linux-kernel - Re: [PATCH v10 01/33] mm: Introduce struct folio

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Date:   Sun, 16 May 2021 19:26:53 +0000
From:   William Kucharski <william.kucharski@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jeff Layton <jlayton@...nel.org>
Subject: Re: [PATCH v10 01/33] mm: Introduce struct folio



> On May 15, 2021, at 2:14 PM, Matthew Wilcox <willy@...radead.org> wrote:
> 
> 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).

I like that comment better, or you could just state bounds checking of
the returned page number is left to the caller; that would cover both the
normal case and possible future usage for calculations outside the base
folio.