}
EXPORT_SYMBOL(default_unplug_io_fn);
+/*
+ * Convienent macros for min/max read-ahead pages.
+ * Note that MAX_RA_PAGES is rounded down, while MIN_RA_PAGES is rounded up.
+ * The latter is necessary for systems with large page size(i.e. 64k).
+ */
+#define MAX_RA_PAGES (VM_MAX_READAHEAD*1024 / PAGE_CACHE_SIZE)
+#define MIN_RA_PAGES DIV_ROUND_UP(VM_MIN_READAHEAD*1024, PAGE_CACHE_SIZE)
+
struct backing_dev_info default_backing_dev_info = {
- .ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE,
+ .ra_pages = MAX_RA_PAGES,
.state = 0,
.capabilities = BDI_CAP_MAP_COPY,
.unplug_io_fn = default_unplug_io_fn,
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;
+ ra->prev_index = -1;
}
EXPORT_SYMBOL_GPL(file_ra_state_init);
-/*
- * 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 reset_ahead_window(struct file_ra_state *ra)
-{
- /*
- * ... but preserve ahead_start + ahead_size value,
- * see 'recheck:' label in page_cache_readahead().
- * Note: We never use ->ahead_size as rvalue without
- * checking ->ahead_start != 0 first.
- */
- ra->ahead_size += ra->ahead_start;
- ra->ahead_start = 0;
-}
-
-static inline void ra_off(struct file_ra_state *ra)
-{
- ra->start = 0;
- ra->flags = 0;
- ra->size = 0;
- reset_ahead_window(ra);
- 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 / 32)
- newsize = newsize * 4;
- else if (newsize <= max / 4)
- newsize = newsize * 2;
- 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))
/**
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
*/
static int
__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
- pgoff_t offset, unsigned long nr_to_read)
+ pgoff_t offset, unsigned long nr_to_read,
+ unsigned long lookahead_size)
{
struct inode *inode = mapping->host;
struct page *page;
if (isize == 0)
goto out;
- end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
+ 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++) {
pgoff_t page_offset = offset + page_idx;
-
+
if (page_offset > end_index)
break;
break;
page->index = page_offset;
list_add(&page->lru, &page_pool);
+ if (page_idx == nr_to_read - lookahead_size)
+ SetPageReadahead(page);
ret++;
}
read_unlock_irq(&mapping->tree_lock);
if (this_chunk > nr_to_read)
this_chunk = nr_to_read;
err = __do_page_cache_readahead(mapping, filp,
- offset, this_chunk);
+ offset, this_chunk, 0);
if (err < 0) {
ret = err;
break;
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.
if (bdi_read_congested(mapping->backing_dev_info))
return -1;
- return __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
+ return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
}
/*
- * 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 successful without switching off
- * readahead mode. Otherwise return failure.
+ * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
+ * sensible upper limit.
*/
-static int
-blockable_page_cache_readahead(struct address_space *mapping, struct file *filp,
- pgoff_t offset, unsigned long nr_to_read,
- struct file_ra_state *ra, int block)
+unsigned long max_sane_readahead(unsigned long nr)
+{
+ return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE)
+ + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
+}
+
+/*
+ * Submit IO for the read-ahead request in file_ra_state.
+ */
+static unsigned long ra_submit(struct file_ra_state *ra,
+ struct address_space *mapping, struct file *filp)
{
int actual;
- if (!block && bdi_read_congested(mapping->backing_dev_info))
- return 0;
+ actual = __do_page_cache_readahead(mapping, filp,
+ ra->start, ra->size, ra->async_size);
+
+ return actual;
+}
- actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
+/*
+ * 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);
- return check_ra_success(ra, nr_to_read, actual);
+ if (newsize <= max / 32)
+ newsize = newsize * 4;
+ else if (newsize <= max / 4)
+ newsize = newsize * 2;
+ else
+ newsize = max;
+
+ return newsize;
}
-static int make_ahead_window(struct address_space *mapping, struct file *filp,
- struct file_ra_state *ra, int force)
+/*
+ * 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)
{
- 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.
- */
- reset_ahead_window(ra);
- }
+ unsigned long cur = ra->size;
+ unsigned long newsize;
- return ret;
+ if (cur < max / 16)
+ newsize = 4 * cur;
+ else
+ newsize = 2 * cur;
+
+ return min(newsize, max);
}
-/**
- * page_cache_readahead - generic adaptive 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 PAGE_CACHE_SIZE units
- * @req_size: hint: total size of the read which the caller is performing in
- * PAGE_CACHE_SIZE units
+/*
+ * 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
*
- * page_cache_readahead() is the main function. If performs the adaptive
- * readahead window size management and submits the readahead I/O.
+ * 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.
*
- * Note that @filp is purely used for passing on to the ->readpage[s]()
- * handler: it may refer to a different file from @mapping (so we may not use
- * @filp->f_mapping or @filp->f_path.dentry->d_inode here).
- * Also, @ra may not be equal to &@filp->f_ra.
+ * 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_index tracks the last visited page 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.
+ */
+
+/*
+ * A minimal readahead algorithm for trivial sequential/random reads.
*/
-unsigned long
-page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
- struct file *filp, pgoff_t offset, unsigned long req_size)
+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)
{
- unsigned long max, newsize;
+ unsigned long max; /* max readahead pages */
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;
+ max = ra->ra_pages;
+ sequential = (offset - ra->prev_index <= 1UL) || (req_size > max);
/*
- * 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
+ * It's the expected callback offset, assume sequential access.
+ * Ramp up sizes, and push forward the readahead window.
*/
- 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;
+ if (offset && (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);
+ ra->async_size = ra->size;
+ goto readit;
}
/*
- * 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
+ * Standalone, small read.
+ * Read as is, and do not pollute the readahead state.
*/
- if (!sequential) {
- ra_off(ra);
- blockable_page_cache_readahead(mapping, filp, offset,
- newsize, ra, 1);
- goto out;
+ if (!hit_readahead_marker && !sequential) {
+ return __do_page_cache_readahead(mapping, filp,
+ offset, req_size, 0);
}
/*
- * If we get here we are doing sequential IO and this was not the first
- * occurence (ie we have an existing window)
+ * It may be one of
+ * - first read on start of file
+ * - sequential cache miss
+ * - oversize random read
+ * Start readahead for it.
*/
- if (ra->ahead_start == 0) { /* no ahead window yet */
- if (!make_ahead_window(mapping, filp, ra, 0))
- goto recheck;
- }
+ ra->start = offset;
+ ra->size = get_init_ra_size(req_size, max);
+ ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
/*
- * 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.
+ * Hit on a marked page without valid readahead state.
+ * E.g. interleaved reads.
+ * Not knowing its readahead pos/size, bet on the minimal possible one.
*/
- if (ra->prev_page >= ra->ahead_start) {
- ra->start = ra->ahead_start;
- ra->size = ra->ahead_size;
- make_ahead_window(mapping, filp, ra, 0);
-recheck:
- /* prev_page shouldn't overrun the ahead window */
- ra->prev_page = min(ra->prev_page,
- ra->ahead_start + ra->ahead_size - 1);
+ if (hit_readahead_marker) {
+ ra->start++;
+ ra->size = get_next_ra_size(ra, max);
}
-out:
- return ra->prev_page + 1;
+readit:
+ return ra_submit(ra, mapping, filp);
}
-EXPORT_SYMBOL_GPL(page_cache_readahead);
-/*
- * 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)
+/**
+ * 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
*
- * 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.
+ * 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 handle_ra_miss(struct address_space *mapping,
- struct file_ra_state *ra, pgoff_t offset)
+void page_cache_sync_readahead(struct address_space *mapping,
+ struct file_ra_state *ra, struct file *filp,
+ pgoff_t offset, unsigned long req_size)
{
- ra->flags |= RA_FLAG_MISS;
- ra->flags &= ~RA_FLAG_INCACHE;
- ra->cache_hit = 0;
+ /* no read-ahead */
+ if (!ra->ra_pages)
+ return;
+
+ /* do read-ahead */
+ ondemand_readahead(mapping, ra, filp, false, offset, req_size);
}
+EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
-/*
- * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
- * sensible upper limit.
- */
-unsigned long max_sane_readahead(unsigned long nr)
+/**
+ * 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_ondemand() 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)
{
- return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE)
- + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
+ /* 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 (bdi_read_congested(mapping->backing_dev_info))
+ return;
+
+ /* do read-ahead */
+ ondemand_readahead(mapping, ra, filp, true, offset, req_size);
}
+EXPORT_SYMBOL_GPL(page_cache_async_readahead);
git.openpandora.org / pandora-kernel.git / blobdiff