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    Subject[PATCH v2 6/8] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting
    Date
    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_requested
    nr_foll_pin_returned

    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_requested") and unpinned ("nr_foll_pin_returned"),
    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.

    Signed-off-by: John Hubbard <jhubbard@nvidia.com>
    ---
    include/linux/mmzone.h | 2 ++
    mm/gup.c | 21 +++++++++++++++++++++
    mm/vmstat.c | 2 ++
    3 files changed, 25 insertions(+)

    diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
    index 462f6873905a..392868bc4763 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_REQUESTED, /* via: pin_user_page(), gup flag: FOLL_PIN */
    + NR_FOLL_PIN_RETURNED, /* pages returned via unpin_user_page() */
    NR_VM_NODE_STAT_ITEMS
    };

    diff --git a/mm/gup.c b/mm/gup.c
    index 03e7a5cfa6a9..d536bda383c4 100644
    --- a/mm/gup.c
    +++ b/mm/gup.c
    @@ -29,6 +29,19 @@ struct follow_page_context {
    unsigned int page_mask;
    };

    +#ifdef CONFIG_DEBUG_VM
    +static inline void __update_proc_vmstat(struct page *page,
    + enum node_stat_item item, int count)
    +{
    + mod_node_page_state(page_pgdat(page), item, count);
    +}
    +#else
    +static inline void __update_proc_vmstat(struct page *page,
    + enum node_stat_item item, int count)
    +{
    +}
    +#endif
    +
    static void hpage_pincount_add(struct page *page, int refs)
    {
    VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
    @@ -86,6 +99,8 @@ 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 +119,7 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
    if (hpage_pincount_available(page))
    hpage_pincount_add(page, refs);

    + __update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, orig_refs);
    return page;
    }

    @@ -159,6 +175,8 @@ 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);
    +
    + __update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, 1);
    }

    return true;
    @@ -179,6 +197,7 @@ static bool __unpin_devmap_managed_user_page(struct page *page)

    count = page_ref_sub_return(page, refs);

    + __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
    @@ -229,6 +248,8 @@ void unpin_user_page(struct page *page)

    if (page_ref_sub_and_test(page, refs))
    __put_page(page);
    +
    + __update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
    }
    EXPORT_SYMBOL(unpin_user_page);

    diff --git a/mm/vmstat.c b/mm/vmstat.c
    index 78d53378db99..b56808bae1b4 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_requested",
    + "nr_foll_pin_returned",

    /* enum writeback_stat_item counters */
    "nr_dirty_threshold",
    --
    2.25.0
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