Revision b91e1302ad9b80c174a4855533f7e3aa2873355e authored by Linus Torvalds on 27 December 2016, 19:40:38 UTC, committed by Linus Torvalds on 29 December 2016, 19:03:15 UTC
In commit 62906027091f ("mm: add PageWaiters indicating tasks are waiting for a page bit") Nick Piggin made our page locking no longer unconditionally touch the hashed page waitqueue, which not only helps performance in general, but is particularly helpful on NUMA machines where the hashed wait queues can bounce around a lot. However, the "clear lock bit atomically and then test the waiters bit" sequence turns out to be much more expensive than it needs to be, because you get a nasty stall when trying to access the same word that just got updated atomically. On architectures where locking is done with LL/SC, this would be trivial to fix with a new primitive that clears one bit and tests another atomically, but that ends up not working on x86, where the only atomic operations that return the result end up being cmpxchg and xadd. The atomic bit operations return the old value of the same bit we changed, not the value of an unrelated bit. On x86, we could put the lock bit in the high bit of the byte, and use "xadd" with that bit (where the overflow ends up not touching other bits), and look at the other bits of the result. However, an even simpler model is to just use a regular atomic "and" to clear the lock bit, and then the sign bit in eflags will indicate the resulting state of the unrelated bit #7. So by moving the PageWaiters bit up to bit #7, we can atomically clear the lock bit and test the waiters bit on x86 too. And architectures with LL/SC (which is all the usual RISC suspects), the particular bit doesn't matter, so they are fine with this approach too. This avoids the extra access to the same atomic word, and thus avoids the costly stall at page unlock time. The only downside is that the interface ends up being a bit odd and specialized: clear a bit in a byte, and test the sign bit. Nick doesn't love the resulting name of the new primitive, but I'd rather make the name be descriptive and very clear about the limitation imposed by trying to work across all relevant architectures than make it be some generic thing that doesn't make the odd semantics explicit. So this introduces the new architecture primitive clear_bit_unlock_is_negative_byte(); and adds the trivial implementation for x86. We have a generic non-optimized fallback (that just does a "clear_bit()"+"test_bit(7)" combination) which can be overridden by any architecture that can do better. According to Nick, Power has the same hickup x86 has, for example, but some other architectures may not even care. All these optimizations mean that my page locking stress-test (which is just executing a lot of small short-lived shell scripts: "make test" in the git source tree) no longer makes our page locking look horribly bad. Before all these optimizations, just the unlock_page() costs were just over 3% of all CPU overhead on "make test". After this, it's down to 0.66%, so just a quarter of the cost it used to be. (The difference on NUMA is bigger, but there this micro-optimization is likely less noticeable, since the big issue on NUMA was not the accesses to 'struct page', but the waitqueue accesses that were already removed by Nick's earlier commit). Acked-by: Nick Piggin <npiggin@gmail.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Bob Peterson <rpeterso@redhat.com> Cc: Steven Whitehouse <swhiteho@redhat.com> Cc: Andrew Lutomirski <luto@kernel.org> Cc: Andreas Gruenbacher <agruenba@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Mel Gorman <mgorman@techsingularity.net> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent 2d706e7
write.c
/* handling of writes to regular files and writing back to the server
*
* Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version
* 2 of the License, or (at your option) any later version.
*/
#include <linux/backing-dev.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <linux/pagemap.h>
#include <linux/writeback.h>
#include <linux/pagevec.h>
#include "internal.h"
static int afs_write_back_from_locked_page(struct afs_writeback *wb,
struct page *page);
/*
* mark a page as having been made dirty and thus needing writeback
*/
int afs_set_page_dirty(struct page *page)
{
_enter("");
return __set_page_dirty_nobuffers(page);
}
/*
* unlink a writeback record because its usage has reached zero
* - must be called with the wb->vnode->writeback_lock held
*/
static void afs_unlink_writeback(struct afs_writeback *wb)
{
struct afs_writeback *front;
struct afs_vnode *vnode = wb->vnode;
list_del_init(&wb->link);
if (!list_empty(&vnode->writebacks)) {
/* if an fsync rises to the front of the queue then wake it
* up */
front = list_entry(vnode->writebacks.next,
struct afs_writeback, link);
if (front->state == AFS_WBACK_SYNCING) {
_debug("wake up sync");
front->state = AFS_WBACK_COMPLETE;
wake_up(&front->waitq);
}
}
}
/*
* free a writeback record
*/
static void afs_free_writeback(struct afs_writeback *wb)
{
_enter("");
key_put(wb->key);
kfree(wb);
}
/*
* dispose of a reference to a writeback record
*/
void afs_put_writeback(struct afs_writeback *wb)
{
struct afs_vnode *vnode = wb->vnode;
_enter("{%d}", wb->usage);
spin_lock(&vnode->writeback_lock);
if (--wb->usage == 0)
afs_unlink_writeback(wb);
else
wb = NULL;
spin_unlock(&vnode->writeback_lock);
if (wb)
afs_free_writeback(wb);
}
/*
* partly or wholly fill a page that's under preparation for writing
*/
static int afs_fill_page(struct afs_vnode *vnode, struct key *key,
loff_t pos, struct page *page)
{
loff_t i_size;
int ret;
int len;
_enter(",,%llu", (unsigned long long)pos);
i_size = i_size_read(&vnode->vfs_inode);
if (pos + PAGE_SIZE > i_size)
len = i_size - pos;
else
len = PAGE_SIZE;
ret = afs_vnode_fetch_data(vnode, key, pos, len, page);
if (ret < 0) {
if (ret == -ENOENT) {
_debug("got NOENT from server"
" - marking file deleted and stale");
set_bit(AFS_VNODE_DELETED, &vnode->flags);
ret = -ESTALE;
}
}
_leave(" = %d", ret);
return ret;
}
/*
* prepare to perform part of a write to a page
*/
int afs_write_begin(struct file *file, struct address_space *mapping,
loff_t pos, unsigned len, unsigned flags,
struct page **pagep, void **fsdata)
{
struct afs_writeback *candidate, *wb;
struct afs_vnode *vnode = AFS_FS_I(file_inode(file));
struct page *page;
struct key *key = file->private_data;
unsigned from = pos & (PAGE_SIZE - 1);
unsigned to = from + len;
pgoff_t index = pos >> PAGE_SHIFT;
int ret;
_enter("{%x:%u},{%lx},%u,%u",
vnode->fid.vid, vnode->fid.vnode, index, from, to);
candidate = kzalloc(sizeof(*candidate), GFP_KERNEL);
if (!candidate)
return -ENOMEM;
candidate->vnode = vnode;
candidate->first = candidate->last = index;
candidate->offset_first = from;
candidate->to_last = to;
INIT_LIST_HEAD(&candidate->link);
candidate->usage = 1;
candidate->state = AFS_WBACK_PENDING;
init_waitqueue_head(&candidate->waitq);
page = grab_cache_page_write_begin(mapping, index, flags);
if (!page) {
kfree(candidate);
return -ENOMEM;
}
*pagep = page;
/* page won't leak in error case: it eventually gets cleaned off LRU */
if (!PageUptodate(page) && len != PAGE_SIZE) {
ret = afs_fill_page(vnode, key, index << PAGE_SHIFT, page);
if (ret < 0) {
kfree(candidate);
_leave(" = %d [prep]", ret);
return ret;
}
SetPageUptodate(page);
}
try_again:
spin_lock(&vnode->writeback_lock);
/* see if this page is already pending a writeback under a suitable key
* - if so we can just join onto that one */
wb = (struct afs_writeback *) page_private(page);
if (wb) {
if (wb->key == key && wb->state == AFS_WBACK_PENDING)
goto subsume_in_current_wb;
goto flush_conflicting_wb;
}
if (index > 0) {
/* see if we can find an already pending writeback that we can
* append this page to */
list_for_each_entry(wb, &vnode->writebacks, link) {
if (wb->last == index - 1 && wb->key == key &&
wb->state == AFS_WBACK_PENDING)
goto append_to_previous_wb;
}
}
list_add_tail(&candidate->link, &vnode->writebacks);
candidate->key = key_get(key);
spin_unlock(&vnode->writeback_lock);
SetPagePrivate(page);
set_page_private(page, (unsigned long) candidate);
_leave(" = 0 [new]");
return 0;
subsume_in_current_wb:
_debug("subsume");
ASSERTRANGE(wb->first, <=, index, <=, wb->last);
if (index == wb->first && from < wb->offset_first)
wb->offset_first = from;
if (index == wb->last && to > wb->to_last)
wb->to_last = to;
spin_unlock(&vnode->writeback_lock);
kfree(candidate);
_leave(" = 0 [sub]");
return 0;
append_to_previous_wb:
_debug("append into %lx-%lx", wb->first, wb->last);
wb->usage++;
wb->last++;
wb->to_last = to;
spin_unlock(&vnode->writeback_lock);
SetPagePrivate(page);
set_page_private(page, (unsigned long) wb);
kfree(candidate);
_leave(" = 0 [app]");
return 0;
/* the page is currently bound to another context, so if it's dirty we
* need to flush it before we can use the new context */
flush_conflicting_wb:
_debug("flush conflict");
if (wb->state == AFS_WBACK_PENDING)
wb->state = AFS_WBACK_CONFLICTING;
spin_unlock(&vnode->writeback_lock);
if (PageDirty(page)) {
ret = afs_write_back_from_locked_page(wb, page);
if (ret < 0) {
afs_put_writeback(candidate);
_leave(" = %d", ret);
return ret;
}
}
/* the page holds a ref on the writeback record */
afs_put_writeback(wb);
set_page_private(page, 0);
ClearPagePrivate(page);
goto try_again;
}
/*
* finalise part of a write to a page
*/
int afs_write_end(struct file *file, struct address_space *mapping,
loff_t pos, unsigned len, unsigned copied,
struct page *page, void *fsdata)
{
struct afs_vnode *vnode = AFS_FS_I(file_inode(file));
loff_t i_size, maybe_i_size;
_enter("{%x:%u},{%lx}",
vnode->fid.vid, vnode->fid.vnode, page->index);
maybe_i_size = pos + copied;
i_size = i_size_read(&vnode->vfs_inode);
if (maybe_i_size > i_size) {
spin_lock(&vnode->writeback_lock);
i_size = i_size_read(&vnode->vfs_inode);
if (maybe_i_size > i_size)
i_size_write(&vnode->vfs_inode, maybe_i_size);
spin_unlock(&vnode->writeback_lock);
}
set_page_dirty(page);
if (PageDirty(page))
_debug("dirtied");
unlock_page(page);
put_page(page);
return copied;
}
/*
* kill all the pages in the given range
*/
static void afs_kill_pages(struct afs_vnode *vnode, bool error,
pgoff_t first, pgoff_t last)
{
struct pagevec pv;
unsigned count, loop;
_enter("{%x:%u},%lx-%lx",
vnode->fid.vid, vnode->fid.vnode, first, last);
pagevec_init(&pv, 0);
do {
_debug("kill %lx-%lx", first, last);
count = last - first + 1;
if (count > PAGEVEC_SIZE)
count = PAGEVEC_SIZE;
pv.nr = find_get_pages_contig(vnode->vfs_inode.i_mapping,
first, count, pv.pages);
ASSERTCMP(pv.nr, ==, count);
for (loop = 0; loop < count; loop++) {
ClearPageUptodate(pv.pages[loop]);
if (error)
SetPageError(pv.pages[loop]);
end_page_writeback(pv.pages[loop]);
}
__pagevec_release(&pv);
} while (first < last);
_leave("");
}
/*
* synchronously write back the locked page and any subsequent non-locked dirty
* pages also covered by the same writeback record
*/
static int afs_write_back_from_locked_page(struct afs_writeback *wb,
struct page *primary_page)
{
struct page *pages[8], *page;
unsigned long count;
unsigned n, offset, to;
pgoff_t start, first, last;
int loop, ret;
_enter(",%lx", primary_page->index);
count = 1;
if (!clear_page_dirty_for_io(primary_page))
BUG();
if (test_set_page_writeback(primary_page))
BUG();
/* find all consecutive lockable dirty pages, stopping when we find a
* page that is not immediately lockable, is not dirty or is missing,
* or we reach the end of the range */
start = primary_page->index;
if (start >= wb->last)
goto no_more;
start++;
do {
_debug("more %lx [%lx]", start, count);
n = wb->last - start + 1;
if (n > ARRAY_SIZE(pages))
n = ARRAY_SIZE(pages);
n = find_get_pages_contig(wb->vnode->vfs_inode.i_mapping,
start, n, pages);
_debug("fgpc %u", n);
if (n == 0)
goto no_more;
if (pages[0]->index != start) {
do {
put_page(pages[--n]);
} while (n > 0);
goto no_more;
}
for (loop = 0; loop < n; loop++) {
page = pages[loop];
if (page->index > wb->last)
break;
if (!trylock_page(page))
break;
if (!PageDirty(page) ||
page_private(page) != (unsigned long) wb) {
unlock_page(page);
break;
}
if (!clear_page_dirty_for_io(page))
BUG();
if (test_set_page_writeback(page))
BUG();
unlock_page(page);
put_page(page);
}
count += loop;
if (loop < n) {
for (; loop < n; loop++)
put_page(pages[loop]);
goto no_more;
}
start += loop;
} while (start <= wb->last && count < 65536);
no_more:
/* we now have a contiguous set of dirty pages, each with writeback set
* and the dirty mark cleared; the first page is locked and must remain
* so, all the rest are unlocked */
first = primary_page->index;
last = first + count - 1;
offset = (first == wb->first) ? wb->offset_first : 0;
to = (last == wb->last) ? wb->to_last : PAGE_SIZE;
_debug("write back %lx[%u..] to %lx[..%u]", first, offset, last, to);
ret = afs_vnode_store_data(wb, first, last, offset, to);
if (ret < 0) {
switch (ret) {
case -EDQUOT:
case -ENOSPC:
mapping_set_error(wb->vnode->vfs_inode.i_mapping, -ENOSPC);
break;
case -EROFS:
case -EIO:
case -EREMOTEIO:
case -EFBIG:
case -ENOENT:
case -ENOMEDIUM:
case -ENXIO:
afs_kill_pages(wb->vnode, true, first, last);
mapping_set_error(wb->vnode->vfs_inode.i_mapping, -EIO);
break;
case -EACCES:
case -EPERM:
case -ENOKEY:
case -EKEYEXPIRED:
case -EKEYREJECTED:
case -EKEYREVOKED:
afs_kill_pages(wb->vnode, false, first, last);
break;
default:
break;
}
} else {
ret = count;
}
_leave(" = %d", ret);
return ret;
}
/*
* write a page back to the server
* - the caller locked the page for us
*/
int afs_writepage(struct page *page, struct writeback_control *wbc)
{
struct afs_writeback *wb;
int ret;
_enter("{%lx},", page->index);
wb = (struct afs_writeback *) page_private(page);
ASSERT(wb != NULL);
ret = afs_write_back_from_locked_page(wb, page);
unlock_page(page);
if (ret < 0) {
_leave(" = %d", ret);
return 0;
}
wbc->nr_to_write -= ret;
_leave(" = 0");
return 0;
}
/*
* write a region of pages back to the server
*/
static int afs_writepages_region(struct address_space *mapping,
struct writeback_control *wbc,
pgoff_t index, pgoff_t end, pgoff_t *_next)
{
struct afs_writeback *wb;
struct page *page;
int ret, n;
_enter(",,%lx,%lx,", index, end);
do {
n = find_get_pages_tag(mapping, &index, PAGECACHE_TAG_DIRTY,
1, &page);
if (!n)
break;
_debug("wback %lx", page->index);
if (page->index > end) {
*_next = index;
put_page(page);
_leave(" = 0 [%lx]", *_next);
return 0;
}
/* at this point we hold neither mapping->tree_lock nor lock on
* the page itself: the page may be truncated or invalidated
* (changing page->mapping to NULL), or even swizzled back from
* swapper_space to tmpfs file mapping
*/
lock_page(page);
if (page->mapping != mapping) {
unlock_page(page);
put_page(page);
continue;
}
if (wbc->sync_mode != WB_SYNC_NONE)
wait_on_page_writeback(page);
if (PageWriteback(page) || !PageDirty(page)) {
unlock_page(page);
continue;
}
wb = (struct afs_writeback *) page_private(page);
ASSERT(wb != NULL);
spin_lock(&wb->vnode->writeback_lock);
wb->state = AFS_WBACK_WRITING;
spin_unlock(&wb->vnode->writeback_lock);
ret = afs_write_back_from_locked_page(wb, page);
unlock_page(page);
put_page(page);
if (ret < 0) {
_leave(" = %d", ret);
return ret;
}
wbc->nr_to_write -= ret;
cond_resched();
} while (index < end && wbc->nr_to_write > 0);
*_next = index;
_leave(" = 0 [%lx]", *_next);
return 0;
}
/*
* write some of the pending data back to the server
*/
int afs_writepages(struct address_space *mapping,
struct writeback_control *wbc)
{
pgoff_t start, end, next;
int ret;
_enter("");
if (wbc->range_cyclic) {
start = mapping->writeback_index;
end = -1;
ret = afs_writepages_region(mapping, wbc, start, end, &next);
if (start > 0 && wbc->nr_to_write > 0 && ret == 0)
ret = afs_writepages_region(mapping, wbc, 0, start,
&next);
mapping->writeback_index = next;
} else if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX) {
end = (pgoff_t)(LLONG_MAX >> PAGE_SHIFT);
ret = afs_writepages_region(mapping, wbc, 0, end, &next);
if (wbc->nr_to_write > 0)
mapping->writeback_index = next;
} else {
start = wbc->range_start >> PAGE_SHIFT;
end = wbc->range_end >> PAGE_SHIFT;
ret = afs_writepages_region(mapping, wbc, start, end, &next);
}
_leave(" = %d", ret);
return ret;
}
/*
* completion of write to server
*/
void afs_pages_written_back(struct afs_vnode *vnode, struct afs_call *call)
{
struct afs_writeback *wb = call->wb;
struct pagevec pv;
unsigned count, loop;
pgoff_t first = call->first, last = call->last;
bool free_wb;
_enter("{%x:%u},{%lx-%lx}",
vnode->fid.vid, vnode->fid.vnode, first, last);
ASSERT(wb != NULL);
pagevec_init(&pv, 0);
do {
_debug("done %lx-%lx", first, last);
count = last - first + 1;
if (count > PAGEVEC_SIZE)
count = PAGEVEC_SIZE;
pv.nr = find_get_pages_contig(call->mapping, first, count,
pv.pages);
ASSERTCMP(pv.nr, ==, count);
spin_lock(&vnode->writeback_lock);
for (loop = 0; loop < count; loop++) {
struct page *page = pv.pages[loop];
end_page_writeback(page);
if (page_private(page) == (unsigned long) wb) {
set_page_private(page, 0);
ClearPagePrivate(page);
wb->usage--;
}
}
free_wb = false;
if (wb->usage == 0) {
afs_unlink_writeback(wb);
free_wb = true;
}
spin_unlock(&vnode->writeback_lock);
first += count;
if (free_wb) {
afs_free_writeback(wb);
wb = NULL;
}
__pagevec_release(&pv);
} while (first <= last);
_leave("");
}
/*
* write to an AFS file
*/
ssize_t afs_file_write(struct kiocb *iocb, struct iov_iter *from)
{
struct afs_vnode *vnode = AFS_FS_I(file_inode(iocb->ki_filp));
ssize_t result;
size_t count = iov_iter_count(from);
_enter("{%x.%u},{%zu},",
vnode->fid.vid, vnode->fid.vnode, count);
if (IS_SWAPFILE(&vnode->vfs_inode)) {
printk(KERN_INFO
"AFS: Attempt to write to active swap file!\n");
return -EBUSY;
}
if (!count)
return 0;
result = generic_file_write_iter(iocb, from);
_leave(" = %zd", result);
return result;
}
/*
* flush the vnode to the fileserver
*/
int afs_writeback_all(struct afs_vnode *vnode)
{
struct address_space *mapping = vnode->vfs_inode.i_mapping;
struct writeback_control wbc = {
.sync_mode = WB_SYNC_ALL,
.nr_to_write = LONG_MAX,
.range_cyclic = 1,
};
int ret;
_enter("");
ret = mapping->a_ops->writepages(mapping, &wbc);
__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
_leave(" = %d", ret);
return ret;
}
/*
* flush any dirty pages for this process, and check for write errors.
* - the return status from this call provides a reliable indication of
* whether any write errors occurred for this process.
*/
int afs_fsync(struct file *file, loff_t start, loff_t end, int datasync)
{
struct inode *inode = file_inode(file);
struct afs_writeback *wb, *xwb;
struct afs_vnode *vnode = AFS_FS_I(inode);
int ret;
_enter("{%x:%u},{n=%pD},%d",
vnode->fid.vid, vnode->fid.vnode, file,
datasync);
ret = filemap_write_and_wait_range(inode->i_mapping, start, end);
if (ret)
return ret;
inode_lock(inode);
/* use a writeback record as a marker in the queue - when this reaches
* the front of the queue, all the outstanding writes are either
* completed or rejected */
wb = kzalloc(sizeof(*wb), GFP_KERNEL);
if (!wb) {
ret = -ENOMEM;
goto out;
}
wb->vnode = vnode;
wb->first = 0;
wb->last = -1;
wb->offset_first = 0;
wb->to_last = PAGE_SIZE;
wb->usage = 1;
wb->state = AFS_WBACK_SYNCING;
init_waitqueue_head(&wb->waitq);
spin_lock(&vnode->writeback_lock);
list_for_each_entry(xwb, &vnode->writebacks, link) {
if (xwb->state == AFS_WBACK_PENDING)
xwb->state = AFS_WBACK_CONFLICTING;
}
list_add_tail(&wb->link, &vnode->writebacks);
spin_unlock(&vnode->writeback_lock);
/* push all the outstanding writebacks to the server */
ret = afs_writeback_all(vnode);
if (ret < 0) {
afs_put_writeback(wb);
_leave(" = %d [wb]", ret);
goto out;
}
/* wait for the preceding writes to actually complete */
ret = wait_event_interruptible(wb->waitq,
wb->state == AFS_WBACK_COMPLETE ||
vnode->writebacks.next == &wb->link);
afs_put_writeback(wb);
_leave(" = %d", ret);
out:
inode_unlock(inode);
return ret;
}
/*
* notification that a previously read-only page is about to become writable
* - if it returns an error, the caller will deliver a bus error signal
*/
int afs_page_mkwrite(struct vm_area_struct *vma, struct page *page)
{
struct afs_vnode *vnode = AFS_FS_I(vma->vm_file->f_mapping->host);
_enter("{{%x:%u}},{%lx}",
vnode->fid.vid, vnode->fid.vnode, page->index);
/* wait for the page to be written to the cache before we allow it to
* be modified */
#ifdef CONFIG_AFS_FSCACHE
fscache_wait_on_page_write(vnode->cache, page);
#endif
_leave(" = 0");
return 0;
}
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