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    Subject[PATCH v2 09/28] gup: Turn hpage_pincount_add() into page_pincount_add()
    Date
    Simplify try_grab_compound_head() and remove an unnecessary
    VM_BUG_ON by handling pages both with and without a pincount field in
    page_pincount_add().

    Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    ---
    mm/gup.c | 33 +++++++++++++++------------------
    1 file changed, 15 insertions(+), 18 deletions(-)

    diff --git a/mm/gup.c b/mm/gup.c
    index dbb1b54d0def..3ed9907f3c8d 100644
    --- a/mm/gup.c
    +++ b/mm/gup.c
    @@ -29,12 +29,23 @@ struct follow_page_context {
    unsigned int page_mask;
    };

    -static void hpage_pincount_add(struct page *page, int refs)
    +/*
    + * When pinning a compound page of order > 1 (which is what
    + * hpage_pincount_available() checks for), use an exact count to track
    + * it, via page_pincount_add/_sub().
    + *
    + * 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.
    + */
    +static void page_pincount_add(struct page *page, int refs)
    {
    - VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
    VM_BUG_ON_PAGE(page != compound_head(page), page);

    - atomic_add(refs, compound_pincount_ptr(page));
    + if (hpage_pincount_available(page))
    + atomic_add(refs, compound_pincount_ptr(page));
    + else
    + page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
    }

    static void hpage_pincount_sub(struct page *page, int refs)
    @@ -150,21 +161,7 @@ struct page *try_grab_compound_head(struct page *page,
    if (!page)
    return NULL;

    - /*
    - * When pinning a compound page of order > 1 (which is what
    - * hpage_pincount_available() checks for), use an exact count to
    - * track it, via hpage_pincount_add/_sub().
    - *
    - * 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);
    - else
    - page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
    -
    + page_pincount_add(page, refs);
    mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
    refs);

    --
    2.33.0
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