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Date:   Wed, 19 Feb 2020 06:41:17 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org,
        linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com,
        ocfs2-devel@oss.oracle.com, linux-xfs@vger.kernel.org
Subject: Re: [PATCH v6 07/19] mm: Put readahead pages in cache earlier
Message-ID: <20200219144117.GP24185@bombadil.infradead.org>
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 <20200217184613.19668-12-willy@infradead.org>
 <e3671faa-dfb3-ceba-3120-a445b2982a95@nvidia.com>
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On Tue, Feb 18, 2020 at 04:01:43PM -0800, John Hubbard wrote:
> How about this instead? It uses the "for" loop fully and more naturally,
> and is easier to read. And it does the same thing:
> 
> static inline struct page *readahead_page(struct readahead_control *rac)
> {
> 	struct page *page;
> 
> 	if (!rac->_nr_pages)
> 		return NULL;
> 
> 	page = xa_load(&rac->mapping->i_pages, rac->_start);
> 	VM_BUG_ON_PAGE(!PageLocked(page), page);
> 	rac->_batch_count = hpage_nr_pages(page);
> 
> 	return page;
> }
> 
> static inline struct page *readahead_next(struct readahead_control *rac)
> {
> 	rac->_nr_pages -= rac->_batch_count;
> 	rac->_start += rac->_batch_count;
> 
> 	return readahead_page(rac);
> }
> 
> #define readahead_for_each(rac, page)			\
> 	for (page = readahead_page(rac); page != NULL;	\
> 	     page = readahead_page(rac))

I'll go you one better ... how about we do this instead:

static inline struct page *readahead_page(struct readahead_control *rac)
{
        struct page *page;

        BUG_ON(rac->_batch_count > rac->_nr_pages);
        rac->_nr_pages -= rac->_batch_count;
        rac->_index += rac->_batch_count;
        rac->_batch_count = 0;

        if (!rac->_nr_pages)
                return NULL;

        page = xa_load(&rac->mapping->i_pages, rac->_index);
        VM_BUG_ON_PAGE(!PageLocked(page), page);
        rac->_batch_count = hpage_nr_pages(page);

        return page;
}

#define readahead_for_each(rac, page)                                   \
        while ((page = readahead_page(rac)))

No more readahead_next() to forget to add to filesystems which don't use
the readahead_for_each() iterator.  Ahem.