/*
* 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/task_io_accounting_ops.h>
#include <linux/pagevec.h>
void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
{
}
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 = MAX_RA_PAGES,
.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_index = -1;
}
EXPORT_SYMBOL_GPL(file_ra_state_init);
#define list_to_page(head) (list_entry((head)->prev, struct page, lru))
/**
* 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;
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) {
put_pages_list(pages);
break;
}
task_io_account_read(PAGE_CACHE_SIZE);
}
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;
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;
}
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);
ret = 0;
out:
return ret;
}
/*
* 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,
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;
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++) {
pgoff_t 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);
if (page_idx == nr_to_read - lookahead_size)
SetPageReadahead(page);
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,
pgoff_t 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, 0);
if (err < 0) {
ret = err;
break;
}
ret += err;
offset += this_chunk;
nr_to_read -= this_chunk;
}
return ret;
}
/*
* 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,
pgoff_t 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, 0);
}
/*
* Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
* sensible upper limit.
*/
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.
*/
unsigned long ra_submit(struct file_ra_state *ra,
struct address_space *mapping, struct file *filp)
{
unsigned long ra_size;
unsigned long la_size;
int actual;
ra_size = ra_readahead_size(ra);
la_size = ra_lookahead_size(ra);
actual = __do_page_cache_readahead(mapping, filp,
ra->ra_index, ra_size, la_size);
return actual;
}
EXPORT_SYMBOL_GPL(ra_submit);
/*
* 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->readahead_index - ra->ra_index;
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:
*
* |-------- last readahead window -------->|
* |-- application walking here -->|
* ======#============|==================#=====================|
* ^la_index ^ra_index ^lookahead_index ^readahead_index
*
* [ra_index, readahead_index) represents the last readahead window.
*
* [la_index, lookahead_index] is where the application would be walking(in
* the common case of cache-cold sequential reads): the last window was
* established when the application was at la_index, and the next window will
* be bring in when the application reaches lookahead_index.
*
* 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 early as the application
* reads on the page at lookahead_index. Normally lookahead_index will be
* equal to ra_index, 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 lookahead_index 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.
*/
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; /* max readahead pages */
pgoff_t ra_index; /* readahead index */
unsigned long ra_size; /* readahead size */
unsigned long la_size; /* lookahead size */
int sequential;
max = ra->ra_pages;
sequential = (offset - ra->prev_index <= 1UL) || (req_size > max);
/*
* Lookahead/readahead hit, assume sequential access.
* Ramp up sizes, and push forward the readahead window.
*/
if (offset && (offset == ra->lookahead_index ||
offset == ra->readahead_index)) {
ra_index = ra->readahead_index;
ra_size = get_next_ra_size(ra, max);
la_size = ra_size;
goto fill_ra;
}
/*
* Standalone, small read.
* Read as is, and do not pollute the readahead state.
*/
if (!hit_readahead_marker && !sequential) {
return __do_page_cache_readahead(mapping, filp,
offset, req_size, 0);
}
/*
* It may be one of
* - first read on start of file
* - sequential cache miss
* - oversize random read
* Start readahead for it.
*/
ra_index = offset;
ra_size = get_init_ra_size(req_size, max);
la_size = ra_size > req_size ? ra_size - req_size : ra_size;
/*
* Hit on a lookahead page without valid readahead state.
* E.g. interleaved reads.
* Not knowing its readahead pos/size, bet on the minimal possible one.
*/
if (hit_readahead_marker) {
ra_index++;
ra_size = min(4 * ra_size, max);
}
fill_ra:
ra_set_index(ra, offset, ra_index);
ra_set_size(ra, ra_size, la_size);
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;
/* 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_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)
{
/* 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);
