The patch titled
Subject: mm/gup: documentation corrections for gup/pup
has been added to the -mm tree. Its filename is
mm-gup-documentation-corrections-for-gup-pup.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-gup-documentation-corrections-for-gup-pup.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-gup-documentation-corrections-for-gup-pup.patch
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next and is updated
there every 3-4 working days
------------------------------------------------------
From: John Hubbard <jhubbard@xxxxxxxxxx>
Subject: 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fallen a little out of date.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@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20210813044133.1536842-2-jhubbard@xxxxxxxxxx
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/gup.c | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)
--- a/mm/gup.c~mm-gup-documentation-corrections-for-gup-pup
+++ a/mm/gup.c
@@ -92,10 +92,17 @@ static inline struct page *try_get_compo
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_compo
* 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_com
*
* 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 pag
* @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
_
Patches currently in -mm which might be from jhubbard@xxxxxxxxxx are
mm-gup-documentation-corrections-for-gup-pup.patch
mm-gup-small-refactoring-simplify-try_grab_page.patch
mm-gup-remove-try_get_page-call-try_get_compound_head-directly.patch
mm-delete-unused-get_kernel_page.patch
