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Date:   Fri, 6 Apr 2018 00:03:50 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>, linux-mips@...ux-mips.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v2 3/3] mm/gup: document return value

__get_user_pages_fast handles errors differently from
get_user_pages_fast: the former always returns the number of pages
pinned, the later might return a negative error code.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 arch/mips/mm/gup.c  | 2 ++
 arch/s390/mm/gup.c  | 2 ++
 arch/sh/mm/gup.c    | 2 ++
 arch/sparc/mm/gup.c | 4 ++++
 mm/gup.c            | 4 +++-
 mm/util.c           | 6 ++++--
 6 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/mips/mm/gup.c b/arch/mips/mm/gup.c
index 1e4658e..5a4875ca 100644
--- a/arch/mips/mm/gup.c
+++ b/arch/mips/mm/gup.c
@@ -178,6 +178,8 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 /*
  * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
  * back to the regular GUP.
+ * Note a difference with get_user_pages_fast: this always returns the
+ * number of pages pinned, 0 if no pages were pinned.
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
diff --git a/arch/s390/mm/gup.c b/arch/s390/mm/gup.c
index 05c8abd..2809d11 100644
--- a/arch/s390/mm/gup.c
+++ b/arch/s390/mm/gup.c
@@ -220,6 +220,8 @@ static inline int gup_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr,
 /*
  * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
  * back to the regular GUP.
+ * Note a difference with get_user_pages_fast: this always returns the
+ * number of pages pinned, 0 if no pages were pinned.
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
diff --git a/arch/sh/mm/gup.c b/arch/sh/mm/gup.c
index 8045b5b..56c86ca 100644
--- a/arch/sh/mm/gup.c
+++ b/arch/sh/mm/gup.c
@@ -160,6 +160,8 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 /*
  * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
  * back to the regular GUP.
+ * Note a difference with get_user_pages_fast: this always returns the
+ * number of pages pinned, 0 if no pages were pinned.
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
diff --git a/arch/sparc/mm/gup.c b/arch/sparc/mm/gup.c
index 5335ba3..ca3eb69 100644
--- a/arch/sparc/mm/gup.c
+++ b/arch/sparc/mm/gup.c
@@ -192,6 +192,10 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
 	return 1;
 }
 
+/*
+ * Note a difference with get_user_pages_fast: this always returns the
+ * number of pages pinned, 0 if no pages were pinned.
+ */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
 {
diff --git a/mm/gup.c b/mm/gup.c
index 8f3a064..5cb5bb1 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1740,7 +1740,9 @@ bool gup_fast_permitted(unsigned long start, int nr_pages, int write)
 
 /*
  * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
- * the regular GUP. It will only return non-negative values.
+ * the regular GUP.
+ * Note a difference with get_user_pages_fast: this always returns the
+ * number of pages pinned, 0 if no pages were pinned.
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
diff --git a/mm/util.c b/mm/util.c
index c125050..db2f005 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -297,8 +297,10 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
 /*
  * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
  * back to the regular GUP.
- * If the architecture not support this function, simply return with no
- * page pinned
+ * Note a difference with get_user_pages_fast: this always returns the
+ * number of pages pinned, 0 if no pages were pinned.
+ * If the architecture does not support this function, simply return with no
+ * pages pinned.
  */
 int __weak __get_user_pages_fast(unsigned long start,
 				 int nr_pages, int write, struct page **pages)
-- 
MST

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