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 KucharskiOn 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 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@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) 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; -}
This is where try_get_page() is removed entirely. thanks, -- John Hubbard NVIDIA
+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
