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userfaultfd: optimize read() and poll() to be O(1) 

This makes read O(1) and poll that was already O(1) becomes lockless.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Cc: Sanidhya Kashyap <sanidhya.gatech@gmail.com>
Cc: zhang.zhanghailiang@huawei.com
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Feiner <pfeiner@google.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Huangpeng (Peter)" <peter.huangpeng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Andrea Arcangeli 2015-09-04 15:46:44 -07:00 committed by Linus Torvalds
parent ba85c702e4
commit 15b726ef04
1 changed files with 112 additions and 75 deletions
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187
fs/userfaultfd.c
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@ -35,7 +35,9 @@ enum userfaultfd_state {
struct userfaultfd_ctx {
/* pseudo fd refcounting */
atomic_t refcount;
/* waitqueue head for the userfaultfd page faults */
/* waitqueue head for the pending (i.e. not read) userfaults */
wait_queue_head_t fault_pending_wqh;
/* waitqueue head for the userfaults */
wait_queue_head_t fault_wqh;
/* waitqueue head for the pseudo fd to wakeup poll/read */
wait_queue_head_t fd_wqh;
@ -52,11 +54,6 @@ struct userfaultfd_ctx {
struct userfaultfd_wait_queue {
struct uffd_msg msg;
wait_queue_t wq;
/*
* Only relevant when queued in fault_wqh and only used by the
* read operation to avoid reading the same userfault twice.
*/
bool pending;
struct userfaultfd_ctx *ctx;
};
@ -263,17 +260,21 @@ int handle_userfault(struct vm_area_struct *vma, unsigned long address,
init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
uwq.wq.private = current;
uwq.msg = userfault_msg(address, flags, reason);
uwq.pending = true;
uwq.ctx = ctx;
spin_lock(&ctx->fault_wqh.lock);
spin_lock(&ctx->fault_pending_wqh.lock);
/*
* After the __add_wait_queue the uwq is visible to userland
* through poll/read().
*/
__add_wait_queue(&ctx->fault_wqh, &uwq.wq);
__add_wait_queue(&ctx->fault_pending_wqh, &uwq.wq);
/*
* The smp_mb() after __set_current_state prevents the reads
* following the spin_unlock to happen before the list_add in
* __add_wait_queue.
*/
set_current_state(TASK_KILLABLE);
spin_unlock(&ctx->fault_wqh.lock);
spin_unlock(&ctx->fault_pending_wqh.lock);
if (likely(!ACCESS_ONCE(ctx->released) &&
!fatal_signal_pending(current))) {
@ -283,11 +284,28 @@ int handle_userfault(struct vm_area_struct *vma, unsigned long address,
}
__set_current_state(TASK_RUNNING);
/* see finish_wait() comment for why list_empty_careful() */
/*
* Here we race with the list_del; list_add in
* userfaultfd_ctx_read(), however because we don't ever run
* list_del_init() to refile across the two lists, the prev
* and next pointers will never point to self. list_add also
* would never let any of the two pointers to point to
* self. So list_empty_careful won't risk to see both pointers
* pointing to self at any time during the list refile. The
* only case where list_del_init() is called is the full
* removal in the wake function and there we don't re-list_add
* and it's fine not to block on the spinlock. The uwq on this
* kernel stack can be released after the list_del_init.
*/
if (!list_empty_careful(&uwq.wq.task_list)) {
spin_lock(&ctx->fault_wqh.lock);
list_del_init(&uwq.wq.task_list);
spin_unlock(&ctx->fault_wqh.lock);
spin_lock(&ctx->fault_pending_wqh.lock);
/*
* No need of list_del_init(), the uwq on the stack
* will be freed shortly anyway.
*/
list_del(&uwq.wq.task_list);
spin_unlock(&ctx->fault_pending_wqh.lock);
}
/*
@ -345,59 +363,38 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
up_write(&mm->mmap_sem);
/*
* After no new page faults can wait on this fault_wqh, flush
* After no new page faults can wait on this fault_*wqh, flush
* the last page faults that may have been already waiting on
* the fault_wqh.
* the fault_*wqh.
*/
spin_lock(&ctx->fault_wqh.lock);
spin_lock(&ctx->fault_pending_wqh.lock);
__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, 0, &range);
__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, 0, &range);
spin_unlock(&ctx->fault_wqh.lock);
spin_unlock(&ctx->fault_pending_wqh.lock);
wake_up_poll(&ctx->fd_wqh, POLLHUP);
userfaultfd_ctx_put(ctx);
return 0;
}
/* fault_wqh.lock must be hold by the caller */
static inline unsigned int find_userfault(struct userfaultfd_ctx *ctx,
struct userfaultfd_wait_queue **uwq)
/* fault_pending_wqh.lock must be hold by the caller */
static inline struct userfaultfd_wait_queue *find_userfault(
struct userfaultfd_ctx *ctx)
{
wait_queue_t *wq;
struct userfaultfd_wait_queue *_uwq;
unsigned int ret = 0;
struct userfaultfd_wait_queue *uwq;
VM_BUG_ON(!spin_is_locked(&ctx->fault_wqh.lock));
VM_BUG_ON(!spin_is_locked(&ctx->fault_pending_wqh.lock));
list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
_uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
if (_uwq->pending) {
ret = POLLIN;
if (!uwq)
/*
* If there's at least a pending and
* we don't care which one it is,
* break immediately and leverage the
* efficiency of the LIFO walk.
*/
break;
/*
* If we need to find which one was pending we
* keep walking until we find the first not
* pending one, so we read() them in FIFO order.
*/
*uwq = _uwq;
} else
/*
* break the loop at the first not pending
* one, there cannot be pending userfaults
* after the first not pending one, because
* all new pending ones are inserted at the
* head and we walk it in LIFO.
*/
break;
}
return ret;
uwq = NULL;
if (!waitqueue_active(&ctx->fault_pending_wqh))
goto out;
/* walk in reverse to provide FIFO behavior to read userfaults */
wq = list_last_entry(&ctx->fault_pending_wqh.task_list,
typeof(*wq), task_list);
uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
out:
return uwq;
}
static unsigned int userfaultfd_poll(struct file *file, poll_table *wait)
@ -417,9 +414,20 @@ static unsigned int userfaultfd_poll(struct file *file, poll_table *wait)
*/
if (unlikely(!(file->f_flags & O_NONBLOCK)))
return POLLERR;
spin_lock(&ctx->fault_wqh.lock);
ret = find_userfault(ctx, NULL);
spin_unlock(&ctx->fault_wqh.lock);
/*
* lockless access to see if there are pending faults
* __pollwait last action is the add_wait_queue but
* the spin_unlock would allow the waitqueue_active to
* pass above the actual list_add inside
* add_wait_queue critical section. So use a full
* memory barrier to serialize the list_add write of
* add_wait_queue() with the waitqueue_active read
* below.
*/
ret = 0;
smp_mb();
if (waitqueue_active(&ctx->fault_pending_wqh))
ret = POLLIN;
return ret;
default:
BUG();
@ -431,27 +439,47 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
{
ssize_t ret;
DECLARE_WAITQUEUE(wait, current);
struct userfaultfd_wait_queue *uwq = NULL;
struct userfaultfd_wait_queue *uwq;
/* always take the fd_wqh lock before the fault_wqh lock */
/* always take the fd_wqh lock before the fault_pending_wqh lock */
spin_lock(&ctx->fd_wqh.lock);
__add_wait_queue(&ctx->fd_wqh, &wait);
for (;;) {
set_current_state(TASK_INTERRUPTIBLE);
spin_lock(&ctx->fault_wqh.lock);
if (find_userfault(ctx, &uwq)) {
spin_lock(&ctx->fault_pending_wqh.lock);
uwq = find_userfault(ctx);
if (uwq) {
/*
* The fault_wqh.lock prevents the uwq to
* disappear from under us.
* The fault_pending_wqh.lock prevents the uwq
* to disappear from under us.
*
* Refile this userfault from
* fault_pending_wqh to fault_wqh, it's not
* pending anymore after we read it.
*
* Use list_del() by hand (as
* userfaultfd_wake_function also uses
* list_del_init() by hand) to be sure nobody
* changes __remove_wait_queue() to use
* list_del_init() in turn breaking the
* !list_empty_careful() check in
* handle_userfault(). The uwq->wq.task_list
* must never be empty at any time during the
* refile, or the waitqueue could disappear
* from under us. The "wait_queue_head_t"
* parameter of __remove_wait_queue() is unused
* anyway.
*/
uwq->pending = false;
list_del(&uwq->wq.task_list);
__add_wait_queue(&ctx->fault_wqh, &uwq->wq);
/* careful to always initialize msg if ret == 0 */
*msg = uwq->msg;
spin_unlock(&ctx->fault_wqh.lock);
spin_unlock(&ctx->fault_pending_wqh.lock);
ret = 0;
break;
}
spin_unlock(&ctx->fault_wqh.lock);
spin_unlock(&ctx->fault_pending_wqh.lock);
if (signal_pending(current)) {
ret = -ERESTARTSYS;
break;
@ -510,10 +538,14 @@ static void __wake_userfault(struct userfaultfd_ctx *ctx,
start = range->start;
end = range->start + range->len;
spin_lock(&ctx->fault_wqh.lock);
spin_lock(&ctx->fault_pending_wqh.lock);
/* wake all in the range and autoremove */
__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, 0, range);
spin_unlock(&ctx->fault_wqh.lock);
if (waitqueue_active(&ctx->fault_pending_wqh))
__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, 0,
range);
if (waitqueue_active(&ctx->fault_wqh))
__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, 0, range);
spin_unlock(&ctx->fault_pending_wqh.lock);
}
static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
@ -534,7 +566,8 @@ static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
* userfaults yet. So we take the spinlock only when we're
* sure we've userfaults to wake.
*/
if (waitqueue_active(&ctx->fault_wqh))
if (waitqueue_active(&ctx->fault_pending_wqh) ||
waitqueue_active(&ctx->fault_wqh))
__wake_userfault(ctx, range);
}
@ -960,14 +993,17 @@ static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
struct userfaultfd_wait_queue *uwq;
unsigned long pending = 0, total = 0;
spin_lock(&ctx->fault_wqh.lock);
list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
spin_lock(&ctx->fault_pending_wqh.lock);
list_for_each_entry(wq, &ctx->fault_pending_wqh.task_list, task_list) {
uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
if (uwq->pending)
pending++;
pending++;
total++;
}
spin_unlock(&ctx->fault_wqh.lock);
list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
total++;
}
spin_unlock(&ctx->fault_pending_wqh.lock);
/*
* If more protocols will be added, there will be all shown
@ -1027,6 +1063,7 @@ static struct file *userfaultfd_file_create(int flags)
goto out;
atomic_set(&ctx->refcount, 1);
init_waitqueue_head(&ctx->fault_pending_wqh);
init_waitqueue_head(&ctx->fault_wqh);
init_waitqueue_head(&ctx->fd_wqh);
ctx->flags = flags;