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Date: Wed, 11 Aug 2021 15:07:16 -0700
From: John Hubbard <jhubbard@...dia.com>
To: William Kucharski <william.kucharski@...cle.com>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Heiko Carstens <hca@...ux.ibm.com>,
Vasily Gorbik <gor@...ux.ibm.com>,
Christian Borntraeger <borntraeger@...ibm.com>,
LKML <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
Christoph Hellwig <hch@....de>,
Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 3/3] mm/gup: Remove try_get_page(), call
try_get_compound_head() directly
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@...dia.com> 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@....de>
>> Cc: Matthew Wilcox <willy@...radead.org>
>> Signed-off-by: John Hubbard <jhubbard@...dia.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;
>> -}
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
>>
>>
>
>
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