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authorJohn Hubbard <jhubbard@nvidia.com>2021-09-03 00:53:48 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2021-09-03 19:58:11 +0300
commit3967db22ba324939762f618f0d654b13317ca7a4 (patch)
tree258356f37a3ac5e5c14dfafba45c487cf076a661 /mm/gup.c
parentbe51eb18b81b2f0a0a562bac0f6fe0f4b05c439e (diff)
downloadlinux-3967db22ba324939762f618f0d654b13317ca7a4.tar.xz
mm/gup: documentation corrections for gup/pup
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 This patch (of 3): The documentation for try_grab_compound_head() and try_grab_page() has fallen a little out of date.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>
Diffstat (limited to 'mm/gup.c')
-rw-r--r--mm/gup.c27
1 files changed, 20 insertions, 7 deletions
diff --git a/mm/gup.c b/mm/gup.c
index 1973bfe59b2e..26ce6bb52044 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -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