#include <linux/kernel.h>
#include <linux/fs.h>
+#include <linux/gfp.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/blkdev.h>
*/
/*
+ * Count contiguously cached pages from @offset-1 to @offset-@max,
+ * this count is a conservative estimation of
+ * - length of the sequential read sequence, or
+ * - thrashing threshold in memory tight systems
+ */
+static pgoff_t count_history_pages(struct address_space *mapping,
+ struct file_ra_state *ra,
+ pgoff_t offset, unsigned long max)
+{
+ pgoff_t head;
+
+ rcu_read_lock();
+ head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
+ rcu_read_unlock();
+
+ return offset - 1 - head;
+}
+
+/*
+ * page cache context based read-ahead
+ */
+static int try_context_readahead(struct address_space *mapping,
+ struct file_ra_state *ra,
+ pgoff_t offset,
+ unsigned long req_size,
+ unsigned long max)
+{
+ pgoff_t size;
+
+ size = count_history_pages(mapping, ra, offset, max);
+
+ /*
+ * no history pages:
+ * it could be a random read
+ */
+ if (!size)
+ return 0;
+
+ /*
+ * starts from beginning of file:
+ * it is a strong indication of long-run stream (or whole-file-read)
+ */
+ if (size >= offset)
+ size *= 2;
+
+ ra->start = offset;
+ ra->size = get_init_ra_size(size + req_size, max);
+ ra->async_size = ra->size;
+
+ return 1;
+}
+
+/*
* A minimal readahead algorithm for trivial sequential/random reads.
*/
static unsigned long
goto initial_readahead;
/*
+ * Query the page cache and look for the traces(cached history pages)
+ * that a sequential stream would leave behind.
+ */
+ if (try_context_readahead(mapping, ra, offset, req_size, max))
+ goto readit;
+
+ /*
* standalone, small random read
* Read as is, and do not pollute the readahead state.
*/
if (!ra->ra_pages)
return;
+ /* be dumb */
+ if (filp && (filp->f_mode & FMODE_RANDOM)) {
+ force_page_cache_readahead(mapping, filp, offset, req_size);
+ return;
+ }
+
/* do read-ahead */
ondemand_readahead(mapping, ra, filp, false, offset, req_size);
}
/* do read-ahead */
ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+
+#ifdef CONFIG_BLOCK
+ /*
+ * Normally the current page is !uptodate and lock_page() will be
+ * immediately called to implicitly unplug the device. However this
+ * is not always true for RAID conifgurations, where data arrives
+ * not strictly in their submission order. In this case we need to
+ * explicitly kick off the IO.
+ */
+ if (PageUptodate(page))
+ blk_run_backing_dev(mapping->backing_dev_info, NULL);
+#endif
}
EXPORT_SYMBOL_GPL(page_cache_async_readahead);