linux-kernel - Re: [PATCH] mm/zsmalloc: don't need to save tag bit in handle

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Date: Tue, 27 Feb 2024 16:16:17 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: minchan@...nel.org, akpm@...ux-foundation.org, hannes@...xchg.org,
 nphamcs@...il.com, yosryahmed@...gle.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH] mm/zsmalloc: don't need to save tag bit in handle

On 2024/2/27 15:52, Sergey Senozhatsky wrote:
> On (24/02/27 03:00), chengming.zhou@...ux.dev wrote:
>>
>> We only need to save the position (pfn + obj_idx) in the handle, don't
>> need to save tag bit in handle. So one more bit can be used as obj_idx.
> 
> [..]
> 
>> mm/zsmalloc: don't need to save tag bit in handle
> 
> Does this mean "don't need to reserve LSB for tag"?
The head of object still need to reverve LSB, to save (handle | OBJ_ALLOCATED_TAG),
only the handle doesn't need to reserve LSB, which save (pfn | obj_idx).

> We still save allocated tag in the handle, that's what
> 
> 	handle |= OBJ_ALLOCATED_TAG;

Yes, this result will be saved in the head of each allocated object.

> 
> does.
> 
>> Actually, the tag bit is only useful in zspage's memory space, to tell
>> if an object is allocated or not.
> 
> I'm not completely sure if I follow this sentence.

What I mean is that only the head of each allocated object need to reverve LSB,
which is used to check if allocated or not.

handle address -> handle (pfn + obj_idx) -> object: (handle | tag), real_object start

I'm not sure if this makes it clearer?

Thanks.