linux-kernel - Re: [PATCH v2] mm: count zeromap read and set for swapout and swapin

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Message-ID: <20241104163402.GA810664@cmpxchg.org>
Date: Mon, 4 Nov 2024 11:34:02 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: David Hildenbrand <david@...hat.com>
Cc: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Barry Song <v-songbaohua@...o.com>,
	Usama Arif <usamaarif642@...il.com>,
	Chengming Zhou <chengming.zhou@...ux.dev>,
	Yosry Ahmed <yosryahmed@...gle.com>, Nhat Pham <nphamcs@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Andi Kleen <ak@...ux.intel.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Chris Li <chrisl@...nel.org>, "Huang, Ying" <ying.huang@...el.com>,
	Kairui Song <kasong@...cent.com>,
	Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH v2] mm: count zeromap read and set for swapout and swapin

On Mon, Nov 04, 2024 at 01:42:08PM +0100, David Hildenbrand wrote:
> On 02.11.24 11:12, Barry Song wrote:
> > @@ -1599,6 +1599,16 @@ The following nested keys are defined.
> >   	  pglazyfreed (npn)
> >   		Amount of reclaimed lazyfree pages
> >   
> > +	  swpin_zero
> > +		Number of pages moved into memory with zero content, meaning no
> > +		copy exists in the backend swapfile, allowing swap-in to avoid
> > +		I/O read overhead.
> > +
> > +	  swpout_zero
> > +		Number of pages moved out of memory with zero content, meaning no
> > +		copy is needed in the backend swapfile, allowing swap-out to avoid
> > +		I/O write overhead.
> 
> Hm, can make it a bit clearer that this is a pure optimization and refer 
> to the other counters?
> 
> swpin_zero
> 	Portion of "pswpin" pages for which I/O was optimized out
> 	because the page content was detected to be zero during swapout.

AFAICS the zeropages currently don't show up in pswpin/pswpout, so
these are independent counters, not subsets.

I'm leaning towards Barry's side on the fixes tag. When zswap handled
the same-filled pages, we would count them in zswpin/out. From a user
POV, especially one using zswap, the behavior didn't change, but the
counts giving insight into this (potentially significant) VM activity
disappeared. This is arguably a regression.

> swpout_zero
> 	Portion of "pswout" pages for which I/O was optimized out
> 	because the page content was detected to be zero.

Are we sure we want to commit to the "zero" in the name here? Until
very recently, zswap optimized all same-filled pages. It's possible
somebody might want to bring that back down the line.

In reference to the above, I'd actually prefer putting them back into
zswpin/zswpout. Sure, they're not handled by zswap.c proper, but this
is arguably just an implementation detail; from a user POV this is
still just (a form of) compression in lieu of IO to the swap backend.

IMO there is no need for coming up with a separate category. Just add
them to zswpin/zswpout and remove the CONFIG_ZSWAP guards from them?