diff --git a/src/kernel/linux/v4.19/mm/readahead.c b/src/kernel/linux/v4.19/mm/readahead.c
new file mode 100644
index 0000000..4e63014
--- /dev/null
+++ b/src/kernel/linux/v4.19/mm/readahead.c
@@ -0,0 +1,608 @@
+/*
+ * mm/readahead.c - address_space-level file readahead.
+ *
+ * Copyright (C) 2002, Linus Torvalds
+ *
+ * 09Apr2002	Andrew Morton
+ *		Initial version.
+ */
+
+#include <linux/kernel.h>
+#include <linux/dax.h>
+#include <linux/gfp.h>
+#include <linux/export.h>
+#include <linux/blkdev.h>
+#include <linux/backing-dev.h>
+#include <linux/task_io_accounting_ops.h>
+#include <linux/pagevec.h>
+#include <linux/pagemap.h>
+#include <linux/syscalls.h>
+#include <linux/file.h>
+#include <linux/mm_inline.h>
+#include <linux/blk-cgroup.h>
+#include <linux/fadvise.h>
+
+#include "internal.h"
+
+/*
+ * Initialise a struct file's readahead state.  Assumes that the caller has
+ * memset *ra to zero.
+ */
+void
+file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
+{
+	ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
+	ra->prev_pos = -1;
+}
+EXPORT_SYMBOL_GPL(file_ra_state_init);
+
+/*
+ * see if a page needs releasing upon read_cache_pages() failure
+ * - the caller of read_cache_pages() may have set PG_private or PG_fscache
+ *   before calling, such as the NFS fs marking pages that are cached locally
+ *   on disk, thus we need to give the fs a chance to clean up in the event of
+ *   an error
+ */
+static void read_cache_pages_invalidate_page(struct address_space *mapping,
+					     struct page *page)
+{
+	if (page_has_private(page)) {
+		if (!trylock_page(page))
+			BUG();
+		page->mapping = mapping;
+		do_invalidatepage(page, 0, PAGE_SIZE);
+		page->mapping = NULL;
+		unlock_page(page);
+	}
+	put_page(page);
+}
+
+/*
+ * release a list of pages, invalidating them first if need be
+ */
+static void read_cache_pages_invalidate_pages(struct address_space *mapping,
+					      struct list_head *pages)
+{
+	struct page *victim;
+
+	while (!list_empty(pages)) {
+		victim = lru_to_page(pages);
+		list_del(&victim->lru);
+		read_cache_pages_invalidate_page(mapping, victim);
+	}
+}
+
+/**
+ * read_cache_pages - populate an address space with some pages & start reads against them
+ * @mapping: the address_space
+ * @pages: The address of a list_head which contains the target pages.  These
+ *   pages have their ->index populated and are otherwise uninitialised.
+ * @filler: callback routine for filling a single page.
+ * @data: private data for the callback routine.
+ *
+ * Hides the details of the LRU cache etc from the filesystems.
+ */
+int read_cache_pages(struct address_space *mapping, struct list_head *pages,
+			int (*filler)(void *, struct page *), void *data)
+{
+	struct page *page;
+	int ret = 0;
+
+	while (!list_empty(pages)) {
+		page = lru_to_page(pages);
+		list_del(&page->lru);
+		if (add_to_page_cache_lru(page, mapping, page->index,
+				readahead_gfp_mask(mapping))) {
+			read_cache_pages_invalidate_page(mapping, page);
+			continue;
+		}
+		put_page(page);
+
+		ret = filler(data, page);
+		if (unlikely(ret)) {
+			read_cache_pages_invalidate_pages(mapping, pages);
+			break;
+		}
+		task_io_account_read(PAGE_SIZE);
+	}
+	return ret;
+}
+
+EXPORT_SYMBOL(read_cache_pages);
+
+static int read_pages(struct address_space *mapping, struct file *filp,
+		struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
+{
+	struct blk_plug plug;
+	unsigned page_idx;
+	int ret;
+
+	blk_start_plug(&plug);
+
+	if (mapping->a_ops->readpages) {
+		ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
+		/* Clean up the remaining pages */
+		put_pages_list(pages);
+		goto out;
+	}
+
+	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
+		struct page *page = lru_to_page(pages);
+		list_del(&page->lru);
+		if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
+			mapping->a_ops->readpage(filp, page);
+		put_page(page);
+	}
+	ret = 0;
+
+out:
+	blk_finish_plug(&plug);
+
+	return ret;
+}
+
+/*
+ * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates
+ * the pages first, then submits them for I/O. This avoids the very bad
+ * behaviour which would occur if page allocations are causing VM writeback.
+ * We really don't want to intermingle reads and writes like that.
+ *
+ * Returns the number of pages requested, or the maximum amount of I/O allowed.
+ */
+unsigned int __do_page_cache_readahead(struct address_space *mapping,
+		struct file *filp, pgoff_t offset, unsigned long nr_to_read,
+		unsigned long lookahead_size)
+{
+	struct inode *inode = mapping->host;
+	struct page *page;
+	unsigned long end_index;	/* The last page we want to read */
+	LIST_HEAD(page_pool);
+	int page_idx;
+	unsigned int nr_pages = 0;
+	loff_t isize = i_size_read(inode);
+	gfp_t gfp_mask = readahead_gfp_mask(mapping);
+
+	if (isize == 0)
+		goto out;
+
+	end_index = ((isize - 1) >> PAGE_SHIFT);
+
+	/*
+	 * Preallocate as many pages as we will need.
+	 */
+	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
+		pgoff_t page_offset = offset + page_idx;
+
+		if (page_offset > end_index)
+			break;
+
+		rcu_read_lock();
+		page = radix_tree_lookup(&mapping->i_pages, page_offset);
+		rcu_read_unlock();
+		if (page && !radix_tree_exceptional_entry(page)) {
+			/*
+			 * Page already present?  Kick off the current batch of
+			 * contiguous pages before continuing with the next
+			 * batch.
+			 */
+			if (nr_pages)
+				read_pages(mapping, filp, &page_pool, nr_pages,
+						gfp_mask);
+			nr_pages = 0;
+			continue;
+		}
+
+		page = __page_cache_alloc(gfp_mask);
+		if (!page)
+			break;
+		page->index = page_offset;
+		list_add(&page->lru, &page_pool);
+		if (page_idx == nr_to_read - lookahead_size)
+			SetPageReadahead(page);
+		nr_pages++;
+	}
+
+	/*
+	 * Now start the IO.  We ignore I/O errors - if the page is not
+	 * uptodate then the caller will launch readpage again, and
+	 * will then handle the error.
+	 */
+	if (nr_pages)
+		read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
+	BUG_ON(!list_empty(&page_pool));
+out:
+	return nr_pages;
+}
+
+/*
+ * Chunk the readahead into 2 megabyte units, so that we don't pin too much
+ * memory at once.
+ */
+int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
+			       pgoff_t offset, unsigned long nr_to_read)
+{
+	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
+	struct file_ra_state *ra = &filp->f_ra;
+	unsigned long max_pages;
+
+	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
+		return -EINVAL;
+
+	/*
+	 * If the request exceeds the readahead window, allow the read to
+	 * be up to the optimal hardware IO size
+	 */
+	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
+	nr_to_read = min(nr_to_read, max_pages);
+	while (nr_to_read) {
+		unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
+
+		if (this_chunk > nr_to_read)
+			this_chunk = nr_to_read;
+		__do_page_cache_readahead(mapping, filp, offset, this_chunk, 0);
+
+		offset += this_chunk;
+		nr_to_read -= this_chunk;
+	}
+	return 0;
+}
+
+/*
+ * Set the initial window size, round to next power of 2 and square
+ * for small size, x 4 for medium, and x 2 for large
+ * for 128k (32 page) max ra
+ * 1-8 page = 32k initial, > 8 page = 128k initial
+ */
+static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
+{
+	unsigned long newsize = roundup_pow_of_two(size);
+
+	if (newsize <= max / 32)
+		newsize = newsize * 4;
+	else if (newsize <= max / 4)
+		newsize = newsize * 2;
+	else
+		newsize = max;
+
+	return newsize;
+}
+
+/*
+ *  Get the previous window size, ramp it up, and
+ *  return it as the new window size.
+ */
+static unsigned long get_next_ra_size(struct file_ra_state *ra,
+						unsigned long max)
+{
+	unsigned long cur = ra->size;
+	unsigned long newsize;
+
+	if (cur < max / 16)
+		newsize = 4 * cur;
+	else
+		newsize = 2 * cur;
+
+	return min(newsize, max);
+}
+
+/*
+ * On-demand readahead design.
+ *
+ * The fields in struct file_ra_state represent the most-recently-executed
+ * readahead attempt:
+ *
+ *                        |<----- async_size ---------|
+ *     |------------------- size -------------------->|
+ *     |==================#===========================|
+ *     ^start             ^page marked with PG_readahead
+ *
+ * To overlap application thinking time and disk I/O time, we do
+ * `readahead pipelining': Do not wait until the application consumed all
+ * readahead pages and stalled on the missing page at readahead_index;
+ * Instead, submit an asynchronous readahead I/O as soon as there are
+ * only async_size pages left in the readahead window. Normally async_size
+ * will be equal to size, for maximum pipelining.
+ *
+ * In interleaved sequential reads, concurrent streams on the same fd can
+ * be invalidating each other's readahead state. So we flag the new readahead
+ * page at (start+size-async_size) with PG_readahead, and use it as readahead
+ * indicator. The flag won't be set on already cached pages, to avoid the
+ * readahead-for-nothing fuss, saving pointless page cache lookups.
+ *
+ * prev_pos tracks the last visited byte in the _previous_ read request.
+ * It should be maintained by the caller, and will be used for detecting
+ * small random reads. Note that the readahead algorithm checks loosely
+ * for sequential patterns. Hence interleaved reads might be served as
+ * sequential ones.
+ *
+ * There is a special-case: if the first page which the application tries to
+ * read happens to be the first page of the file, it is assumed that a linear
+ * read is about to happen and the window is immediately set to the initial size
+ * based on I/O request size and the max_readahead.
+ *
+ * The code ramps up the readahead size aggressively at first, but slow down as
+ * it approaches max_readhead.
+ */
+
+/*
+ * 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,
+				   pgoff_t offset, unsigned long max)
+{
+	pgoff_t head;
+
+	rcu_read_lock();
+	head = page_cache_prev_hole(mapping, 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, offset, max);
+
+	/*
+	 * not enough history pages:
+	 * it could be a random read
+	 */
+	if (size <= req_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 = min(size + req_size, max);
+	ra->async_size = 1;
+
+	return 1;
+}
+
+/*
+ * A minimal readahead algorithm for trivial sequential/random reads.
+ */
+static unsigned long
+ondemand_readahead(struct address_space *mapping,
+		   struct file_ra_state *ra, struct file *filp,
+		   bool hit_readahead_marker, pgoff_t offset,
+		   unsigned long req_size)
+{
+	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
+	unsigned long max_pages = ra->ra_pages;
+	unsigned long add_pages;
+	pgoff_t prev_offset;
+
+	/*
+	 * If the request exceeds the readahead window, allow the read to
+	 * be up to the optimal hardware IO size
+	 */
+	if (req_size > max_pages && bdi->io_pages > max_pages)
+		max_pages = min(req_size, bdi->io_pages);
+
+	/*
+	 * start of file
+	 */
+	if (!offset)
+		goto initial_readahead;
+
+	/*
+	 * It's the expected callback offset, assume sequential access.
+	 * Ramp up sizes, and push forward the readahead window.
+	 */
+	if ((offset == (ra->start + ra->size - ra->async_size) ||
+	     offset == (ra->start + ra->size))) {
+		ra->start += ra->size;
+		ra->size = get_next_ra_size(ra, max_pages);
+		ra->async_size = ra->size;
+		goto readit;
+	}
+
+	/*
+	 * Hit a marked page without valid readahead state.
+	 * E.g. interleaved reads.
+	 * Query the pagecache for async_size, which normally equals to
+	 * readahead size. Ramp it up and use it as the new readahead size.
+	 */
+	if (hit_readahead_marker) {
+		pgoff_t start;
+
+		rcu_read_lock();
+		start = page_cache_next_hole(mapping, offset + 1, max_pages);
+		rcu_read_unlock();
+
+		if (!start || start - offset > max_pages)
+			return 0;
+
+		ra->start = start;
+		ra->size = start - offset;	/* old async_size */
+		ra->size += req_size;
+		ra->size = get_next_ra_size(ra, max_pages);
+		ra->async_size = ra->size;
+		goto readit;
+	}
+
+	/*
+	 * oversize read
+	 */
+	if (req_size > max_pages)
+		goto initial_readahead;
+
+	/*
+	 * sequential cache miss
+	 * trivial case: (offset - prev_offset) == 1
+	 * unaligned reads: (offset - prev_offset) == 0
+	 */
+	prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
+	if (offset - prev_offset <= 1UL)
+		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_pages))
+		goto readit;
+
+	/*
+	 * standalone, small random read
+	 * Read as is, and do not pollute the readahead state.
+	 */
+	return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
+
+initial_readahead:
+	ra->start = offset;
+	ra->size = get_init_ra_size(req_size, max_pages);
+	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
+
+readit:
+	/*
+	 * Will this read hit the readahead marker made by itself?
+	 * If so, trigger the readahead marker hit now, and merge
+	 * the resulted next readahead window into the current one.
+	 * Take care of maximum IO pages as above.
+	 */
+	if (offset == ra->start && ra->size == ra->async_size) {
+		add_pages = get_next_ra_size(ra, max_pages);
+		if (ra->size + add_pages <= max_pages) {
+			ra->async_size = add_pages;
+			ra->size += add_pages;
+		} else {
+			ra->size = max_pages;
+			ra->async_size = max_pages >> 1;
+		}
+	}
+
+	return ra_submit(ra, mapping, filp);
+}
+
+/**
+ * page_cache_sync_readahead - generic file readahead
+ * @mapping: address_space which holds the pagecache and I/O vectors
+ * @ra: file_ra_state which holds the readahead state
+ * @filp: passed on to ->readpage() and ->readpages()
+ * @offset: start offset into @mapping, in pagecache page-sized units
+ * @req_size: hint: total size of the read which the caller is performing in
+ *            pagecache pages
+ *
+ * page_cache_sync_readahead() should be called when a cache miss happened:
+ * it will submit the read.  The readahead logic may decide to piggyback more
+ * pages onto the read request if access patterns suggest it will improve
+ * performance.
+ */
+void page_cache_sync_readahead(struct address_space *mapping,
+			       struct file_ra_state *ra, struct file *filp,
+			       pgoff_t offset, unsigned long req_size)
+{
+	/* no read-ahead */
+	if (!ra->ra_pages)
+		return;
+
+	if (blk_cgroup_congested())
+		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);
+}
+EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
+
+/**
+ * page_cache_async_readahead - file readahead for marked pages
+ * @mapping: address_space which holds the pagecache and I/O vectors
+ * @ra: file_ra_state which holds the readahead state
+ * @filp: passed on to ->readpage() and ->readpages()
+ * @page: the page at @offset which has the PG_readahead flag set
+ * @offset: start offset into @mapping, in pagecache page-sized units
+ * @req_size: hint: total size of the read which the caller is performing in
+ *            pagecache pages
+ *
+ * page_cache_async_readahead() should be called when a page is used which
+ * has the PG_readahead flag; this is a marker to suggest that the application
+ * has used up enough of the readahead window that we should start pulling in
+ * more pages.
+ */
+void
+page_cache_async_readahead(struct address_space *mapping,
+			   struct file_ra_state *ra, struct file *filp,
+			   struct page *page, pgoff_t offset,
+			   unsigned long req_size)
+{
+	/* no read-ahead */
+	if (!ra->ra_pages)
+		return;
+
+	/*
+	 * Same bit is used for PG_readahead and PG_reclaim.
+	 */
+	if (PageWriteback(page))
+		return;
+
+	ClearPageReadahead(page);
+
+	/*
+	 * Defer asynchronous read-ahead on IO congestion.
+	 */
+	if (inode_read_congested(mapping->host))
+		return;
+
+	if (blk_cgroup_congested())
+		return;
+
+	/* do read-ahead */
+	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+}
+EXPORT_SYMBOL_GPL(page_cache_async_readahead);
+
+ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
+{
+	ssize_t ret;
+	struct fd f;
+
+	ret = -EBADF;
+	f = fdget(fd);
+	if (!f.file || !(f.file->f_mode & FMODE_READ))
+		goto out;
+
+	/*
+	 * The readahead() syscall is intended to run only on files
+	 * that can execute readahead. If readahead is not possible
+	 * on this file, then we must return -EINVAL.
+	 */
+	ret = -EINVAL;
+	if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
+	    !S_ISREG(file_inode(f.file)->i_mode))
+		goto out;
+
+	ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
+out:
+	fdput(f);
+	return ret;
+}
+
+SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
+{
+	return ksys_readahead(fd, offset, count);
+}