linux-kernel - Re: [PATCHv2 2/2] zram: drop max_zpage_size and use zs_huge_class

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Date:   Tue, 13 Mar 2018 23:18:13 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [PATCHv2 2/2] zram: drop max_zpage_size and use
 zs_huge_class_size()

On (03/13/18 22:58), Minchan Kim wrote:
> > > If it is static, we can do this in zram_init? I believe it's more readable in that
> > > it's never changed betweens zram instances.
> > 
> > We need to have at least one pool, because pool decides where the
> > watermark is. At zram_init() stage we don't have a pool yet. We
> > zs_create_pool() in zram_meta_alloc() so that's why I put
> > zs_huge_class_size() there. I'm not in love with it, but that's
> > the only place where we can have it.
> 
> Fair enough. Then what happens if client calls zs_huge_class_size
> without creating zs_create_pool?

Will receive 0.
One of the version was returning SIZE_MAX in such case.

size_t zs_huge_class_size(void)
 {
+	if (unlikely(!huge_class_size))
+		return SIZE_MAX;
 	return huge_class_size;
 }

> I think we should make zs_huge_class_size has a zs_pool as argument.

Can do, but the param will be unused. May be we can do something
like below instead:

 size_t zs_huge_class_size(void)
 {
+	if (unlikely(!huge_class_size))
+		return 3 * PAGE_SIZE / 4;
 	return huge_class_size;
 }

Should do no harm (unless I'm missing something).

	-ss