linux-kernel - Re: [PATCH] mm: kill frontswap

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Date:   Mon, 17 Jul 2023 17:51:05 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Nhat Pham <nphamcs@...il.com>,
        Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        konrad.wilk@...cle.com, vitaly.wool@...sulko.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: kill frontswap

On Mon, Jul 17, 2023 at 05:07:46PM +0100, Matthew Wilcox wrote:
> On Mon, Jul 17, 2023 at 12:02:27PM -0400, Johannes Weiner wrote:
> > -	pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s%s\n",
> > +	pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s\n",
> >  		p->pages<<(PAGE_SHIFT-10), name->name, p->prio,
> >  		nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10),
> >  		(p->flags & SWP_SOLIDSTATE) ? "SS" : "",
> >  		(p->flags & SWP_DISCARDABLE) ? "D" : "",
> >  		(p->flags & SWP_AREA_DISCARD) ? "s" : "",
> > -		(p->flags & SWP_PAGE_DISCARD) ? "c" : "",
> > -		(frontswap_map) ? "FS" : "");
> > +		(p->flags & SWP_PAGE_DISCARD) ? "c" : "");
> 
> Is it useful information (to the sysadmin) that zswap is enabled?  If so,
> we might want to print a "Z" instead of a "FS" here (would need to
> know whether enabling zswap succeeded ...)

I don't think it is needed in this particular place.

zswap prints to dmesg when it is initialized. And it's easy to runtime
check in sysfs or in /proc/meminfo if it's enabled - that's where I
would look first if I checked a system that's showing zswap issues -
not in the dmesg that may have long wrapped since swapon.

There is also a warning to dmesg if zswap fails to enable for a given
swapfile. But with the current page allocator implementation this
can't actually fail.