linux-kernel - Re: [PATCH v1 2/3] mm: Implement folio_remove_rmap

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Date:   Mon, 17 Jul 2023 16:56:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 2/3] mm: Implement folio_remove_rmap_range()

On Mon, Jul 17, 2023 at 04:49:19PM +0100, Ryan Roberts wrote:
> > We're still doing one atomic op per page on the folio's nr_pages_mapped
> > ... is it possible to batch this and use atomic_sub_return_relaxed()?
> 
> Good spot, something like this:
> 
> 	} else {
> 		for (; nr != 0; nr--, page++) {
> 			/* Is this the page's last map to be removed? */
> 			last = atomic_add_negative(-1, &page->_mapcount);
> 			if (last)
> 				nr_unmapped++;
> 		}
> 
> 		/* Pages still mapped if folio mapped entirely */
> 		nr_mapped = atomic_sub_return_relaxed(nr_unmapped, mapped);
> 		if (nr_mapped >= COMPOUND_MAPPED)
> 			nr_unmapped = 0;
> 	}

I think that's right, but my eyes always go slightly crossed trying to
read the new mapcount scheme.