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linux/fs/mpage.c
Tejun Heo 2a81490811 writeback: implement foreign cgroup inode detection 
As concurrent write sharing of an inode is expected to be very rare
and memcg only tracks page ownership on first-use basis severely
confining the usefulness of such sharing, cgroup writeback tracks
ownership per-inode.  While the support for concurrent write sharing
of an inode is deemed unnecessary, an inode being written to by
different cgroups at different points in time is a lot more common,
and, more importantly, charging only by first-use can too readily lead
to grossly incorrect behaviors (single foreign page can lead to
gigabytes of writeback to be incorrectly attributed).

To resolve this issue, cgroup writeback detects the majority dirtier
of an inode and will transfer the ownership to it.  To avoid
unnnecessary oscillation, the detection mechanism keeps track of
history and gives out the switch verdict only if the foreign usage
pattern is stable over a certain amount of time and/or writeback
attempts.

The detection mechanism has fairly low space and computation overhead.
It adds 8 bytes to struct inode (one int and two u16's) and minimal
amount of calculation per IO.  The detection mechanism converges to
the correct answer usually in several seconds of IO time when there's
a clear majority dirtier.  Even when there isn't, it can reach an
acceptable answer fairly quickly under most circumstances.

Please see wb_detach_inode() for more details.

This patch only implements detection.  Following patches will
implement actual switching.

v2: wbc_account_io() now checks whether the wbc is associated with a
    wb before dereferencing it.  This can happen when pageout() is
    writing pages directly without going through the usual writeback
    path.  As pageout() path is single-threaded, we don't want it to
    be blocked behind a slow cgroup and ultimately want it to delegate
    actual writing to the usual writeback path.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
2015-06-02 08:40:20 -06:00

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/*
* fs/mpage.c
*
* Copyright (C) 2002, Linus Torvalds.
*
* Contains functions related to preparing and submitting BIOs which contain
* multiple pagecache pages.
*
* 15May2002 Andrew Morton
* Initial version
* 27Jun2002 axboe@suse.de
* use bio_add_page() to build bio's just the right size
*/
#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/mm.h>
#include <linux/kdev_t.h>
#include <linux/gfp.h>
#include <linux/bio.h>
#include <linux/fs.h>
#include <linux/buffer_head.h>
#include <linux/blkdev.h>
#include <linux/highmem.h>
#include <linux/prefetch.h>
#include <linux/mpage.h>
#include <linux/writeback.h>
#include <linux/backing-dev.h>
#include <linux/pagevec.h>
#include <linux/cleancache.h>
#include "internal.h"
/*
* I/O completion handler for multipage BIOs.
*
* The mpage code never puts partial pages into a BIO (except for end-of-file).
* If a page does not map to a contiguous run of blocks then it simply falls
* back to block_read_full_page().
*
* Why is this? If a page's completion depends on a number of different BIOs
* which can complete in any order (or at the same time) then determining the
* status of that page is hard. See end_buffer_async_read() for the details.
* There is no point in duplicating all that complexity.
*/
static void mpage_end_io(struct bio *bio, int err)
{
struct bio_vec *bv;
int i;
bio_for_each_segment_all(bv, bio, i) {
struct page *page = bv->bv_page;
page_endio(page, bio_data_dir(bio), err);
}
bio_put(bio);
}
static struct bio *mpage_bio_submit(int rw, struct bio *bio)
{
bio->bi_end_io = mpage_end_io;
guard_bio_eod(rw, bio);
submit_bio(rw, bio);
return NULL;
}
static struct bio *
mpage_alloc(struct block_device *bdev,
sector_t first_sector, int nr_vecs,
gfp_t gfp_flags)
{
struct bio *bio;
bio = bio_alloc(gfp_flags, nr_vecs);
if (bio == NULL && (current->flags & PF_MEMALLOC)) {
while (!bio && (nr_vecs /= 2))
bio = bio_alloc(gfp_flags, nr_vecs);
}
if (bio) {
bio->bi_bdev = bdev;
bio->bi_iter.bi_sector = first_sector;
}
return bio;
}
/*
* support function for mpage_readpages. The fs supplied get_block might
* return an up to date buffer. This is used to map that buffer into
* the page, which allows readpage to avoid triggering a duplicate call
* to get_block.
*
* The idea is to avoid adding buffers to pages that don't already have
* them. So when the buffer is up to date and the page size == block size,
* this marks the page up to date instead of adding new buffers.
*/
static void
map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
{
struct inode *inode = page->mapping->host;
struct buffer_head *page_bh, *head;
int block = 0;
if (!page_has_buffers(page)) {
/*
* don't make any buffers if there is only one buffer on
* the page and the page just needs to be set up to date
*/
if (inode->i_blkbits == PAGE_CACHE_SHIFT &&
buffer_uptodate(bh)) {
SetPageUptodate(page);
return;
}
create_empty_buffers(page, 1 << inode->i_blkbits, 0);
}
head = page_buffers(page);
page_bh = head;
do {
if (block == page_block) {
page_bh->b_state = bh->b_state;
page_bh->b_bdev = bh->b_bdev;
page_bh->b_blocknr = bh->b_blocknr;
break;
}
page_bh = page_bh->b_this_page;
block++;
} while (page_bh != head);
}
/*
* This is the worker routine which does all the work of mapping the disk
* blocks and constructs largest possible bios, submits them for IO if the
* blocks are not contiguous on the disk.
*
* We pass a buffer_head back and forth and use its buffer_mapped() flag to
* represent the validity of its disk mapping and to decide when to do the next
* get_block() call.
*/
static struct bio *
do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
sector_t *last_block_in_bio, struct buffer_head *map_bh,
unsigned long *first_logical_block, get_block_t get_block)
{
struct inode *inode = page->mapping->host;
const unsigned blkbits = inode->i_blkbits;
const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
const unsigned blocksize = 1 << blkbits;
sector_t block_in_file;
sector_t last_block;
sector_t last_block_in_file;
sector_t blocks[MAX_BUF_PER_PAGE];
unsigned page_block;
unsigned first_hole = blocks_per_page;
struct block_device *bdev = NULL;
int length;
int fully_mapped = 1;
unsigned nblocks;
unsigned relative_block;
if (page_has_buffers(page))
goto confused;
block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
last_block = block_in_file + nr_pages * blocks_per_page;
last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
if (last_block > last_block_in_file)
last_block = last_block_in_file;
page_block = 0;
/*
* Map blocks using the result from the previous get_blocks call first.
*/
nblocks = map_bh->b_size >> blkbits;
if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
block_in_file < (*first_logical_block + nblocks)) {
unsigned map_offset = block_in_file - *first_logical_block;
unsigned last = nblocks - map_offset;
for (relative_block = 0; ; relative_block++) {
if (relative_block == last) {
clear_buffer_mapped(map_bh);
break;
}
if (page_block == blocks_per_page)
break;
blocks[page_block] = map_bh->b_blocknr + map_offset +
relative_block;
page_block++;
block_in_file++;
}
bdev = map_bh->b_bdev;
}
/*
* Then do more get_blocks calls until we are done with this page.
*/
map_bh->b_page = page;
while (page_block < blocks_per_page) {
map_bh->b_state = 0;
map_bh->b_size = 0;
if (block_in_file < last_block) {
map_bh->b_size = (last_block-block_in_file) << blkbits;
if (get_block(inode, block_in_file, map_bh, 0))
goto confused;
*first_logical_block = block_in_file;
}
if (!buffer_mapped(map_bh)) {
fully_mapped = 0;
if (first_hole == blocks_per_page)
first_hole = page_block;
page_block++;
block_in_file++;
continue;
}
/* some filesystems will copy data into the page during
* the get_block call, in which case we don't want to
* read it again. map_buffer_to_page copies the data
* we just collected from get_block into the page's buffers
* so readpage doesn't have to repeat the get_block call
*/
if (buffer_uptodate(map_bh)) {
map_buffer_to_page(page, map_bh, page_block);
goto confused;
}
if (first_hole != blocks_per_page)
goto confused; /* hole -> non-hole */
/* Contiguous blocks? */
if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
goto confused;
nblocks = map_bh->b_size >> blkbits;
for (relative_block = 0; ; relative_block++) {
if (relative_block == nblocks) {
clear_buffer_mapped(map_bh);
break;
} else if (page_block == blocks_per_page)
break;
blocks[page_block] = map_bh->b_blocknr+relative_block;
page_block++;
block_in_file++;
}
bdev = map_bh->b_bdev;
}
if (first_hole != blocks_per_page) {
zero_user_segment(page, first_hole << blkbits, PAGE_CACHE_SIZE);
if (first_hole == 0) {
SetPageUptodate(page);
unlock_page(page);
goto out;
}
} else if (fully_mapped) {
SetPageMappedToDisk(page);
}
if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
cleancache_get_page(page) == 0) {
SetPageUptodate(page);
goto confused;
}
/*
* This page will go to BIO. Do we need to send this BIO off first?
*/
if (bio && (*last_block_in_bio != blocks[0] - 1))
bio = mpage_bio_submit(READ, bio);
alloc_new:
if (bio == NULL) {
if (first_hole == blocks_per_page) {
if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
page))
goto out;
}
bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
min_t(int, nr_pages, bio_get_nr_vecs(bdev)),
GFP_KERNEL);
if (bio == NULL)
goto confused;
}
length = first_hole << blkbits;
if (bio_add_page(bio, page, length, 0) < length) {
bio = mpage_bio_submit(READ, bio);
goto alloc_new;
}
relative_block = block_in_file - *first_logical_block;
nblocks = map_bh->b_size >> blkbits;
if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
(first_hole != blocks_per_page))
bio = mpage_bio_submit(READ, bio);
else
*last_block_in_bio = blocks[blocks_per_page - 1];
out:
return bio;
confused:
if (bio)
bio = mpage_bio_submit(READ, bio);
if (!PageUptodate(page))
block_read_full_page(page, get_block);
else
unlock_page(page);
goto out;
}
/**
* mpage_readpages - 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.
* The page at @pages->prev has the lowest file offset, and reads should be
* issued in @pages->prev to @pages->next order.
* @nr_pages: The number of pages at *@pages
* @get_block: The filesystem's block mapper function.
*
* This function walks the pages and the blocks within each page, building and
* emitting large BIOs.
*
* If anything unusual happens, such as:
*
* - encountering a page which has buffers
* - encountering a page which has a non-hole after a hole
* - encountering a page with non-contiguous blocks
*
* then this code just gives up and calls the buffer_head-based read function.
* It does handle a page which has holes at the end - that is a common case:
* the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
*
* BH_Boundary explanation:
*
* There is a problem. The mpage read code assembles several pages, gets all
* their disk mappings, and then submits them all. That's fine, but obtaining
* the disk mappings may require I/O. Reads of indirect blocks, for example.
*
* So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
* submitted in the following order:
* 12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
*
* because the indirect block has to be read to get the mappings of blocks
* 13,14,15,16. Obviously, this impacts performance.
*
* So what we do it to allow the filesystem's get_block() function to set
* BH_Boundary when it maps block 11. BH_Boundary says: mapping of the block
* after this one will require I/O against a block which is probably close to
* this one. So you should push what I/O you have currently accumulated.
*
* This all causes the disk requests to be issued in the correct order.
*/
int
mpage_readpages(struct address_space *mapping, struct list_head *pages,
unsigned nr_pages, get_block_t get_block)
{
struct bio *bio = NULL;
unsigned page_idx;
sector_t last_block_in_bio = 0;
struct buffer_head map_bh;
unsigned long first_logical_block = 0;
map_bh.b_state = 0;
map_bh.b_size = 0;
for (page_idx = 0; page_idx < nr_pages; page_idx++) {
struct page *page = list_entry(pages->prev, struct page, lru);
prefetchw(&page->flags);
list_del(&page->lru);
if (!add_to_page_cache_lru(page, mapping,
page->index, GFP_KERNEL)) {
bio = do_mpage_readpage(bio, page,
nr_pages - page_idx,
&last_block_in_bio, &map_bh,
&first_logical_block,
get_block);
}
page_cache_release(page);
}
BUG_ON(!list_empty(pages));
if (bio)
mpage_bio_submit(READ, bio);
return 0;
}
EXPORT_SYMBOL(mpage_readpages);
/*
* This isn't called much at all
*/
int mpage_readpage(struct page *page, get_block_t get_block)
{
struct bio *bio = NULL;
sector_t last_block_in_bio = 0;
struct buffer_head map_bh;
unsigned long first_logical_block = 0;
map_bh.b_state = 0;
map_bh.b_size = 0;
bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
&map_bh, &first_logical_block, get_block);
if (bio)
mpage_bio_submit(READ, bio);
return 0;
}
EXPORT_SYMBOL(mpage_readpage);
/*
* Writing is not so simple.
*
* If the page has buffers then they will be used for obtaining the disk
* mapping. We only support pages which are fully mapped-and-dirty, with a
* special case for pages which are unmapped at the end: end-of-file.
*
* If the page has no buffers (preferred) then the page is mapped here.
*
* If all blocks are found to be contiguous then the page can go into the
* BIO. Otherwise fall back to the mapping's writepage().
*
* FIXME: This code wants an estimate of how many pages are still to be
* written, so it can intelligently allocate a suitably-sized BIO. For now,
* just allocate full-size (16-page) BIOs.
*/
struct mpage_data {
struct bio *bio;
sector_t last_block_in_bio;
get_block_t *get_block;
unsigned use_writepage;
};
/*
* We have our BIO, so we can now mark the buffers clean. Make
* sure to only clean buffers which we know we'll be writing.
*/
static void clean_buffers(struct page *page, unsigned first_unmapped)
{
unsigned buffer_counter = 0;
struct buffer_head *bh, *head;
if (!page_has_buffers(page))
return;
head = page_buffers(page);
bh = head;
do {
if (buffer_counter++ == first_unmapped)
break;
clear_buffer_dirty(bh);
bh = bh->b_this_page;
} while (bh != head);
/*
* we cannot drop the bh if the page is not uptodate or a concurrent
* readpage would fail to serialize with the bh and it would read from
* disk before we reach the platter.
*/
if (buffer_heads_over_limit && PageUptodate(page))
try_to_free_buffers(page);
}
static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
void *data)
{
struct mpage_data *mpd = data;
struct bio *bio = mpd->bio;
struct address_space *mapping = page->mapping;
struct inode *inode = page->mapping->host;
const unsigned blkbits = inode->i_blkbits;
unsigned long end_index;
const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
sector_t last_block;
sector_t block_in_file;
sector_t blocks[MAX_BUF_PER_PAGE];
unsigned page_block;
unsigned first_unmapped = blocks_per_page;
struct block_device *bdev = NULL;
int boundary = 0;
sector_t boundary_block = 0;
struct block_device *boundary_bdev = NULL;
int length;
struct buffer_head map_bh;
loff_t i_size = i_size_read(inode);
int ret = 0;
if (page_has_buffers(page)) {
struct buffer_head *head = page_buffers(page);
struct buffer_head *bh = head;
/* If they're all mapped and dirty, do it */
page_block = 0;
do {
BUG_ON(buffer_locked(bh));
if (!buffer_mapped(bh)) {
/*
* unmapped dirty buffers are created by
* __set_page_dirty_buffers -> mmapped data
*/
if (buffer_dirty(bh))
goto confused;
if (first_unmapped == blocks_per_page)
first_unmapped = page_block;
continue;
}
if (first_unmapped != blocks_per_page)
goto confused; /* hole -> non-hole */
if (!buffer_dirty(bh) || !buffer_uptodate(bh))
goto confused;
if (page_block) {
if (bh->b_blocknr != blocks[page_block-1] + 1)
goto confused;
}
blocks[page_block++] = bh->b_blocknr;
boundary = buffer_boundary(bh);
if (boundary) {
boundary_block = bh->b_blocknr;
boundary_bdev = bh->b_bdev;
}
bdev = bh->b_bdev;
} while ((bh = bh->b_this_page) != head);
if (first_unmapped)
goto page_is_mapped;
/*
* Page has buffers, but they are all unmapped. The page was
* created by pagein or read over a hole which was handled by
* block_read_full_page(). If this address_space is also
* using mpage_readpages then this can rarely happen.
*/
goto confused;
}
/*
* The page has no buffers: map it to disk
*/
BUG_ON(!PageUptodate(page));
block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
last_block = (i_size - 1) >> blkbits;
map_bh.b_page = page;
for (page_block = 0; page_block < blocks_per_page; ) {
map_bh.b_state = 0;
map_bh.b_size = 1 << blkbits;
if (mpd->get_block(inode, block_in_file, &map_bh, 1))
goto confused;
if (buffer_new(&map_bh))
unmap_underlying_metadata(map_bh.b_bdev,
map_bh.b_blocknr);
if (buffer_boundary(&map_bh)) {
boundary_block = map_bh.b_blocknr;
boundary_bdev = map_bh.b_bdev;
}
if (page_block) {
if (map_bh.b_blocknr != blocks[page_block-1] + 1)
goto confused;
}
blocks[page_block++] = map_bh.b_blocknr;
boundary = buffer_boundary(&map_bh);
bdev = map_bh.b_bdev;
if (block_in_file == last_block)
break;
block_in_file++;
}
BUG_ON(page_block == 0);
first_unmapped = page_block;
page_is_mapped:
end_index = i_size >> PAGE_CACHE_SHIFT;
if (page->index >= end_index) {
/*
* The page straddles i_size. It must be zeroed out on each
* and every writepage invocation because it may be mmapped.
* "A file is mapped in multiples of the page size. For a file
* that is not a multiple of the page size, the remaining memory
* is zeroed when mapped, and writes to that region are not
* written out to the file."
*/
unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
if (page->index > end_index || !offset)
goto confused;
zero_user_segment(page, offset, PAGE_CACHE_SIZE);
}
/*
* This page will go to BIO. Do we need to send this BIO off first?
*/
if (bio && mpd->last_block_in_bio != blocks[0] - 1)
bio = mpage_bio_submit(WRITE, bio);
alloc_new:
if (bio == NULL) {
if (first_unmapped == blocks_per_page) {
if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
page, wbc)) {
clean_buffers(page, first_unmapped);
goto out;
}
}
bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
if (bio == NULL)
goto confused;
wbc_init_bio(wbc, bio);
}
/*
* Must try to add the page before marking the buffer clean or
* the confused fail path above (OOM) will be very confused when
* it finds all bh marked clean (i.e. it will not write anything)
*/
wbc_account_io(wbc, page, PAGE_SIZE);
length = first_unmapped << blkbits;
if (bio_add_page(bio, page, length, 0) < length) {
bio = mpage_bio_submit(WRITE, bio);
goto alloc_new;
}
clean_buffers(page, first_unmapped);
BUG_ON(PageWriteback(page));
set_page_writeback(page);
unlock_page(page);
if (boundary || (first_unmapped != blocks_per_page)) {
bio = mpage_bio_submit(WRITE, bio);
if (boundary_block) {
write_boundary_block(boundary_bdev,
boundary_block, 1 << blkbits);
}
} else {
mpd->last_block_in_bio = blocks[blocks_per_page - 1];
}
goto out;
confused:
if (bio)
bio = mpage_bio_submit(WRITE, bio);
if (mpd->use_writepage) {
ret = mapping->a_ops->writepage(page, wbc);
} else {
ret = -EAGAIN;
goto out;
}
/*
* The caller has a ref on the inode, so *mapping is stable
*/
mapping_set_error(mapping, ret);
out:
mpd->bio = bio;
return ret;
}
/**
* mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
* @mapping: address space structure to write
* @wbc: subtract the number of written pages from *@wbc->nr_to_write
* @get_block: the filesystem's block mapper function.
* If this is NULL then use a_ops->writepage. Otherwise, go
* direct-to-BIO.
*
* This is a library function, which implements the writepages()
* address_space_operation.
*
* If a page is already under I/O, generic_writepages() skips it, even
* if it's dirty. This is desirable behaviour for memory-cleaning writeback,
* but it is INCORRECT for data-integrity system calls such as fsync(). fsync()
* and msync() need to guarantee that all the data which was dirty at the time
* the call was made get new I/O started against them. If wbc->sync_mode is
* WB_SYNC_ALL then we were called for data integrity and we must wait for
* existing IO to complete.
*/
int
mpage_writepages(struct address_space *mapping,
struct writeback_control *wbc, get_block_t get_block)
{
struct blk_plug plug;
int ret;
blk_start_plug(&plug);
if (!get_block)
ret = generic_writepages(mapping, wbc);
else {
struct mpage_data mpd = {
.bio = NULL,
.last_block_in_bio = 0,
.get_block = get_block,
.use_writepage = 1,
};
ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
if (mpd.bio)
mpage_bio_submit(WRITE, mpd.bio);
}
blk_finish_plug(&plug);
return ret;
}
EXPORT_SYMBOL(mpage_writepages);
int mpage_writepage(struct page *page, get_block_t get_block,
struct writeback_control *wbc)
{
struct mpage_data mpd = {
.bio = NULL,
.last_block_in_bio = 0,
.get_block = get_block,
.use_writepage = 0,
};
int ret = __mpage_writepage(page, wbc, &mpd);
if (mpd.bio)
mpage_bio_submit(WRITE, mpd.bio);
return ret;
}
EXPORT_SYMBOL(mpage_writepage);
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