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mm/gup: documentation corrections for gup/pup

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Patch series "A few gup refactorings and documentation updates", v3.

While reviewing some of the other things going on around gup.c, I noticed
that the documentation was wrong for a few of the routines that I wrote.
And then I noticed that there was some significant code duplication too.
So this fixes those issues.

This is not entirely risk-free, but after looking closely at this, I think
it's actually a useful improvement, getting rid of the code duplication
here.

This patch (of 3):

The documentation for try_grab_compound_head() and try_grab_page() has
fallen a little out of date.  Update and clarify a few points.

Also make it kerneldoc-correct, by adding @args documentation.

Link: https://lkml.kernel.org/r/20210813044133.1536842-1-jhubbard@nvidia.com
Link: https://lkml.kernel.org/r/20210813044133.1536842-2-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
John Hubbard 2021-09-02 14:53:48 -07:00 committed by Linus Torvalds
parent be51eb18b8
commit 3967db22ba
1 changed files with 20 additions and 7 deletions
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27
mm/gup.c
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@ -92,10 +92,17 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
return head;
}
/*
/**
* try_grab_compound_head() - attempt to elevate a page's refcount, by a
* flags-dependent amount.
*
* Even though the name includes "compound_head", this function is still
* appropriate for callers that have a non-compound @page to get.
*
* @page: pointer to page to be grabbed
* @refs: the value to (effectively) add to the page's refcount
* @flags: gup flags: these are the FOLL_* flag values.
*
* "grab" names in this file mean, "look at flags to decide whether to use
* FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
*
@ -103,8 +110,14 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
* same time. (That's true throughout the get_user_pages*() and
* pin_user_pages*() APIs.) Cases:
*
* FOLL_GET: page's refcount will be incremented by 1.
* FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
* FOLL_GET: page's refcount will be incremented by @refs.
*
* FOLL_PIN on compound pages that are > two pages long: page's refcount will
* be incremented by @refs, and page[2].hpage_pinned_refcount will be
* incremented by @refs * GUP_PIN_COUNTING_BIAS.
*
* FOLL_PIN on normal pages, or compound pages that are two pages long:
* page's refcount will be incremented by @refs * GUP_PIN_COUNTING_BIAS.
*
* Return: head page (with refcount appropriately incremented) for success, or
* NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
@ -141,6 +154,8 @@ __maybe_unused struct page *try_grab_compound_head(struct page *page,
*
* However, be sure to *also* increment the normal page refcount
* field at least once, so that the page really is pinned.
* That's why the refcount from the earlier
* try_get_compound_head() is left intact.
*/
if (hpage_pincount_available(page))
hpage_pincount_add(page, refs);
@ -184,10 +199,8 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
* @flags: gup flags: these are the FOLL_* flag values.
*
* Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
* time. Cases:
*
* FOLL_GET: page's refcount will be incremented by 1.
* FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
* time. Cases: please see the try_grab_compound_head() documentation, with
* "refs=1".
*
* Return: true for success, or if no action was required (if neither FOLL_PIN
* nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or