Re: [PATCH v2 3/3] mm/gup: Remove try_get_page(), call try_get_compound_head() directly

From: John Hubbard
Date: Wed Aug 11 2021 - 18:07:28 EST

On 8/11/21 1:35 PM, William Kucharski wrote:
I agree that try_get_page() should probably be removed entirely; is there
a reason you didn't in v2 of the patch?

Hi William,

This patch *does* remove try_get_page() entirely! Look below. I'll reply
inline, below, to show where that happens.

I'm also curious why you changed try_get_compound_head() into a routine
from an inline.

It was part of the change to make it available to callers outside of
gup.c. try_get_compound_head() is slightly messy and doesn't like to
live in mm.h, because it calls page_cache_add_speculative(), which lives
in linux-pagemap.h, which, in turn, has its own set of different headers
that it pulls in.

So, leaving it in gup.c, and exposing it to the other callers as a
non-static function, seemed appropriate here.

If you want to retain try_get_page() it should be an inline as well, especially
in its current implementation.

William Kucharski

On Aug 11, 2021, at 1:05 AM, John Hubbard <jhubbard@xxxxxxxxxx> wrote:

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

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

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@xxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
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)
page = phys_to_page(addr);
- if (unlikely(!try_get_page(page)))
+ if (unlikely(try_get_compound_head(page, 1) == NULL))
rc = arch_make_page_accessible(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;

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;

This is where try_get_page() is removed entirely.

John Hubbard

+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)

- * 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);