[v12,04/22] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages
diff mbox series

Message ID 20200107224558.2362728-5-jhubbard@nvidia.com
State New
Headers show
Series
  • mm/gup: prereqs to track dma-pinned pages: FOLL_PIN
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Commit Message

John Hubbard Jan. 7, 2020, 10:45 p.m. UTC
An upcoming patch changes and complicates the refcounting and
especially the "put page" aspects of it. In order to keep
everything clean, refactor the devmap page release routines:

* Rename put_devmap_managed_page() to page_is_devmap_managed(),
  and limit the functionality to "read only": return a bool,
  with no side effects.

* Add a new routine, put_devmap_managed_page(), to handle
  decrementing the refcount for ZONE_DEVICE pages.

* Change callers (just release_pages() and put_page()) to check
  page_is_devmap_managed() before calling the new
  put_devmap_managed_page() routine. This is a performance
  point: put_page() is a hot path, so we need to avoid non-
  inline function calls where possible.

* Rename __put_devmap_managed_page() to free_devmap_managed_page(),
  and limit the functionality to unconditionally freeing a devmap
  page.

This is originally based on a separate patch by Ira Weiny, which
applied to an early version of the put_user_page() experiments.
Since then, Jérôme Glisse suggested the refactoring described above.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Suggested-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h | 18 +++++++++++++-----
 mm/memremap.c      | 15 +--------------
 mm/swap.c          | 27 ++++++++++++++++++++++++++-
 3 files changed, 40 insertions(+), 20 deletions(-)

Comments

Christoph Hellwig Jan. 15, 2020, 3:23 p.m. UTC | #1
On Tue, Jan 07, 2020 at 02:45:40PM -0800, John Hubbard wrote:
> An upcoming patch changes and complicates the refcounting and
> especially the "put page" aspects of it. In order to keep
> everything clean, refactor the devmap page release routines:
> 
> * Rename put_devmap_managed_page() to page_is_devmap_managed(),
>   and limit the functionality to "read only": return a bool,
>   with no side effects.
> 
> * Add a new routine, put_devmap_managed_page(), to handle
>   decrementing the refcount for ZONE_DEVICE pages.
> 
> * Change callers (just release_pages() and put_page()) to check
>   page_is_devmap_managed() before calling the new
>   put_devmap_managed_page() routine. This is a performance
>   point: put_page() is a hot path, so we need to avoid non-
>   inline function calls where possible.
> 
> * Rename __put_devmap_managed_page() to free_devmap_managed_page(),
>   and limit the functionality to unconditionally freeing a devmap
>   page.
> 
> This is originally based on a separate patch by Ira Weiny, which
> applied to an early version of the put_user_page() experiments.
> Since then, Jérôme Glisse suggested the refactoring described above.

I'm really not sold on this scheme.  Note that I think it is
particularly bad, but it also doesn't seem any better than what
we had before, and it introduced quite a bit more code.
John Hubbard Jan. 15, 2020, 9:19 p.m. UTC | #2
On 1/15/20 7:23 AM, Christoph Hellwig wrote:
...
> 
> I'm really not sold on this scheme.  Note that I think it is
> particularly bad, but it also doesn't seem any better than what
> we had before, and it introduced quite a bit more code.
> 

Hi Christoph,

All by itself, yes. But the very next patch (which needs a little 
rework for other reasons, so not included here) needs to reuse some of 
these functions within __unpin_devmap_managed_user_page():

    page_is_devmap_managed()
    free_devmap_managed_page()

That patch was posted as part of the v11 series [1], and it did this:

+#ifdef CONFIG_DEV_PAGEMAP_OPS
+static bool __unpin_devmap_managed_user_page(struct page *page)
+{
+	int count;
+
+	if (!page_is_devmap_managed(page))
+		return false;
+
+	count = page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
+
+	__update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
+	/*
+	 * devmap page refcounts are 1-based, rather than 0-based: if
+	 * refcount is 1, then the page is free and the refcount is
+	 * stable because nobody holds a reference on the page.
+	 */
+	if (count == 1)
+		free_devmap_managed_page(page);
+	else if (!count)
+		__put_page(page);
+
+	return true;
+}
+#else
+static bool __unpin_devmap_managed_user_page(struct page *page)
+{
+	return false;
+}
+#endif /* CONFIG_DEV_PAGEMAP_OPS */
+
+/**
+ * unpin_user_page() - release a dma-pinned page
+ * @page:            pointer to page to be released
+ *
+ * Pages that were pinned via pin_user_pages*() must be released via either
+ * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
+ * that such pages can be separately tracked and uniquely handled. In
+ * particular, interactions with RDMA and filesystems need special handling.
+ */
+void unpin_user_page(struct page *page)
+{
+	page = compound_head(page);
+
+	/*
+	 * For devmap managed pages we need to catch refcount transition from
+	 * GUP_PIN_COUNTING_BIAS to 1, when refcount reach one it means the
+	 * page is free and we need to inform the device driver through
+	 * callback. See include/linux/memremap.h and HMM for details.
+	 */
+	if (__unpin_devmap_managed_user_page(page))
+		return;
+
+	if (page_ref_sub_and_test(page, GUP_PIN_COUNTING_BIAS))
+		__put_page(page);
+
+	__update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
+}
+EXPORT_SYMBOL(unpin_user_page);


[1] https://lore.kernel.org/r/20191216222537.491123-24-jhubbard@nvidia.com  
    [PATCH v11 23/25] mm/gup: track FOLL_PIN pages

thanks,
Christoph Hellwig Jan. 16, 2020, 9:37 a.m. UTC | #3
On Wed, Jan 15, 2020 at 01:19:41PM -0800, John Hubbard wrote:
> On 1/15/20 7:23 AM, Christoph Hellwig wrote:
> ...
> > 
> > I'm really not sold on this scheme.  Note that I think it is
> > particularly bad, but it also doesn't seem any better than what
> > we had before, and it introduced quite a bit more code.
> > 
> 
> Hi Christoph,
> 
> All by itself, yes. But the very next patch (which needs a little 
> rework for other reasons, so not included here) needs to reuse some of 
> these functions within __unpin_devmap_managed_user_page():

Well, then combine it with the series that actually does the change.

Also my vaguely recollection is that we had some idea on how to get rid
of the off by one refcounting for the zone device pages, which would be
a much better outcome.
John Hubbard Jan. 16, 2020, 8:30 p.m. UTC | #4
On 1/16/20 1:37 AM, Christoph Hellwig wrote:
> On Wed, Jan 15, 2020 at 01:19:41PM -0800, John Hubbard wrote:
>> On 1/15/20 7:23 AM, Christoph Hellwig wrote:
>> ...
>>>
>>> I'm really not sold on this scheme.  Note that I think it is
>>> particularly bad, but it also doesn't seem any better than what
>>> we had before, and it introduced quite a bit more code.
>>>
>>
>> Hi Christoph,
>>
>> All by itself, yes. But the very next patch (which needs a little
>> rework for other reasons, so not included here) needs to reuse some of
>> these functions within __unpin_devmap_managed_user_page():
> 
> Well, then combine it with the series that actually does the change.


OK, that makes sense. I just double-checked with a quick test run, that it
doesn't have dependencies with the rest of this series, and it came out clean,
so:

Andrew, could you please remove just this one patch from mmotm and linux-next?


> 
> Also my vaguely recollection is that we had some idea on how to get rid
> of the off by one refcounting for the zone device pages, which would be
> a much better outcome.
> 

Yes, I recall that Dan Williams mentioned it, but I don't think he provided
any details yet.


thanks,

Patch
diff mbox series

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 80a9162b406c..e2032ff640eb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -952,9 +952,10 @@  static inline bool is_zone_device_page(const struct page *page)
 #endif
 
 #ifdef CONFIG_DEV_PAGEMAP_OPS
-void __put_devmap_managed_page(struct page *page);
+void free_devmap_managed_page(struct page *page);
 DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
-static inline bool put_devmap_managed_page(struct page *page)
+
+static inline bool page_is_devmap_managed(struct page *page)
 {
 	if (!static_branch_unlikely(&devmap_managed_key))
 		return false;
@@ -963,7 +964,6 @@  static inline bool put_devmap_managed_page(struct page *page)
 	switch (page->pgmap->type) {
 	case MEMORY_DEVICE_PRIVATE:
 	case MEMORY_DEVICE_FS_DAX:
-		__put_devmap_managed_page(page);
 		return true;
 	default:
 		break;
@@ -971,11 +971,17 @@  static inline bool put_devmap_managed_page(struct page *page)
 	return false;
 }
 
+void put_devmap_managed_page(struct page *page);
+
 #else /* CONFIG_DEV_PAGEMAP_OPS */
-static inline bool put_devmap_managed_page(struct page *page)
+static inline bool page_is_devmap_managed(struct page *page)
 {
 	return false;
 }
+
+static inline void put_devmap_managed_page(struct page *page)
+{
+}
 #endif /* CONFIG_DEV_PAGEMAP_OPS */
 
 static inline bool is_device_private_page(const struct page *page)
@@ -1028,8 +1034,10 @@  static inline void put_page(struct page *page)
 	 * need to inform the device driver through callback. See
 	 * include/linux/memremap.h and HMM for details.
 	 */
-	if (put_devmap_managed_page(page))
+	if (page_is_devmap_managed(page)) {
+		put_devmap_managed_page(page);
 		return;
+	}
 
 	if (put_page_testzero(page))
 		__put_page(page);
diff --git a/mm/memremap.c b/mm/memremap.c
index f915d074ac20..4c723d2049d5 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -411,20 +411,8 @@  struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
 EXPORT_SYMBOL_GPL(get_dev_pagemap);
 
 #ifdef CONFIG_DEV_PAGEMAP_OPS
-void __put_devmap_managed_page(struct page *page)
+void free_devmap_managed_page(struct page *page)
 {
-	int count = page_ref_dec_return(page);
-
-	/* still busy */
-	if (count > 1)
-		return;
-
-	/* only triggered by the dev_pagemap shutdown path */
-	if (count == 0) {
-		__put_page(page);
-		return;
-	}
-
 	/* notify page idle for dax */
 	if (!is_device_private_page(page)) {
 		wake_up_var(&page->_refcount);
@@ -461,5 +449,4 @@  void __put_devmap_managed_page(struct page *page)
 	page->mapping = NULL;
 	page->pgmap->ops->page_free(page);
 }
-EXPORT_SYMBOL(__put_devmap_managed_page);
 #endif /* CONFIG_DEV_PAGEMAP_OPS */
diff --git a/mm/swap.c b/mm/swap.c
index 5341ae93861f..cf39d24ada2a 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -813,8 +813,10 @@  void release_pages(struct page **pages, int nr)
 			 * processing, and instead, expect a call to
 			 * put_page_testzero().
 			 */
-			if (put_devmap_managed_page(page))
+			if (page_is_devmap_managed(page)) {
+				put_devmap_managed_page(page);
 				continue;
+			}
 		}
 
 		page = compound_head(page);
@@ -1102,3 +1104,26 @@  void __init swap_setup(void)
 	 * _really_ don't want to cluster much more
 	 */
 }
+
+#ifdef CONFIG_DEV_PAGEMAP_OPS
+void put_devmap_managed_page(struct page *page)
+{
+	int count;
+
+	if (WARN_ON_ONCE(!page_is_devmap_managed(page)))
+		return;
+
+	count = page_ref_dec_return(page);
+
+	/*
+	 * devmap page refcounts are 1-based, rather than 0-based: if
+	 * refcount is 1, then the page is free and the refcount is
+	 * stable because nobody holds a reference on the page.
+	 */
+	if (count == 1)
+		free_devmap_managed_page(page);
+	else if (!count)
+		__put_page(page);
+}
+EXPORT_SYMBOL(put_devmap_managed_page);
+#endif