("kernel BUG at
 mm/zswap.c:1005!" and "Oops: invalid opcode: 0000")

On Mon, Sep 2, 2024 at 1:58 AM Piotr Oniszczuk
<piotr.oniszczuk@...il.com> wrote:
>
>
>
> > Wiadomość napisana przez Yosry Ahmed <yosryahmed@...gle.com> w dniu 31.08.2024, o godz. 19:23:
> >
> > On Sat, Aug 31, 2024 at 2:41 AM Piotr Oniszczuk
> > <piotr.oniszczuk@...il.com> wrote:
> >>
> >>
> >>
> >>> Wiadomość napisana przez Yosry Ahmed <yosryahmed@...gle.com> w dniu 29.08.2024, o godz. 23:54:
> >>>
> >>> I also noticed that you are using z3fold as the zpool. Is the problem
> >>> reproducible with zsmalloc? I wouldn't be surprised if there's a
> >>> z3fold bug somewhere.
> >>>
> >>
> >> Hmm - yesterday i recompiled 6.9.12 with zsmalloc and …. after 16h of continuous tests I can’t reproduce issue.
> >> With zsmalloc 6.9.12 looks to me like stable.
> >
> > Interesting, and a little bit what I hoped for tbh.
>
> :-)
>
> I tested mainline 6.10.7 with 26h test and also it is stable with zsmalloc
>
> >
> >>
> >> With this - what will be your advice to move forward?
> >
> > Well, it's possible that some zswap change was not fully compatible
> > with z3fold, or surfaced a dormant bug in z3fold. Either way, my
> > recommendation is to use zsmalloc.
> > I have been trying to deprecate
>
> IMHO - isn’t bug in this report + difficulties to reproduce->fix enough to depreciate z3fold?

I would say this bug report is yet another reason why we should deprecate it.

>
> > z3fold, and honestly you are the only person I have seen use z3fold in
> > a while -- which is probably why no one else reported such a problem.
>
> Well - in fact this is ArchLinux - not me.
> I’m using Arch and kernel in builder machine with ArchLinux config + packaging

According to [1], zsmalloc should be the default allocator for zswap
on ArchLinux. Anyway, I initially thought that no one was using z3fold
and it was bitrot, but apparently some people are using it and it's
actively harming them.

[1]https://wiki.archlinux.org/title/Zswap

>
> >
> >> Is there any possibility/way to avoid bisecting? (due limited time from my side)
> >
> > So unless you have a reason to specifically use z3fold or avoid
> > zsmalloc, please use zsmalloc. It should be better for you anyway. I
>
> I see benefits already: on very memory demanding qtwebkit compile:
> z3fold: swap frequently gets 6..8G from 16G available
> zsmalloc: can’t see more than 1..2G
>
> > doubt that you (or anyone) wants to spend time debugging a z3fold
> > problem :)
>
> lets depreciate it!

I tried deprecating it before [2] and performed some analysis [3], but
there was some.. resistance. Maybe I will try again and use this bug
report as yet another argument for deprecating z3fold :)

[2] https://lore.kernel.org/linux-mm/20240112193103.3798287-1-yosryahmed@google.com/
[3] https://lore.kernel.org/lkml/CAJD7tkbRF6od-2x_L8-A1QL3=2Ww13sCj4S3i4bNndqF+3+_Vg@mail.gmail.com/