[PATCH 0/3] vmevent: Some fixes + a new event type

[PATCH 0/3] vmevent: Some fixes + a new event type

[Anton Vorontsov]

Hi all,

This small patch set fixes a bug in the vmevent core, plus makes the
vmevent-test buildable w/o unneded SDL library.

Plus, we add a new 'cross' event type: the event will trigger whenever
a value crosses a user-specified threshold. It works two-way, i.e. when
a value crosses the threshold from a lesser values side to a greater 
values side, and vice versa.

We use the event type in an userspace low-memory killer: we get a
notification when memory becomes low, so we start freeing memory by
killing unneeded processes, and we get notification when memory hits
the threshold from another side, so we know that we freed enough of
memory.

The patches are against

	git://github.com/penberg/linux.git vmevent/core

Thanks!

-- 
Anton Vorontsov
Email: cbouatmailru@gmail.com

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[PATCH 1/3] vmevent: Should not grab mutex in the atomic context

[Anton Vorontsov]

vmevent grabs a mutex in the atomic context, and so this pops up:

BUG: sleeping function called from invalid context at kernel/mutex.c:271
in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
1 lock held by swapper/0/0:
 #0:  (&watch->timer){+.-...}, at: [<ffffffff8103eb80>] call_timer_fn+0x0/0xf0
Pid: 0, comm: swapper/0 Not tainted 3.2.0+ #6
Call Trace:
 <IRQ>  [<ffffffff8102f5da>] __might_sleep+0x12a/0x1e0
 [<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
 [<ffffffff81321f2c>] mutex_lock_nested+0x3c/0x340
 [<ffffffff81064b33>] ? lock_acquire+0xa3/0xc0
 [<ffffffff8103eb80>] ? internal_add_timer+0x110/0x110
 [<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
 [<ffffffff810bda21>] vmevent_timer_fn+0x91/0xf0
 [<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
 [<ffffffff8103ebf5>] call_timer_fn+0x75/0xf0
 [<ffffffff8103eb80>] ? internal_add_timer+0x110/0x110
 [<ffffffff81062fdd>] ? trace_hardirqs_on_caller+0x7d/0x120
 [<ffffffff8103ee9f>] run_timer_softirq+0x10f/0x1e0
 [<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
 [<ffffffff81038d90>] __do_softirq+0xb0/0x160
 [<ffffffff8105eb0f>] ? tick_program_event+0x1f/0x30
 [<ffffffff8132642c>] call_softirq+0x1c/0x26
 [<ffffffff810036d5>] do_softirq+0x85/0xc0

This patch fixes the issue by removing the mutex and making the logic
lock-free.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 mm/vmevent.c |   35 ++++++++++++-----------------------
 1 files changed, 12 insertions(+), 23 deletions(-)

diff --git a/mm/vmevent.c b/mm/vmevent.c
index 1847b56..a56174f 100644
--- a/mm/vmevent.c
+++ b/mm/vmevent.c
@@ -1,4 +1,5 @@
 #include <linux/anon_inodes.h>
+#include <linux/atomic.h>
 #include <linux/vmevent.h>
 #include <linux/syscalls.h>
 #include <linux/timer.h>
@@ -23,7 +24,7 @@ struct vmevent_watch {
 	struct vmevent_config		config;
 
 	struct mutex			mutex;
-	bool				pending;
+	atomic_t			pending;
 
 	/*
 	 * Attributes that are exported as part of delivered VM events.
@@ -103,20 +104,18 @@ static void vmevent_sample(struct vmevent_watch *watch)
 {
 	int i;
 
+	if (atomic_read(&watch->pending))
+		return;
 	if (!vmevent_match(watch))
 		return;
 
-	mutex_lock(&watch->mutex);
-
-	watch->pending = true;
-
 	for (i = 0; i < watch->nr_attrs; i++) {
 		struct vmevent_attr *attr = &watch->sample_attrs[i];
 
 		attr->value = vmevent_sample_attr(watch, attr);
 	}
 
-	mutex_unlock(&watch->mutex);
+	atomic_set(&watch->pending, 1);
 }
 
 static void vmevent_timer_fn(unsigned long data)
@@ -125,7 +124,7 @@ static void vmevent_timer_fn(unsigned long data)
 
 	vmevent_sample(watch);
 
-	if (watch->pending)
+	if (atomic_read(&watch->pending))
 		wake_up(&watch->waitq);
 	mod_timer(&watch->timer, jiffies +
 			nsecs_to_jiffies64(watch->config.sample_period_ns));
@@ -148,13 +147,9 @@ static unsigned int vmevent_poll(struct file *file, poll_table *wait)
 
 	poll_wait(file, &watch->waitq, wait);
 
-	mutex_lock(&watch->mutex);
-
-	if (watch->pending)
+	if (atomic_read(&watch->pending))
 		events |= POLLIN;
 
-	mutex_unlock(&watch->mutex);
-
 	return events;
 }
 
@@ -171,15 +166,13 @@ static ssize_t vmevent_read(struct file *file, char __user *buf, size_t count, l
 	if (count < size)
 		return -EINVAL;
 
-	mutex_lock(&watch->mutex);
-
-	if (!watch->pending)
-		goto out_unlock;
+	if (!atomic_read(&watch->pending))
+		goto out;
 
 	event = kmalloc(size, GFP_KERNEL);
 	if (!event) {
 		ret = -ENOMEM;
-		goto out_unlock;
+		goto out;
 	}
 
 	for (i = 0; i < watch->nr_attrs; i++) {
@@ -195,14 +188,10 @@ static ssize_t vmevent_read(struct file *file, char __user *buf, size_t count, l
 
 	ret = count;
 
-	watch->pending = false;
-
+	atomic_set(&watch->pending, 0);
 out_free:
 	kfree(event);
-
-out_unlock:
-	mutex_unlock(&watch->mutex);
-
+out:
 	return ret;
 }
 
-- 
1.7.7.6

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Re: [PATCH 1/3] vmevent: Should not grab mutex in the atomic context

[Pekka Enberg]

On Mon, 2012-04-09 at 03:38 +0400, Anton Vorontsov wrote:
> vmevent grabs a mutex in the atomic context, and so this pops up:
> 
> BUG: sleeping function called from invalid context at kernel/mutex.c:271
> in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
> 1 lock held by swapper/0/0:
>  #0:  (&watch->timer){+.-...}, at: [<ffffffff8103eb80>] call_timer_fn+0x0/0xf0
> Pid: 0, comm: swapper/0 Not tainted 3.2.0+ #6
> Call Trace:
>  <IRQ>  [<ffffffff8102f5da>] __might_sleep+0x12a/0x1e0
>  [<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
>  [<ffffffff81321f2c>] mutex_lock_nested+0x3c/0x340
>  [<ffffffff81064b33>] ? lock_acquire+0xa3/0xc0
>  [<ffffffff8103eb80>] ? internal_add_timer+0x110/0x110
>  [<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
>  [<ffffffff810bda21>] vmevent_timer_fn+0x91/0xf0
>  [<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
>  [<ffffffff8103ebf5>] call_timer_fn+0x75/0xf0
>  [<ffffffff8103eb80>] ? internal_add_timer+0x110/0x110
>  [<ffffffff81062fdd>] ? trace_hardirqs_on_caller+0x7d/0x120
>  [<ffffffff8103ee9f>] run_timer_softirq+0x10f/0x1e0
>  [<ffffffff810bd990>] ? vmevent_match+0xe0/0xe0
>  [<ffffffff81038d90>] __do_softirq+0xb0/0x160
>  [<ffffffff8105eb0f>] ? tick_program_event+0x1f/0x30
>  [<ffffffff8132642c>] call_softirq+0x1c/0x26
>  [<ffffffff810036d5>] do_softirq+0x85/0xc0
> 
> This patch fixes the issue by removing the mutex and making the logic
> lock-free.
> 
> Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

What guarantees that there's only one thread writing to struct
vmevent_attr::value in vmevent_sample() now that the mutex is gone?

> ---
>  mm/vmevent.c |   35 ++++++++++++-----------------------
>  1 files changed, 12 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/vmevent.c b/mm/vmevent.c
> index 1847b56..a56174f 100644
> --- a/mm/vmevent.c
> +++ b/mm/vmevent.c
> @@ -1,4 +1,5 @@
>  #include <linux/anon_inodes.h>
> +#include <linux/atomic.h>
>  #include <linux/vmevent.h>
>  #include <linux/syscalls.h>
>  #include <linux/timer.h>
> @@ -23,7 +24,7 @@ struct vmevent_watch {
>  	struct vmevent_config		config;
>  
>  	struct mutex			mutex;
> -	bool				pending;
> +	atomic_t			pending;
>  
>  	/*
>  	 * Attributes that are exported as part of delivered VM events.
> @@ -103,20 +104,18 @@ static void vmevent_sample(struct vmevent_watch *watch)
>  {
>  	int i;
>  
> +	if (atomic_read(&watch->pending))
> +		return;
>  	if (!vmevent_match(watch))
>  		return;
>  
> -	mutex_lock(&watch->mutex);
> -
> -	watch->pending = true;
> -
>  	for (i = 0; i < watch->nr_attrs; i++) {
>  		struct vmevent_attr *attr = &watch->sample_attrs[i];
>  
>  		attr->value = vmevent_sample_attr(watch, attr);
>  	}
>  
> -	mutex_unlock(&watch->mutex);
> +	atomic_set(&watch->pending, 1);
>  }
>  
>  static void vmevent_timer_fn(unsigned long data)
> @@ -125,7 +124,7 @@ static void vmevent_timer_fn(unsigned long data)
>  
>  	vmevent_sample(watch);
>  
> -	if (watch->pending)
> +	if (atomic_read(&watch->pending))
>  		wake_up(&watch->waitq);
>  	mod_timer(&watch->timer, jiffies +
>  			nsecs_to_jiffies64(watch->config.sample_period_ns));
> @@ -148,13 +147,9 @@ static unsigned int vmevent_poll(struct file *file, poll_table *wait)
>  
>  	poll_wait(file, &watch->waitq, wait);
>  
> -	mutex_lock(&watch->mutex);
> -
> -	if (watch->pending)
> +	if (atomic_read(&watch->pending))
>  		events |= POLLIN;
>  
> -	mutex_unlock(&watch->mutex);
> -
>  	return events;
>  }
>  
> @@ -171,15 +166,13 @@ static ssize_t vmevent_read(struct file *file, char __user *buf, size_t count, l
>  	if (count < size)
>  		return -EINVAL;
>  
> -	mutex_lock(&watch->mutex);
> -
> -	if (!watch->pending)
> -		goto out_unlock;
> +	if (!atomic_read(&watch->pending))
> +		goto out;
>  
>  	event = kmalloc(size, GFP_KERNEL);
>  	if (!event) {
>  		ret = -ENOMEM;
> -		goto out_unlock;
> +		goto out;
>  	}
>  
>  	for (i = 0; i < watch->nr_attrs; i++) {
> @@ -195,14 +188,10 @@ static ssize_t vmevent_read(struct file *file, char __user *buf, size_t count, l
>  
>  	ret = count;
>  
> -	watch->pending = false;
> -
> +	atomic_set(&watch->pending, 0);
>  out_free:
>  	kfree(event);
> -
> -out_unlock:
> -	mutex_unlock(&watch->mutex);
> -
> +out:
>  	return ret;
>  }
>  


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Re: [PATCH 1/3] vmevent: Should not grab mutex in the atomic context

[Anton Vorontsov]

On Mon, Apr 09, 2012 at 11:40:31AM +0300, Pekka Enberg wrote:
> On Mon, 2012-04-09 at 03:38 +0400, Anton Vorontsov wrote:
> > vmevent grabs a mutex in the atomic context, and so this pops up:
> > 
> > BUG: sleeping function called from invalid context at kernel/mutex.c:271
> > in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
[...]
> > This patch fixes the issue by removing the mutex and making the logic
> > lock-free.
> > 
> > Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
> 
> What guarantees that there's only one thread writing to struct
> vmevent_attr::value in vmevent_sample() now that the mutex is gone?

Well, it is called from the timer function, which has the same guaranties
as an interrupt handler: it can have only one execution thread (unlike
bare softirq handler), so we don't need to worry about racing w/
ourselves?

If you're concerned about several instances of timers accessing the 
same vmevent_watch, I don't really see how it is possible, as we
allocate vmevent_watch together w/ the timer instance in vmevent_fd(),
so there is always one timer per vmevent_watch.

Thanks,

-- 
Anton Vorontsov
Email: cbouatmailru@gmail.com

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Re: [PATCH 1/3] vmevent: Should not grab mutex in the atomic context

[Pekka Enberg]

> On Mon, Apr 09, 2012 at 11:40:31AM +0300, Pekka Enberg wrote:
>> On Mon, 2012-04-09 at 03:38 +0400, Anton Vorontsov wrote:
>> > vmevent grabs a mutex in the atomic context, and so this pops up:
>> >
>> > BUG: sleeping function called from invalid context at kernel/mutex.c:271
>> > in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
> [...]
>> > This patch fixes the issue by removing the mutex and making the logic
>> > lock-free.
>> >
>> > Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
>>
>> What guarantees that there's only one thread writing to struct
>> vmevent_attr::value in vmevent_sample() now that the mutex is gone?

On Mon, Apr 9, 2012 at 3:29 PM, Anton Vorontsov
<anton.vorontsov@linaro.org> wrote:
> Well, it is called from the timer function, which has the same guaranties
> as an interrupt handler: it can have only one execution thread (unlike
> bare softirq handler), so we don't need to worry about racing w/
> ourselves?
>
> If you're concerned about several instances of timers accessing the
> same vmevent_watch, I don't really see how it is possible, as we
> allocate vmevent_watch together w/ the timer instance in vmevent_fd(),
> so there is always one timer per vmevent_watch.

Makes sense. A big fat comment on top of vmevent_sample() explaining
all this would be helpful... ;-)

                        Pekka

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[PATCH 2/3] vmevent-test: No need for SDL library

[Anton Vorontsov]

panacea:~/src/linux/linux-vmevent/tools/testing/vmevent$ make
cc -O3 -g -std=gnu99 -Wcast-align -Wformat -Wformat-security
-Wformat-y2k -Wshadow -Winit-self -Wpacked -Wredundant-decls
-Wstrict-aliasing=3 -Wswitch-default -Wno-system-headers -Wundef
-Wwrite-strings -Wbad-function-cast -Wmissing-declarations
-Wmissing-prototypes -Wnested-externs -Wold-style-definition
-Wstrict-prototypes -Wdeclaration-after-statement  -lSDL  vmevent-test.c
-o vmevent-test
/usr/bin/ld: cannot find -lSDL
collect2: ld returned 1 exit status
make: *** [vmevent-test] Error 1

This patch fixes the issue.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 tools/testing/vmevent/Makefile |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/tools/testing/vmevent/Makefile b/tools/testing/vmevent/Makefile
index 5b5505f..d14b5c9 100644
--- a/tools/testing/vmevent/Makefile
+++ b/tools/testing/vmevent/Makefile
@@ -20,7 +20,6 @@ WARNINGS += -Wstrict-prototypes
 WARNINGS += -Wdeclaration-after-statement
 
 CFLAGS  = -O3 -g -std=gnu99 $(WARNINGS)
-LDFLAGS = -lSDL
 
 PROGRAMS = vmevent-test
 
-- 
1.7.7.6

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Re: [PATCH 2/3] vmevent-test: No need for SDL library

[Pekka Enberg]

On Mon, 9 Apr 2012, Anton Vorontsov wrote:
> panacea:~/src/linux/linux-vmevent/tools/testing/vmevent$ make
> cc -O3 -g -std=gnu99 -Wcast-align -Wformat -Wformat-security
> -Wformat-y2k -Wshadow -Winit-self -Wpacked -Wredundant-decls
> -Wstrict-aliasing=3 -Wswitch-default -Wno-system-headers -Wundef
> -Wwrite-strings -Wbad-function-cast -Wmissing-declarations
> -Wmissing-prototypes -Wnested-externs -Wold-style-definition
> -Wstrict-prototypes -Wdeclaration-after-statement  -lSDL  vmevent-test.c
> -o vmevent-test
> /usr/bin/ld: cannot find -lSDL
> collect2: ld returned 1 exit status
> make: *** [vmevent-test] Error 1
> 
> This patch fixes the issue.
> 
> Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

Applied, thanks!

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[PATCH 3/3] vmevent: Implement cross event type

[Anton Vorontsov]

This patch implements a new event type, it will trigger whenever a
value crosses a user-specified threshold. It works two-way, i.e. when
a value crosses the threshold from a lesser values side to a greater
values side, and vice versa.

We use the event type in an userspace low-memory killer: we get a
notification when memory becomes low, so we start freeing memory by
killing unneeded processes, and we get notification when memory hits
the threshold from another side, so we know that we freed enough of
memory.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 include/linux/vmevent.h              |    9 +++++++++
 mm/vmevent.c                         |   21 +++++++++++++++++++++
 tools/testing/vmevent/vmevent-test.c |   15 ++++++++++-----
 3 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/include/linux/vmevent.h b/include/linux/vmevent.h
index 64357e4..00cc04f 100644
--- a/include/linux/vmevent.h
+++ b/include/linux/vmevent.h
@@ -22,6 +22,15 @@ enum {
 	 * Sample value is less than user-specified value
 	 */
 	VMEVENT_ATTR_STATE_VALUE_LT	= (1UL << 0),
+	/*
+	 * Sample value crossed user-specified value
+	 */
+	VMEVENT_ATTR_STATE_VALUE_CROSS	= (1UL << 2),
+
+	/* Last saved state, used internally by the kernel. */
+	__VMEVENT_ATTR_STATE_LAST	= (1UL << 30),
+	/* Not first sample, used internally by the kernel. */
+	__VMEVENT_ATTR_STATE_NFIRST	= (1UL << 31),
 };
 
 struct vmevent_attr {
diff --git a/mm/vmevent.c b/mm/vmevent.c
index a56174f..f8fd2d6 100644
--- a/mm/vmevent.c
+++ b/mm/vmevent.c
@@ -1,5 +1,6 @@
 #include <linux/anon_inodes.h>
 #include <linux/atomic.h>
+#include <linux/compiler.h>
 #include <linux/vmevent.h>
 #include <linux/syscalls.h>
 #include <linux/timer.h>
@@ -94,6 +95,26 @@ static bool vmevent_match(struct vmevent_watch *watch)
 		if (attr->state & VMEVENT_ATTR_STATE_VALUE_LT) {
 			if (value < attr->value)
 				return true;
+		} else if (attr->state & VMEVENT_ATTR_STATE_VALUE_CROSS) {
+			bool fst = !(attr->state & __VMEVENT_ATTR_STATE_NFIRST);
+			bool old = attr->state & __VMEVENT_ATTR_STATE_LAST;
+			bool new = value < attr->value;
+			bool chg = old ^ new;
+			bool ret = chg;
+
+			/*
+			 * This is not 'lt' or 'gt' match, so on the first
+			 * sample assume we crossed the threshold.
+			 */
+			if (unlikely(fst)) {
+				attr->state |= __VMEVENT_ATTR_STATE_NFIRST;
+				ret = true;
+			}
+
+			attr->state &= ~__VMEVENT_ATTR_STATE_LAST;
+			attr->state |= new ? __VMEVENT_ATTR_STATE_LAST : 0;
+
+			return ret;
 		}
 	}
 
diff --git a/tools/testing/vmevent/vmevent-test.c b/tools/testing/vmevent/vmevent-test.c
index 534f827..39e93af 100644
--- a/tools/testing/vmevent/vmevent-test.c
+++ b/tools/testing/vmevent/vmevent-test.c
@@ -33,20 +33,25 @@ int main(int argc, char *argv[])
 
 	config = (struct vmevent_config) {
 		.sample_period_ns	= 1000000000L,
-		.counter		= 4,
+		.counter		= 5,
 		.attrs			= {
-			[0]			= {
+			{
+				.type	= VMEVENT_ATTR_NR_FREE_PAGES,
+				.state	= VMEVENT_ATTR_STATE_VALUE_CROSS,
+				.value	= phys_pages / 2,
+			},
+			{
 				.type	= VMEVENT_ATTR_NR_FREE_PAGES,
 				.state	= VMEVENT_ATTR_STATE_VALUE_LT,
 				.value	= phys_pages,
 			},
-			[1]			= {
+			{
 				.type	= VMEVENT_ATTR_NR_AVAIL_PAGES,
 			},
-			[2]			= {
+			{
 				.type	= VMEVENT_ATTR_NR_SWAP_PAGES,
 			},
-			[3]			= {
+			{
 				.type	= 0xffff, /* invalid */
 			},
 		},
-- 
1.7.7.6

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Re: [PATCH 3/3] vmevent: Implement cross event type

[Pekka Enberg]

On Mon, Apr 9, 2012 at 2:38 AM, Anton Vorontsov
<anton.vorontsov@linaro.org> wrote:
> This patch implements a new event type, it will trigger whenever a
> value crosses a user-specified threshold. It works two-way, i.e. when
> a value crosses the threshold from a lesser values side to a greater
> values side, and vice versa.
>
> We use the event type in an userspace low-memory killer: we get a
> notification when memory becomes low, so we start freeing memory by
> killing unneeded processes, and we get notification when memory hits
> the threshold from another side, so we know that we freed enough of
> memory.
>
> Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
> ---
>  include/linux/vmevent.h              |    9 +++++++++
>  mm/vmevent.c                         |   21 +++++++++++++++++++++
>  tools/testing/vmevent/vmevent-test.c |   15 ++++++++++-----
>  3 files changed, 40 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/vmevent.h b/include/linux/vmevent.h
> index 64357e4..00cc04f 100644
> --- a/include/linux/vmevent.h
> +++ b/include/linux/vmevent.h
> @@ -22,6 +22,15 @@ enum {
>         * Sample value is less than user-specified value
>         */
>        VMEVENT_ATTR_STATE_VALUE_LT     = (1UL << 0),
> +       /*
> +        * Sample value crossed user-specified value
> +        */
> +       VMEVENT_ATTR_STATE_VALUE_CROSS  = (1UL << 2),
> +
> +       /* Last saved state, used internally by the kernel. */
> +       __VMEVENT_ATTR_STATE_LAST       = (1UL << 30),
> +       /* Not first sample, used internally by the kernel. */
> +       __VMEVENT_ATTR_STATE_NFIRST     = (1UL << 31),
>  };
>
>  struct vmevent_attr {
> diff --git a/mm/vmevent.c b/mm/vmevent.c
> index a56174f..f8fd2d6 100644
> --- a/mm/vmevent.c
> +++ b/mm/vmevent.c
> @@ -1,5 +1,6 @@
>  #include <linux/anon_inodes.h>
>  #include <linux/atomic.h>
> +#include <linux/compiler.h>
>  #include <linux/vmevent.h>
>  #include <linux/syscalls.h>
>  #include <linux/timer.h>
> @@ -94,6 +95,26 @@ static bool vmevent_match(struct vmevent_watch *watch)
>                if (attr->state & VMEVENT_ATTR_STATE_VALUE_LT) {
>                        if (value < attr->value)
>                                return true;
> +               } else if (attr->state & VMEVENT_ATTR_STATE_VALUE_CROSS) {
> +                       bool fst = !(attr->state & __VMEVENT_ATTR_STATE_NFIRST);
> +                       bool old = attr->state & __VMEVENT_ATTR_STATE_LAST;
> +                       bool new = value < attr->value;
> +                       bool chg = old ^ new;
> +                       bool ret = chg;
> +
> +                       /*
> +                        * This is not 'lt' or 'gt' match, so on the first
> +                        * sample assume we crossed the threshold.
> +                        */
> +                       if (unlikely(fst)) {
> +                               attr->state |= __VMEVENT_ATTR_STATE_NFIRST;
> +                               ret = true;
> +                       }
> +
> +                       attr->state &= ~__VMEVENT_ATTR_STATE_LAST;
> +                       attr->state |= new ? __VMEVENT_ATTR_STATE_LAST : 0;
> +
> +                       return ret;
>                }
>        }
>

Can't we implement this by specifying both VMEVENT_ATTR_STATE_VALUE_LT
and VMEVENT_ATTR_STATE_VALUE_GT in userspace? I assume the problem
with current approach is that you get more than one notifications,
right? We can implement a "single-shot" flag to deal with that.

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