linux-kernel - Re: [PATCH v1 1/2] zsmalloc: add allocated objects counter for subpage

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Message-ID: <20230620111635.gztldehfzvuzkdnj@cab-wsm-0029881>
Date:   Tue, 20 Jun 2023 11:16:36 +0000
From:   Alexey Romanov <AVRomanov@...rdevices.ru>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
CC:     "minchan@...nel.org" <minchan@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>
Subject: Re: [PATCH v1 1/2] zsmalloc: add allocated objects counter for
 subpage

Hello Sergey! Thank you for feedback.

On Tue, Jun 20, 2023 at 07:36:29PM +0900, Sergey Senozhatsky wrote:
> On (23/06/19 17:35), Alexey Romanov wrote:
> > 
> > We use a variable of type unsigned int to store the offset
> > of the first object at the subpage In turn, the offset cannot
> > exceed the size of PAGE_SIZE, which is usually 4096. Thus,
> > 12 bits are enough to store the offset.
> 
> [..]
> 
> > If the page size is 4Kb
> 
> It's certainly not a given. PAGE_SIZE is architecture specific.
> PAGE_SIZE_16KB and PAGE_SIZE_64KB would be simple examples, but there
> are, apparently, architectures that even can have PAGE_SIZE_256KB.

Sure. 

As far I understand at the moment the maximum size of the page
(according to Kconfig info in linux sources) is 256Kb. In this case,
we need maximum 18 bits for storing offset. And 2^18 / 32 = 8192 bytes
(13 bits, I think u16 is OK for such purpose) for storing allocated
objects counter. 

If sizeof(unsigned int) >= 32 bits the this will be enough for us. 
Of course, in rare cases this will not be the case. But it seems that
zram and kernel already has similiar places. For example, if page size
is 256 Kb and sizeof(unsigned int) = 16 bits (2 byte), zram will not
wotk on such system, because we can't store offset. But such case is
very rare, most systems have unsigned int over 32 bits. 

Therefore, I think that my idea is still applicable, we just need to
change the counter type. What do you think?

-- 
Thank you,
Alexey