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    SubjectRe: [PATCH v2 10/28] gup: Turn hpage_pincount_sub() into page_pincount_sub()
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    On 1/9/22 20:23, Matthew Wilcox (Oracle) wrote:
    > Remove an unnecessary VM_BUG_ON by handling pages both with and without
    > a pincount field in page_pincount_sub().
    >
    > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    > ---
    > mm/gup.c | 15 +++++++--------
    > 1 file changed, 7 insertions(+), 8 deletions(-)
    >
    > diff --git a/mm/gup.c b/mm/gup.c
    > index 3ed9907f3c8d..aed48de3912e 100644
    > --- a/mm/gup.c
    > +++ b/mm/gup.c
    > @@ -48,12 +48,15 @@ static void page_pincount_add(struct page *page, int refs)
    > page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
    > }
    >
    > -static void hpage_pincount_sub(struct page *page, int refs)

    OK, this is a correct transformation. But it does make the function, as
    small as it is, rather hard to understand. It is now factored out at a
    strange place, as evidenced in the difficulty in documenting it. Here's
    my best attempt so far at documenting it here:

    /*
    * Subtract @refs from the compound_pincount, if applicable. Otherwise, do
    * nothing. And in all cases, return the number of pages that should be passed
    * to put_page_refs().
    */

    ...which I think is worth adding, if we can't find a better factoring.

    Either way, it's correct, so:


    Reviewed-by: John Hubbard <jhubbard@nvidia.com>


    thanks,
    --
    John Hubbard
    NVIDIA

    > +static int page_pincount_sub(struct page *page, int refs)
    > {
    > - VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
    > VM_BUG_ON_PAGE(page != compound_head(page), page);
    >
    > - atomic_sub(refs, compound_pincount_ptr(page));
    > + if (hpage_pincount_available(page))
    > + atomic_sub(refs, compound_pincount_ptr(page));
    > + else
    > + refs *= GUP_PIN_COUNTING_BIAS;
    > + return refs;
    > }
    >
    > /* Equivalent to calling put_page() @refs times. */
    > @@ -177,11 +180,7 @@ 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
    > - refs *= GUP_PIN_COUNTING_BIAS;
    > + refs = page_pincount_sub(page, refs);
    > }
    >
    > put_page_refs(page, refs);

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