On May 15, 2021, at 2:14 PM, Matthew Wilcox [off-list ref] wrote:

On Sat, May 15, 2021 at 10:55:19AM +0000, William Kucharski wrote:

quoted

quoted

+/**
+ * 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.