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

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Date: Wed, 28 Feb 2024 10:54:37 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Chengming Zhou <chengming.zhou@...ux.dev>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 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 (24/02/27 16:16), Chengming Zhou wrote:
> 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).

Correct.

> > 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.

Right, that's what I was talking about.

> >> 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?

Yes, thanks. I think separating handle and object header in the commit
message will be helpful.