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Date:   Tue, 9 May 2023 13:44:01 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Sergey Senozhatsky <senozhatsky@chromium.org>
Cc:     Nhat Pham <nphamcs@gmail.com>, Minchan Kim <minchan@kernel.org>,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, ngupta@vflare.org,
        sjenning@redhat.com, ddstreet@ieee.org, vitaly.wool@konsulko.com,
        kernel-team@meta.com
Subject: Re: [PATCH] zsmalloc: move LRU update from zs_map_object() to
 zs_malloc()
Message-ID: <20230509174401.GA18828@cmpxchg.org>
References: <20230505185054.2417128-1-nphamcs@gmail.com>
 <20230506030140.GC3281499@google.com>
 <20230508140658.GA3421@cmpxchg.org>
 <CAKEwX=MtunOe6A--SG3ud-gUFg3bXFJgG4csgwHeZFAEqjCgHg@mail.gmail.com>
 <20230509030030.GD11511@google.com>
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On Tue, May 09, 2023 at 12:00:30PM +0900, Sergey Senozhatsky wrote:
> On (23/05/08 09:00), Nhat Pham wrote:
> > > The deeper bug here is that zs_map_object() tries to add the page to
> > > the LRU list while the shrinker has it isolated for reclaim. This is
> > > way too sutble and error prone. Even if it worked now, it'll cause
> > > corruption issues down the line.
> > >
> > > For example, Nhat is adding a secondary entry point to reclaim.
> > > Reclaim expects that a page that's on the LRU is also on the fullness
> > > list, so this would lead to a double remove_zspage() and BUG_ON().
> > >
> > > This patch doesn't just fix the crash, it eliminates the deeper LRU
> > > isolation issue and makes the code more robust and simple.
> > 
> > I agree. IMO, less unnecessary concurrent interaction is always a
> > win for developers' and maintainers' cognitive load.
> 
> Thanks for all the explanations.
> 
> > As a side benefit - this also gets rid of the inelegant check
> > (mm == ZS_MM_WO). The fact that we had to include a
> > a multi-paragraph explanation for a 3-line piece of code
> > should have been a red flag.
> 
> Minchan had some strong opinion on that, so we need to hear from him
> before we decide how do we fix it.

I'd be happy if he could validate the fix. But this fixes a crash, so
the clock is ticking.

I will also say, his was a design preference. One we agreed to only
very reluctantly: https://lore.kernel.org/lkml/Y3f6habiVuV9LMcu@google.com/

Now we have a crash that is a direct result of it, and which cost us
(and apparently is still costing us) time and energy to resolve.

Unless somebody surfaces a real technical problem with the fix, I'd
say let's do it our way this time.