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    SubjectRe: [PATCH 26/94] Maple Tree: Add new data structure
    On Wed, Apr 28, 2021 at 03:36:02PM +0000, Liam Howlett wrote:
    > +/*
    > + * mas_set_alloc_req() - Set the requested number of allocations.
    > + * @mas: the maple state
    > + * @count: the number of allocations.
    > + *
    > + * If @mas->alloc has bit 1 set (0x1) or @mas->alloc is %NULL, then there are no
    > + * nodes allocated and @mas->alloc should be set to count << 1 | 1. If there is
    > + * already nodes allocated, then @mas->alloc->request_count stores the request.
    > + */
    > +static inline void mas_set_alloc_req(struct ma_state *mas, unsigned long count)
    > +{
    > + if (!mas->alloc || ((unsigned long)mas->alloc & 0x1)) {
    > + if (!count)
    > + mas->alloc = NULL;
    > + else
    > + mas->alloc = (struct maple_alloc *)(((count) << 1U) | 1U);
    > + return;
    > + }
    > +
    > + mas->alloc->request_count = count;
    > +}
    > +
    > +/*
    > + * mas_alloc_req() - get the requested number of allocations.
    > + * @mas: The maple state
    > + *
    > + * The alloc count is either stored directly in @mas, or in
    > + * @mas->alloc->request_count if there is at least one node allocated.
    > + *
    > + * Return: The allocation request count.
    > + */
    > +static inline unsigned int mas_alloc_req(const struct ma_state *mas)
    > +{
    > + if ((unsigned long)mas->alloc & 0x1)
    > + return (unsigned long)(mas->alloc) >> 1;
    > + else if (mas->alloc)
    > + return mas->alloc->request_count;
    > + return 0;
    > +}


    I'm confuse.. and the comments fail to eludicate *why* the code is the
    way it is, they simply explain exactly what the code does, which I can
    already tell from reading the code.

    Why can't we have ->request_count unconditionally be the requested
    count, and have ->alloc be NULL or not. Why do we play games with low
    pointer bits here? AFAICT there's no actual benefit to doing so.

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