linux-kernel - Re: [PATCH v10 16/33] mm: Add folio

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Date:   Tue, 18 May 2021 19:45:32 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     akpm@...ux-foundation.org
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Jeff Layton <jlayton@...nel.org>
Subject: Re: [PATCH v10 16/33] mm: Add folio_mapcount

On Tue, May 11, 2021 at 10:47:18PM +0100, Matthew Wilcox (Oracle) wrote:
> This is the folio equivalent of page_mapcount().
[...]
>  
> +/**
> + * folio_mapcount - The number of mappings of this folio.
> + * @folio: The folio.
> + *
> + * The result includes the number of times any of the pages in the
> + * folio are mapped to userspace.

I thought it did, but it doesn't.  It returns the number of times
the head/base page of this folio is mapped into userspace, which is not
a terribly useful concept.  I suspect this should call total_mapcount()
instead.  Looking through the complete set of patches, it's only used
in debugging code (unaccount_page_cache_page() and dump_page()).
I'm going to withdraw this patch from the next submission until I've
had the chance to think about it some more.