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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-17 02:20:36 +0400
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-17 02:20:36 +0400
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /mm/readahead.c
downloadlinux-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.xz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
Diffstat (limited to 'mm/readahead.c')
-rw-r--r--mm/readahead.c557
1 files changed, 557 insertions, 0 deletions
diff --git a/mm/readahead.c b/mm/readahead.c
new file mode 100644
index 000000000000..b840e7c6ea74
--- /dev/null
+++ b/mm/readahead.c
@@ -0,0 +1,557 @@
+/*
+ * mm/readahead.c - address_space-level file readahead.
+ *
+ * Copyright (C) 2002, Linus Torvalds
+ *
+ * 09Apr2002 akpm@zip.com.au
+ * Initial version.
+ */
+
+#include <linux/kernel.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/blkdev.h>
+#include <linux/backing-dev.h>
+#include <linux/pagevec.h>
+
+void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
+{
+}
+EXPORT_SYMBOL(default_unplug_io_fn);
+
+struct backing_dev_info default_backing_dev_info = {
+ .ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE,
+ .state = 0,
+ .capabilities = BDI_CAP_MAP_COPY,
+ .unplug_io_fn = default_unplug_io_fn,
+};
+EXPORT_SYMBOL_GPL(default_backing_dev_info);
+
+/*
+ * 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 = mapping->backing_dev_info->ra_pages;
+ ra->prev_page = -1;
+}
+
+/*
+ * Return max readahead size for this inode in number-of-pages.
+ */
+static inline unsigned long get_max_readahead(struct file_ra_state *ra)
+{
+ return ra->ra_pages;
+}
+
+static inline unsigned long get_min_readahead(struct file_ra_state *ra)
+{
+ return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+}
+
+static inline void ra_off(struct file_ra_state *ra)
+{
+ ra->start = 0;
+ ra->flags = 0;
+ ra->size = 0;
+ ra->ahead_start = 0;
+ ra->ahead_size = 0;
+ return;
+}
+
+/*
+ * 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 / 64)
+ newsize = newsize * newsize;
+ else if (newsize <= max / 4)
+ newsize = max / 4;
+ else
+ newsize = max;
+ return newsize;
+}
+
+/*
+ * Set the new window size, this is called only when I/O is to be submitted,
+ * not for each call to readahead. If a cache miss occured, reduce next I/O
+ * size, else increase depending on how close to max we are.
+ */
+static inline unsigned long get_next_ra_size(struct file_ra_state *ra)
+{
+ unsigned long max = get_max_readahead(ra);
+ unsigned long min = get_min_readahead(ra);
+ unsigned long cur = ra->size;
+ unsigned long newsize;
+
+ if (ra->flags & RA_FLAG_MISS) {
+ ra->flags &= ~RA_FLAG_MISS;
+ newsize = max((cur - 2), min);
+ } else if (cur < max / 16) {
+ newsize = 4 * cur;
+ } else {
+ newsize = 2 * cur;
+ }
+ return min(newsize, max);
+}
+
+#define list_to_page(head) (list_entry((head)->prev, struct page, lru))
+
+/**
+ * read_cache_pages - populate an address space with some pages, and
+ * 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;
+ struct pagevec lru_pvec;
+ int ret = 0;
+
+ pagevec_init(&lru_pvec, 0);
+
+ while (!list_empty(pages)) {
+ page = list_to_page(pages);
+ list_del(&page->lru);
+ if (add_to_page_cache(page, mapping, page->index, GFP_KERNEL)) {
+ page_cache_release(page);
+ continue;
+ }
+ ret = filler(data, page);
+ if (!pagevec_add(&lru_pvec, page))
+ __pagevec_lru_add(&lru_pvec);
+ if (ret) {
+ while (!list_empty(pages)) {
+ struct page *victim;
+
+ victim = list_to_page(pages);
+ list_del(&victim->lru);
+ page_cache_release(victim);
+ }
+ break;
+ }
+ }
+ pagevec_lru_add(&lru_pvec);
+ return ret;
+}
+
+EXPORT_SYMBOL(read_cache_pages);
+
+static int read_pages(struct address_space *mapping, struct file *filp,
+ struct list_head *pages, unsigned nr_pages)
+{
+ unsigned page_idx;
+ struct pagevec lru_pvec;
+ int ret = 0;
+
+ if (mapping->a_ops->readpages) {
+ ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
+ goto out;
+ }
+
+ pagevec_init(&lru_pvec, 0);
+ for (page_idx = 0; page_idx < nr_pages; page_idx++) {
+ struct page *page = list_to_page(pages);
+ list_del(&page->lru);
+ if (!add_to_page_cache(page, mapping,
+ page->index, GFP_KERNEL)) {
+ mapping->a_ops->readpage(filp, page);
+ if (!pagevec_add(&lru_pvec, page))
+ __pagevec_lru_add(&lru_pvec);
+ } else {
+ page_cache_release(page);
+ }
+ }
+ pagevec_lru_add(&lru_pvec);
+out:
+ return ret;
+}
+
+/*
+ * Readahead design.
+ *
+ * The fields in struct file_ra_state represent the most-recently-executed
+ * readahead attempt:
+ *
+ * start: Page index at which we started the readahead
+ * size: Number of pages in that read
+ * Together, these form the "current window".
+ * Together, start and size represent the `readahead window'.
+ * prev_page: The page which the readahead algorithm most-recently inspected.
+ * It is mainly used to detect sequential file reading.
+ * If page_cache_readahead sees that it is again being called for
+ * a page which it just looked at, it can return immediately without
+ * making any state changes.
+ * ahead_start,
+ * ahead_size: Together, these form the "ahead window".
+ * ra_pages: The externally controlled max readahead for this fd.
+ *
+ * When readahead is in the off state (size == 0), readahead is disabled.
+ * In this state, prev_page is used to detect the resumption of sequential I/O.
+ *
+ * The readahead code manages two windows - the "current" and the "ahead"
+ * windows. The intent is that while the application is walking the pages
+ * in the current window, I/O is underway on the ahead window. When the
+ * current window is fully traversed, it is replaced by the ahead window
+ * and the ahead window is invalidated. When this copying happens, the
+ * new current window's pages are probably still locked. So
+ * we submit a new batch of I/O immediately, creating a new ahead window.
+ *
+ * So:
+ *
+ * ----|----------------|----------------|-----
+ * ^start ^start+size
+ * ^ahead_start ^ahead_start+ahead_size
+ *
+ * ^ When this page is read, we submit I/O for the
+ * ahead window.
+ *
+ * A `readahead hit' occurs when a read request is made against a page which is
+ * the next sequential page. Ahead window calculations are done only when it
+ * is time to submit a new IO. The code ramps up the size agressively at first,
+ * but slow down as it approaches max_readhead.
+ *
+ * Any seek/ramdom IO will result in readahead being turned off. It will resume
+ * at the first sequential access.
+ *
+ * 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.
+ *
+ * This function is to be called for every read request, rather than when
+ * it is time to perform readahead. It is called only once for the entire I/O
+ * regardless of size unless readahead is unable to start enough I/O to satisfy
+ * the request (I/O request > max_readahead).
+ */
+
+/*
+ * do_page_cache_readahead actually reads a chunk of disk. It allocates all
+ * the pages first, then submits them all 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.
+ *
+ * do_page_cache_readahead() returns -1 if it encountered request queue
+ * congestion.
+ */
+static int
+__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
+ unsigned long offset, unsigned long nr_to_read)
+{
+ 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;
+ int ret = 0;
+ loff_t isize = i_size_read(inode);
+
+ if (isize == 0)
+ goto out;
+
+ end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
+
+ /*
+ * Preallocate as many pages as we will need.
+ */
+ read_lock_irq(&mapping->tree_lock);
+ for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
+ unsigned long page_offset = offset + page_idx;
+
+ if (page_offset > end_index)
+ break;
+
+ page = radix_tree_lookup(&mapping->page_tree, page_offset);
+ if (page)
+ continue;
+
+ read_unlock_irq(&mapping->tree_lock);
+ page = page_cache_alloc_cold(mapping);
+ read_lock_irq(&mapping->tree_lock);
+ if (!page)
+ break;
+ page->index = page_offset;
+ list_add(&page->lru, &page_pool);
+ ret++;
+ }
+ read_unlock_irq(&mapping->tree_lock);
+
+ /*
+ * 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 (ret)
+ read_pages(mapping, filp, &page_pool, ret);
+ BUG_ON(!list_empty(&page_pool));
+out:
+ return ret;
+}
+
+/*
+ * 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,
+ unsigned long offset, unsigned long nr_to_read)
+{
+ int ret = 0;
+
+ if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
+ return -EINVAL;
+
+ while (nr_to_read) {
+ int err;
+
+ unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
+
+ if (this_chunk > nr_to_read)
+ this_chunk = nr_to_read;
+ err = __do_page_cache_readahead(mapping, filp,
+ offset, this_chunk);
+ if (err < 0) {
+ ret = err;
+ break;
+ }
+ ret += err;
+ offset += this_chunk;
+ nr_to_read -= this_chunk;
+ }
+ return ret;
+}
+
+/*
+ * Check how effective readahead is being. If the amount of started IO is
+ * less than expected then the file is partly or fully in pagecache and
+ * readahead isn't helping.
+ *
+ */
+static inline int check_ra_success(struct file_ra_state *ra,
+ unsigned long nr_to_read, unsigned long actual)
+{
+ if (actual == 0) {
+ ra->cache_hit += nr_to_read;
+ if (ra->cache_hit >= VM_MAX_CACHE_HIT) {
+ ra_off(ra);
+ ra->flags |= RA_FLAG_INCACHE;
+ return 0;
+ }
+ } else {
+ ra->cache_hit=0;
+ }
+ return 1;
+}
+
+/*
+ * This version skips the IO if the queue is read-congested, and will tell the
+ * block layer to abandon the readahead if request allocation would block.
+ *
+ * force_page_cache_readahead() will ignore queue congestion and will block on
+ * request queues.
+ */
+int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
+ unsigned long offset, unsigned long nr_to_read)
+{
+ if (bdi_read_congested(mapping->backing_dev_info))
+ return -1;
+
+ return __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
+}
+
+/*
+ * Read 'nr_to_read' pages starting at page 'offset'. If the flag 'block'
+ * is set wait till the read completes. Otherwise attempt to read without
+ * blocking.
+ * Returns 1 meaning 'success' if read is succesfull without switching off
+ * readhaead mode. Otherwise return failure.
+ */
+static int
+blockable_page_cache_readahead(struct address_space *mapping, struct file *filp,
+ unsigned long offset, unsigned long nr_to_read,
+ struct file_ra_state *ra, int block)
+{
+ int actual;
+
+ if (!block && bdi_read_congested(mapping->backing_dev_info))
+ return 0;
+
+ actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
+
+ return check_ra_success(ra, nr_to_read, actual);
+}
+
+static int make_ahead_window(struct address_space *mapping, struct file *filp,
+ struct file_ra_state *ra, int force)
+{
+ int block, ret;
+
+ ra->ahead_size = get_next_ra_size(ra);
+ ra->ahead_start = ra->start + ra->size;
+
+ block = force || (ra->prev_page >= ra->ahead_start);
+ ret = blockable_page_cache_readahead(mapping, filp,
+ ra->ahead_start, ra->ahead_size, ra, block);
+
+ if (!ret && !force) {
+ /* A read failure in blocking mode, implies pages are
+ * all cached. So we can safely assume we have taken
+ * care of all the pages requested in this call.
+ * A read failure in non-blocking mode, implies we are
+ * reading more pages than requested in this call. So
+ * we safely assume we have taken care of all the pages
+ * requested in this call.
+ *
+ * Just reset the ahead window in case we failed due to
+ * congestion. The ahead window will any way be closed
+ * in case we failed due to excessive page cache hits.
+ */
+ ra->ahead_start = 0;
+ ra->ahead_size = 0;
+ }
+
+ return ret;
+}
+
+/*
+ * page_cache_readahead is the main function. If performs the adaptive
+ * readahead window size management and submits the readahead I/O.
+ */
+unsigned long
+page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
+ struct file *filp, unsigned long offset,
+ unsigned long req_size)
+{
+ unsigned long max, newsize;
+ int sequential;
+
+ /*
+ * We avoid doing extra work and bogusly perturbing the readahead
+ * window expansion logic.
+ */
+ if (offset == ra->prev_page && --req_size)
+ ++offset;
+
+ /* Note that prev_page == -1 if it is a first read */
+ sequential = (offset == ra->prev_page + 1);
+ ra->prev_page = offset;
+
+ max = get_max_readahead(ra);
+ newsize = min(req_size, max);
+
+ /* No readahead or sub-page sized read or file already in cache */
+ if (newsize == 0 || (ra->flags & RA_FLAG_INCACHE))
+ goto out;
+
+ ra->prev_page += newsize - 1;
+
+ /*
+ * Special case - first read at start of file. We'll assume it's
+ * a whole-file read and grow the window fast. Or detect first
+ * sequential access
+ */
+ if (sequential && ra->size == 0) {
+ ra->size = get_init_ra_size(newsize, max);
+ ra->start = offset;
+ if (!blockable_page_cache_readahead(mapping, filp, offset,
+ ra->size, ra, 1))
+ goto out;
+
+ /*
+ * If the request size is larger than our max readahead, we
+ * at least want to be sure that we get 2 IOs in flight and
+ * we know that we will definitly need the new I/O.
+ * once we do this, subsequent calls should be able to overlap
+ * IOs,* thus preventing stalls. so issue the ahead window
+ * immediately.
+ */
+ if (req_size >= max)
+ make_ahead_window(mapping, filp, ra, 1);
+
+ goto out;
+ }
+
+ /*
+ * Now handle the random case:
+ * partial page reads and first access were handled above,
+ * so this must be the next page otherwise it is random
+ */
+ if (!sequential) {
+ ra_off(ra);
+ blockable_page_cache_readahead(mapping, filp, offset,
+ newsize, ra, 1);
+ goto out;
+ }
+
+ /*
+ * If we get here we are doing sequential IO and this was not the first
+ * occurence (ie we have an existing window)
+ */
+
+ if (ra->ahead_start == 0) { /* no ahead window yet */
+ if (!make_ahead_window(mapping, filp, ra, 0))
+ goto out;
+ }
+ /*
+ * Already have an ahead window, check if we crossed into it.
+ * If so, shift windows and issue a new ahead window.
+ * Only return the #pages that are in the current window, so that
+ * we get called back on the first page of the ahead window which
+ * will allow us to submit more IO.
+ */
+ if (ra->prev_page >= ra->ahead_start) {
+ ra->start = ra->ahead_start;
+ ra->size = ra->ahead_size;
+ make_ahead_window(mapping, filp, ra, 0);
+ }
+
+out:
+ return ra->prev_page + 1;
+}
+
+/*
+ * handle_ra_miss() is called when it is known that a page which should have
+ * been present in the pagecache (we just did some readahead there) was in fact
+ * not found. This will happen if it was evicted by the VM (readahead
+ * thrashing)
+ *
+ * Turn on the cache miss flag in the RA struct, this will cause the RA code
+ * to reduce the RA size on the next read.
+ */
+void handle_ra_miss(struct address_space *mapping,
+ struct file_ra_state *ra, pgoff_t offset)
+{
+ ra->flags |= RA_FLAG_MISS;
+ ra->flags &= ~RA_FLAG_INCACHE;
+}
+
+/*
+ * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
+ * sensible upper limit.
+ */
+unsigned long max_sane_readahead(unsigned long nr)
+{
+ unsigned long active;
+ unsigned long inactive;
+ unsigned long free;
+
+ __get_zone_counts(&active, &inactive, &free, NODE_DATA(numa_node_id()));
+ return min(nr, (inactive + free) / 2);
+}