OK, here is v2 with changes as recommended by Christoph Hellwig and
Matthew Wilcox (thanks!):
* Changed refs to @refs, and added some more documentation as well.
* Completely removed try_get_page(). (I'm adding more people and lists
to Cc, because of those call site changes.)
* Reversed the logic in try_grab_page() to make it a touch more
readable.
Also, this has been rebased to today's linux-next (next-20210810), and
re-tested on that.
Here is the v1 cover letter, edited slightly to keep up with the latest
story.
While reviewing some of the other things going on around gup.c, I
noticed that the documentation was wrong for a few of the routines that
I wrote. And then I noticed that there was some significant code
duplication too. So this fixes those issues.
This is not entirely risk-free, but after looking closely at this, I
think it's actually a useful improvement, getting rid of the code
duplication here.
However, it is possible I've overlooked something. I did some local LTP
and other testing on an x86 test machine but failed to find any problems
yet.
And the original v1 is here:
https://lore.kernel.org/r/[email protected]
John Hubbard (3):
mm/gup: documentation corrections for gup/pup
mm/gup: small refactoring: simplify try_grab_page()
mm/gup: Remove try_get_page(), call try_get_compound_head() directly
arch/s390/mm/fault.c | 2 +-
fs/pipe.c | 2 +-
include/linux/mm.h | 10 +-----
mm/gup.c | 79 ++++++++++++++++++++++----------------------
4 files changed, 43 insertions(+), 50 deletions(-)
--
2.32.0
try_grab_page() does the same thing as try_grab_compound_head(...,
refs=1, ...), just with a different API. So there is a lot of code
duplication there.
Change try_grab_page() to call try_grab_compound_head(), while keeping
the API contract identical for callers.
Cc: Christoph Hellwig <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
---
mm/gup.c | 31 +++----------------------------
1 file changed, 3 insertions(+), 28 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 52f08e3177e9..64798d6b5043 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -208,35 +208,10 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
*/
bool __must_check try_grab_page(struct page *page, unsigned int flags)
{
- WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == (FOLL_GET | FOLL_PIN));
+ if (!(flags & (FOLL_GET | FOLL_PIN)))
+ return true;
- if (flags & FOLL_GET)
- return try_get_page(page);
- else if (flags & FOLL_PIN) {
- int refs = 1;
-
- page = compound_head(page);
-
- if (WARN_ON_ONCE(page_ref_count(page) <= 0))
- return false;
-
- if (hpage_pincount_available(page))
- hpage_pincount_add(page, 1);
- else
- refs = GUP_PIN_COUNTING_BIAS;
-
- /*
- * Similar to try_grab_compound_head(): even if using the
- * hpage_pincount_add/_sub() routines, be sure to
- * *also* increment the normal page refcount field at least
- * once, so that the page really is pinned.
- */
- page_ref_add(page, refs);
-
- mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
- }
-
- return true;
+ return try_grab_compound_head(page, 1, flags) != NULL;
}
/**
--
2.32.0
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 <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
---
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
The documentation for try_grab_compound_head() and try_grab_page() has
fallen a little out of date. Update and clarify a few points.
Also make it kerneldoc-correct, by adding @args documentation.
Cc: Christoph Hellwig <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
---
mm/gup.c | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 2630ed1bb4f4..52f08e3177e9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -92,10 +92,17 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
return head;
}
-/*
+/**
* try_grab_compound_head() - attempt to elevate a page's refcount, by a
* flags-dependent amount.
*
+ * 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 grabbed
+ * @refs: the value to (effectively) add to the page's refcount
+ * @flags: gup flags: these are the FOLL_* flag values.
+ *
* "grab" names in this file mean, "look at flags to decide whether to use
* FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
*
@@ -103,8 +110,14 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
* same time. (That's true throughout the get_user_pages*() and
* pin_user_pages*() APIs.) Cases:
*
- * FOLL_GET: page's refcount will be incremented by 1.
- * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ * FOLL_GET: page's refcount will be incremented by @refs.
+ *
+ * FOLL_PIN on compound pages that are > two pages long: page's refcount will
+ * be incremented by @refs, and page[2].hpage_pinned_refcount will be
+ * incremented by @refs * GUP_PIN_COUNTING_BIAS.
+ *
+ * FOLL_PIN on normal pages, or compound pages that are two pages long:
+ * page's refcount will be incremented by @refs * GUP_PIN_COUNTING_BIAS.
*
* Return: head page (with refcount appropriately incremented) for success, or
* NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
@@ -141,6 +154,8 @@ __maybe_unused struct page *try_grab_compound_head(struct page *page,
*
* However, be sure to *also* increment the normal page refcount
* field at least once, so that the page really is pinned.
+ * That's why the refcount from the earlier
+ * try_get_compound_head() is left intact.
*/
if (hpage_pincount_available(page))
hpage_pincount_add(page, refs);
@@ -184,10 +199,8 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
* @flags: gup flags: these are the FOLL_* flag values.
*
* Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
- * time. Cases:
- *
- * FOLL_GET: page's refcount will be incremented by 1.
- * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ * time. Cases: please see the try_grab_compound_head() documentation, with
+ * "refs=1".
*
* Return: true for success, or if no action was required (if neither FOLL_PIN
* nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
--
2.32.0
I agree that try_get_page() should probably be removed entirely; is there
a reason you didn't in v2 of the patch?
I'm also curious why you changed try_get_compound_head() into a routine
from an inline.
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 <[email protected]> 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 <[email protected]>
> Cc: Matthew Wilcox <[email protected]>
> Signed-off-by: John Hubbard <[email protected]>
> ---
> 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
>
>
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 <[email protected]> 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 <[email protected]>
>> Cc: Matthew Wilcox <[email protected]>
>> Signed-off-by: John Hubbard <[email protected]>
>> ---
>> 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
>>
>>
>
>
> On Aug 11, 2021, at 4:07 PM, John Hubbard <[email protected]> wrote:
>
> 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.
Ah, my bad. I was conflating it with try_grab_page() in patch 2/3, which
also seems like it should be an inline, but given your explanation re:
try_get_compound_head() it makes perfect sense.
For the series:
Reviewed-by: William Kucharski <[email protected]>
On Wed, Aug 11, 2021 at 12:05:42AM -0700, John Hubbard wrote:
> - if (unlikely(!try_get_page(page)))
> + if (unlikely(try_get_compound_head(page, 1) == NULL))
Why not a simple ! instead of the == NULL?
Otherwise looks good:
Reviewed-by: Christoph Hellwig <[email protected]>
Looks good,
Reviewed-by: Christoph Hellwig <[email protected]>
Note: the __maybe_unused on try_grab_compound_head should be dropped now
that there is always a user.
On 8/12/21 2:22 AM, Christoph Hellwig wrote:
> On Wed, Aug 11, 2021 at 12:05:42AM -0700, John Hubbard wrote:
>> - if (unlikely(!try_get_page(page)))
>> + if (unlikely(try_get_compound_head(page, 1) == NULL))
>
> Why not a simple ! instead of the == NULL?
I'll fix those up and post a v4.
>
> Otherwise looks good:
>
> Reviewed-by: Christoph Hellwig <[email protected]>
>
Thanks for the reviews!
thanks,
--
John Hubbard
NVIDIA
On 8/12/21 2:20 AM, Christoph Hellwig wrote:
> Looks good,
>
> Reviewed-by: Christoph Hellwig <[email protected]>
>
> Note: the __maybe_unused on try_grab_compound_head should be dropped now
> that there is always a user.
Good point, fixed in v3.
thanks,
--
John Hubbard
NVIDIA