linux-kernel - Re: [PATCH] zsmalloc: move LRU update from zs_map_object() to zs

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Message-ID: <20230509030030.GD11511@google.com>
Date:   Tue, 9 May 2023 12:00:30 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Nhat Pham <nphamcs@...il.com>, Minchan Kim <minchan@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, ngupta@...are.org,
        sjenning@...hat.com, ddstreet@...e.org, vitaly.wool@...sulko.com,
        kernel-team@...a.com
Subject: Re: [PATCH] zsmalloc: move LRU update from zs_map_object() to
 zs_malloc()

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.