Re: [PATCH] MM: handle THP in swap_*page_fs() - count_vm_events()

From: "NeilBrown" <neilb@suse.de>
To: "Matthew Wilcox" <willy@infradead.org>
Cc: "Miaohe Lin" <linmiaohe@huawei.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Geert Uytterhoeven" <geert+renesas@glider.be>,
	"Christoph Hellwig" <hch@lst.de>,
	linux-nfs@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] MM: handle THP in swap_*page_fs() - count_vm_events()
Date: Mon, 02 May 2022 15:28:49 +1000	[thread overview]
Message-ID: <165146932944.24404.17790836056748683378@noble.neil.brown.name> (raw)
In-Reply-To: <Ym9pLhqtf61AVrZG@casper.infradead.org>

On Mon, 02 May 2022, Matthew Wilcox wrote:
> On Mon, May 02, 2022 at 02:57:46PM +1000, NeilBrown wrote:
> > @@ -390,9 +392,9 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
> >  			struct page *page = sio->bvec[p].bv_page;
> >  
> >  			SetPageUptodate(page);
> > +			count_swpout_vm_event(page);
> >  			unlock_page(page);
> >  		}
> > -		count_vm_events(PSWPIN, sio->pages);
> 
> Surely that should be count_swpIN_vm_event?
> 
I'm not having a good day....

Certainly shouldn't be swpout.  There isn't a count_swpin_vm_event().

swap_readpage() only counts once for each page no matter how big it is.
While swap_writepage() counts one for each PAGE_SIZE written.

And we have THP_SWPOUT but not THP_SWPIN

And I cannot find where any of these counters are documents, so I cannot
say what is "correct".

Well.... arch/s390/appldata/appldata_mem.c says
	u64 pswpin;		/* pages swapped in  */
	u64 pswpout;		/* pages swapped out */

but that isn't exactly unambiguous, and is for s390 which doesn't
support THP_SWAP

Ho hum.  I guess I put that back as it was.

Thanks for the review!!!

NeilBrown