linux-kernel - Re: [PATCH v2 16/25] iomap: Support large pages in read paths

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Message-ID: <20200212081340.GD24497@infradead.org>
Date:   Wed, 12 Feb 2020 00:13:40 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 16/25] iomap: Support large pages in read paths

> +/*
> + * Estimate the number of vectors we need based on the current page size;
> + * if we're wrong we'll end up doing an overly large allocation or needing
> + * to do a second allocation, neither of which is a big deal.
> + */
> +static unsigned int iomap_nr_vecs(struct page *page, loff_t length)
> +{
> +	return (length + thp_size(page) - 1) >> page_shift(page);
> +}

With the multipage bvecs a huge page (or any physically contigous piece
of memory) will always use one or less (if merged into the previous)
bio_vec.  So this can be simplified and optimized.