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From:   John Hubbard <jhubbard@nvidia.com>
To:     Andrew Morton <akpm@linux-foundation.org>
CC:     Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        <linux-s390@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Christoph Hellwig <hch@lst.de>,
        Matthew Wilcox <willy@infradead.org>
Subject: [PATCH v2 3/3] mm/gup: Remove try_get_page(), call try_get_compound_head() directly
Date:   Wed, 11 Aug 2021 00:05:42 -0700
Message-ID: <20210811070542.3403116-4-jhubbard@nvidia.com>
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try_get_page() is very similar to try_get_compound_head(), and in fact
try_get_page() has fallen a little behind in terms of maintenance:
try_get_compound_head() handles speculative page references more
thoroughly.

There are only two try_get_page() callsites, so just call
try_get_compound_head() directly from those, and remove try_get_page()
entirely.

Also, seeing as how this changes try_get_compound_head() into a
non-static function, provide some kerneldoc documentation for it.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 arch/s390/mm/fault.c |  2 +-
 fs/pipe.c            |  2 +-
 include/linux/mm.h   | 10 +---------
 mm/gup.c             | 21 +++++++++++++++++----
 4 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 212632d57db9..fe1d2c1dbe3b 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -822,7 +822,7 @@ void do_secure_storage_access(struct pt_regs *regs)
 		break;
 	case KERNEL_FAULT:
 		page = phys_to_page(addr);
-		if (unlikely(!try_get_page(page)))
+		if (unlikely(try_get_compound_head(page, 1) == NULL))
 			break;
 		rc = arch_make_page_accessible(page);
 		put_page(page);
diff --git a/fs/pipe.c b/fs/pipe.c
index 8e6ef62aeb1c..06ba9df37410 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -191,7 +191,7 @@ EXPORT_SYMBOL(generic_pipe_buf_try_steal);
  */
 bool generic_pipe_buf_get(struct pipe_inode_info *pipe, struct pipe_buffer *buf)
 {
-	return try_get_page(buf->page);
+	return try_get_compound_head(buf->page, 1) != NULL;
 }
 EXPORT_SYMBOL(generic_pipe_buf_get);
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ce8fc0fd6d6e..cd00d1222235 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1207,15 +1207,7 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags);
 __maybe_unused struct page *try_grab_compound_head(struct page *page, int refs,
 						   unsigned int flags);
 
-
-static inline __must_check bool try_get_page(struct page *page)
-{
-	page = compound_head(page);
-	if (WARN_ON_ONCE(page_ref_count(page) <= 0))
-		return false;
-	page_ref_inc(page);
-	return true;
-}
+struct page *try_get_compound_head(struct page *page, int refs);
 
 /**
  * folio_put - Decrement the reference count on a folio.
diff --git a/mm/gup.c b/mm/gup.c
index 64798d6b5043..c2d19d370c99 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -62,11 +62,24 @@ static void put_page_refs(struct page *page, int refs)
 	put_page(page);
 }
 
-/*
- * Return the compound head page with ref appropriately incremented,
- * or NULL if that failed.
+/**
+ * try_get_compound_head() - return the compound head page with refcount
+ * appropriately incremented, or NULL if that failed.
+ *
+ * This handles potential refcount overflow correctly. It also works correctly
+ * for various lockless get_user_pages()-related callers, due to the use of
+ * page_cache_add_speculative().
+ *
+ * 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 gotten
+ * @refs:  the value to add to the page's refcount
+ *
+ * Return: head page (with refcount appropriately incremented) for success, or
+ * NULL upon failure.
  */
-static inline struct page *try_get_compound_head(struct page *page, int refs)
+struct page *try_get_compound_head(struct page *page, int refs)
 {
 	struct page *head = compound_head(page);
 
-- 
2.32.0