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/*
* 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++)
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