Re: [PATCH v6 08/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting

[Jan Kara]

On Mon 10-02-20 16:15:32, John Hubbard wrote:
> Now that pages are "DMA-pinned" via pin_user_page*(), and unpinned via
> unpin_user_pages*(), we need some visibility into whether all of this is
> working correctly.
> 
> Add two new fields to /proc/vmstat:
> 
>     nr_foll_pin_acquired
>     nr_foll_pin_released
> 
> These are documented in Documentation/core-api/pin_user_pages.rst.
> They represent the number of pages (since boot time) that have been
> pinned ("nr_foll_pin_acquired") and unpinned ("nr_foll_pin_released"),
> via pin_user_pages*() and unpin_user_pages*().
> 
> In the absence of long-running DMA or RDMA operations that hold pages
> pinned, the above two fields will normally be equal to each other.
> 
> Also: update Documentation/core-api/pin_user_pages.rst, to remove an
> earlier (now confirmed untrue) claim about a performance problem with
> /proc/vmstat.
> 
> Also: updated Documentation/core-api/pin_user_pages.rst to rename the
> new /proc/vmstat entries, to the names listed here.
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>

The patch looks good to me now. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  Documentation/core-api/pin_user_pages.rst | 33 +++++++++++++++++++----
>  include/linux/mmzone.h                    |  2 ++
>  mm/gup.c                                  | 13 +++++++++
>  mm/vmstat.c                               |  2 ++
>  4 files changed, 45 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
> index 7e5dd8b1b3f2..5c8a5f89756b 100644
> --- a/Documentation/core-api/pin_user_pages.rst
> +++ b/Documentation/core-api/pin_user_pages.rst
> @@ -208,12 +208,35 @@ has the following new calls to exercise the new pin*() wrapper functions:
>  You can monitor how many total dma-pinned pages have been acquired and released
>  since the system was booted, via two new /proc/vmstat entries: ::
>  
> -    /proc/vmstat/nr_foll_pin_requested
> -    /proc/vmstat/nr_foll_pin_requested
> +    /proc/vmstat/nr_foll_pin_acquired
> +    /proc/vmstat/nr_foll_pin_released
>  
> -Those are both going to show zero, unless CONFIG_DEBUG_VM is set. This is
> -because there is a noticeable performance drop in unpin_user_page(), when they
> -are activated.
> +Under normal conditions, these two values will be equal unless there are any
> +long-term [R]DMA pins in place, or during pin/unpin transitions.
> +
> +* nr_foll_pin_acquired: This is the number of logical pins that have been
> +  acquired since the system was powered on. For huge pages, the head page is
> +  pinned once for each page (head page and each tail page) within the huge page.
> +  This follows the same sort of behavior that get_user_pages() uses for huge
> +  pages: the head page is refcounted once for each tail or head page in the huge
> +  page, when get_user_pages() is applied to a huge page.
> +
> +* nr_foll_pin_released: The number of logical pins that have been released since
> +  the system was powered on. Note that pages are released (unpinned) on a
> +  PAGE_SIZE granularity, even if the original pin was applied to a huge page.
> +  Becaused of the pin count behavior described above in "nr_foll_pin_acquired",
> +  the accounting balances out, so that after doing this::
> +
> +    pin_user_pages(huge_page);
> +    for (each page in huge_page)
> +        unpin_user_page(page);
> +
> +...the following is expected::
> +
> +    nr_foll_pin_released == nr_foll_pin_acquired
> +
> +(...unless it was already out of balance due to a long-term RDMA pin being in
> +place.)
>  
>  References
>  ==========
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 462f6873905a..4bca42eeb439 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -243,6 +243,8 @@ enum node_stat_item {
>  	NR_DIRTIED,		/* page dirtyings since bootup */
>  	NR_WRITTEN,		/* page writings since bootup */
>  	NR_KERNEL_MISC_RECLAIMABLE,	/* reclaimable non-slab kernel pages */
> +	NR_FOLL_PIN_ACQUIRED,	/* via: pin_user_page(), gup flag: FOLL_PIN */
> +	NR_FOLL_PIN_RELEASED,	/* pages returned via unpin_user_page() */
>  	NR_VM_NODE_STAT_ITEMS
>  };
>  
> diff --git a/mm/gup.c b/mm/gup.c
> index 4d0d94405639..441f7a48f370 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -86,6 +86,8 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
>  	if (flags & FOLL_GET)
>  		return try_get_compound_head(page, refs);
>  	else if (flags & FOLL_PIN) {
> +		int orig_refs = refs;
> +
>  		/*
>  		 * When pinning a compound page of order > 1 (which is what
>  		 * hpage_pincount_available() checks for), use an exact count to
> @@ -104,6 +106,9 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
>  		if (hpage_pincount_available(page))
>  			hpage_pincount_add(page, refs);
>  
> +		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
> +				    orig_refs);
> +
>  		return page;
>  	}
>  
> @@ -158,6 +163,8 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
>  		 * 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;
> @@ -178,6 +185,7 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
>  
>  	count = page_ref_sub_return(page, refs);
>  
> +	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
>  	/*
>  	 * devmap page refcounts are 1-based, rather than 0-based: if
>  	 * refcount is 1, then the page is free and the refcount is
> @@ -228,6 +236,8 @@ void unpin_user_page(struct page *page)
>  
>  	if (page_ref_sub_and_test(page, refs))
>  		__put_page(page);
> +
> +	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
>  }
>  EXPORT_SYMBOL(unpin_user_page);
>  
> @@ -2259,6 +2269,9 @@ static int record_subpages(struct page *page, unsigned long addr,
>  static void put_compound_head(struct page *page, int refs, unsigned int flags)
>  {
>  	if (flags & FOLL_PIN) {
> +		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
> +				    refs);
> +
>  		if (hpage_pincount_available(page))
>  			hpage_pincount_sub(page, refs);
>  		else
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 78d53378db99..c9c0d71f917f 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1168,6 +1168,8 @@ const char * const vmstat_text[] = {
>  	"nr_dirtied",
>  	"nr_written",
>  	"nr_kernel_misc_reclaimable",
> +	"nr_foll_pin_acquired",
> +	"nr_foll_pin_released",
>  
>  	/* enum writeback_stat_item counters */
>  	"nr_dirty_threshold",
> -- 
> 2.25.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR