Re: [PATCH RFC 4/7] zram: Introduce recompress sysfs knob

From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Barry Song <21cnbao@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
	Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Nitin Gupta <ngupta@vflare.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH RFC 4/7] zram: Introduce recompress sysfs knob
Date: Mon, 5 Sep 2022 18:53:25 +0900	[thread overview]
Message-ID: <YxXHFZ1PJI3h6BZY@google.com> (raw)
In-Reply-To: <CAGsJ_4yciyk49cTk9A4di2sM=io-4z5oi4Q7P-SUOG-MGJGcQg@mail.gmail.com>

On (22/09/05 21:21), Barry Song wrote:
> > 3) HUGE pages recompression is activated by `huge` mode
> >
> >         echo huge > /sys/block/zram0/recompress
> 
> Thanks for developing this interesting feature. It seems reasonable for cold
> pages. But for a huge page,  do you have some data to show that the hugepage
> is not compressed by lzo/lz4 so we need zstd further? i assume the size of
> the huge page you are talking about is 2MB?

Oh, yeah, this is the lingo we use in zram. We used "huge" object and "huge"
size class in zsmalloc and the term "huge" transitioned to zram, but zram
operates with pages not objects, so huge zsmalloc object is "huge zram page".

> on second thoughts, it seems you mean hugepage is those pages
> whose compressed data is big? if so, can you please avoid using
> "huge page" as it is quite misleading in linux. we are using hugepage
> for pages larger than 4KB.

Yes, you are right. And I wish I could use a different term, but...
this is what zram has been using for many years:
Documentation/admin-guide/blockdev/zram.rst

And we already accept "huge" and "huge pages", and so, in sysfs knobs
(zram device attrs), so the confusing term, unfortunately, is there
forever.