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Date:   Sat, 15 May 2021 21:14:13 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     William Kucharski <william.kucharski@oracle.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jeff Layton <jlayton@kernel.org>
Subject: Re: [PATCH v10 01/33] mm: Introduce struct folio
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References: <20210511214735.1836149-1-willy@infradead.org>
 <20210511214735.1836149-2-willy@infradead.org>
 <0FF7A37F-80A8-4B49-909D-6234ADA8A25C@oracle.com>
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On Sat, May 15, 2021 at 10:55:19AM +0000, William Kucharski wrote:
> > +/**
> > + * folio_page - Return a page from a folio.
> > + * @folio: The folio.
> > + * @n: The page number to return.
> > + *
> > + * @n is relative to the start of the folio.  It should be between
> > + * 0 and folio_nr_pages(@folio) - 1, but this is not checked for.
> 
> Please add a statement noting WHY @n isn't checked since you state it
> should be. Something like "...but this is not checked for because this is
> a hot path."

Hmm ... how about this:

/**
 * folio_page - Return a page from a folio.
 * @folio: The folio.
 * @n: The page number to return.
 *
 * @n is relative to the start of the folio.  This function does not
 * check that the page number lies within @folio; the caller is presumed
 * to have a reference to the page.
 */
#define folio_page(folio, n)    nth_page(&(folio)->page, n)

It occurred to me that it is actually useful (under some circumstances)
for referring to a page outside the base folio.  For example when
dealing with bios that have merged consecutive pages together into a
single bvec (ok, bios don't use folios, but it would be reasonable if
they did in future).