linux-kernel - Re: Zram writeback feature unstable with heavy swap utilization

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Message-ID: <20180725025525.GD16271@bombadil.infradead.org>
Date:   Tue, 24 Jul 2018 19:55:25 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Tino Lehnig <tino.lehnig@...tabo.de>, ngupta@...are.org,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Zram writeback feature unstable with heavy swap utilization -
 BUG: Bad page state in process...

On Wed, Jul 25, 2018 at 11:51:06AM +0900, Minchan Kim wrote:
> On Tue, Jul 24, 2018 at 07:35:32PM -0700, Matthew Wilcox wrote:
> > There is NOTHING in a union with _refcount.
> 
> Confusing. Matthew, what am I missing?
> 
> Before:
> 
> counters consumes 8 bytes
> units and _refcount consumes each 4 bytes so memory space is not overlapped.

Correct.  _mapcount is at offset 24, as is units.  _refcount is at offset 28.

>         union {
>                 unsigned long counters;
>                 struct {
>                         union {
>                                 atomic_t _mapcount;
>                                 int units;                      /* SLOB */
>                         };
>                         atomic_t _refcount;
>                 };
>         };
> 
> 
> After:
> 
> Now, units is overlapped with _refcount and _mapcount.

No.  units is at offset 24, as is _mapcount.  But _refcount is still at
offset 28.

>         union {
>                 unsigned long counters;
>                 int units;                      /* SLOB */
>                 struct {                        /* Page cache */
>                         atomic_t _mapcount;
>                         atomic_t _refcount;
>                 };
>         };