linux-kernel - Re: [PATCH v2 02/25] mm: Optimise find

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Date:   Wed, 12 Feb 2020 09:52:50 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/25] mm: Optimise find_subpage for !THP

On Wed, Feb 12, 2020 at 05:02:00AM -0800, Matthew Wilcox wrote:
> > Can you add comments describing the use case of this function and why
> > it does all these checks?  It looks like black magic to me.
> 
> Would this help?
> 
> -static inline struct page *find_subpage(struct page *page, pgoff_t offset)
> +/*
> + * Given the page we found in the page cache, return the page corresponding
> + * to this offset in the file
> + */
> +static inline struct page *find_subpage(struct page *head, pgoff_t offset)
>  {
> -       if (PageHuge(page))
> -               return page;
> +       /* HugeTLBfs wants the head page regardless */
> +       if (PageHuge(head))
> +               return head;
>  
> -       VM_BUG_ON_PAGE(PageTail(page), page);
> +       VM_BUG_ON_PAGE(PageTail(head), head);
>  
> -       return page + (offset & (hpage_nr_pages(page) - 1));
> +       return head + (offset & (hpage_nr_pages(head) - 1));

Much better.