linux-kernel - Re: [PATCHv2 0/4] zsmalloc: make zspage chain size configurable

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Date:   Sun, 15 Jan 2023 14:32:50 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCHv2 0/4] zsmalloc: make zspage chain size configurable

On (23/01/15 13:21), Sergey Senozhatsky wrote:
> On (23/01/14 13:34), Mike Kravetz wrote:
> > I did the following:
> > 
> > - Start with clean v6.2-rc3
> >   Perform echo, did not see issue
> > 
> > - Applied your 5 patches (includes the zsmalloc: turn chain size config option
> >   into UL constant patch).  Took default value for ZSMALLOC_CHAIN_SIZE of 8.
> >   Performed echo, recreated issue.
> > 
> > - Changed ZSMALLOC_CHAIN_SIZE to 1.
> >   Perform echo, did not see issue
> 
> The patch set basically just adjusts $NUM in calculate_zspage_chain_size():
> 
> 		for (i = 1; i <= $NUM; i++)
> 
> It changes default 4 to 8. Can't really see how this can cause problems.

OK, I guess it overflows zspage isolated counter, which is a 3 bit
integer, so the max chain-size we can have is b111 == 7.

We probably need something like below (this should not increase sizeof
zspage):

---

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 290053e648b0..86b742a613ee 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -129,7 +129,7 @@
 #define HUGE_BITS      1
 #define FULLNESS_BITS  2
 #define CLASS_BITS     8
-#define ISOLATED_BITS  3
+#define ISOLATED_BITS  5
 #define MAGIC_VAL_BITS 8
 
 #define MAX(a, b) ((a) >= (b) ? (a) : (b))