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Message-ID: <20200212091743.GC25573@quack2.suse.cz>
Date: Wed, 12 Feb 2020 10:17:43 +0100
From: Jan Kara <jack@...e.cz>
To: John Hubbard <jhubbard@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
Christoph Hellwig <hch@...radead.org>,
Dan Williams <dan.j.williams@...el.com>,
Dave Chinner <david@...morbit.com>,
Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
Jason Gunthorpe <jgg@...pe.ca>,
Jonathan Corbet <corbet@....net>,
Jérôme Glisse <jglisse@...hat.com>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Michal Hocko <mhocko@...e.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Shuah Khan <shuah@...nel.org>,
Vlastimil Babka <vbabka@...e.cz>,
Matthew Wilcox <willy@...radead.org>,
linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-kselftest@...r.kernel.org, linux-rdma@...r.kernel.org,
linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v6 08/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN)
reporting
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@...ux.intel.com>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
The patch looks good to me now. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>
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@...e.com>
SUSE Labs, CR
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