[PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

[PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

[David Hildenbrand]

Let's update the documentation that any signal is sufficient, and
add a comment that not only checking for fatal signals is historical
baggage: changing it now could break existing user space. although
unlikely.

For example, when an app provides a custom SIGALRM handler and triggers
memory offlining, the timeout cmd would no longer stop memory offlining,
because SIGALRM would no longer be considered a fatal signal.

Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 Documentation/admin-guide/mm/memory-hotplug.rst | 2 +-
 mm/memory_hotplug.c                             | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
index 1b02fe5807cc..bd77841041af 100644
--- a/Documentation/admin-guide/mm/memory-hotplug.rst
+++ b/Documentation/admin-guide/mm/memory-hotplug.rst
@@ -669,7 +669,7 @@ when still encountering permanently unmovable pages within ZONE_MOVABLE
 (-> BUG), memory offlining will keep retrying until it eventually succeeds.
 
 When offlining is triggered from user space, the offlining context can be
-terminated by sending a fatal signal. A timeout based offlining can easily be
+terminated by sending a signal. A timeout based offlining can easily be
 implemented via::
 
 	% timeout $TIMEOUT offline_block | failure_handling
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 3f231cf1b410..7cfd13c91568 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1843,6 +1843,11 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
 	do {
 		pfn = start_pfn;
 		do {
+			/*
+			 * Historically we always checked for any signal and
+			 * can't limit it to fatal signals without eventually
+			 * breaking user space.
+			 */
 			if (signal_pending(current)) {
 				ret = -EINTR;
 				reason = "signal backoff";
-- 
2.41.0

Re: [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

[Michal Hocko]

On Tue 11-07-23 19:40:50, David Hildenbrand wrote:
> Let's update the documentation that any signal is sufficient, and
> add a comment that not only checking for fatal signals is historical
> baggage: changing it now could break existing user space. although
> unlikely.
> 
> For example, when an app provides a custom SIGALRM handler and triggers
> memory offlining, the timeout cmd would no longer stop memory offlining,
> because SIGALRM would no longer be considered a fatal signal.

Yes, and it is likely goot to mention here that this is an antipattern
for many other kernel operations like IO (e.g. write) but it is a long
term behavior that somebody might depend on and it is safer to reflect
the documentation to the realitity rather than other way around (which
would be imho better).

> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  Documentation/admin-guide/mm/memory-hotplug.rst | 2 +-
>  mm/memory_hotplug.c                             | 5 +++++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> index 1b02fe5807cc..bd77841041af 100644
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> @@ -669,7 +669,7 @@ when still encountering permanently unmovable pages within ZONE_MOVABLE
>  (-> BUG), memory offlining will keep retrying until it eventually succeeds.
>  
>  When offlining is triggered from user space, the offlining context can be
> -terminated by sending a fatal signal. A timeout based offlining can easily be
> +terminated by sending a signal. A timeout based offlining can easily be
>  implemented via::
>  
>  	% timeout $TIMEOUT offline_block | failure_handling
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 3f231cf1b410..7cfd13c91568 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1843,6 +1843,11 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>  	do {
>  		pfn = start_pfn;
>  		do {
> +			/*
> +			 * Historically we always checked for any signal and
> +			 * can't limit it to fatal signals without eventually
> +			 * breaking user space.
> +			 */
>  			if (signal_pending(current)) {
>  				ret = -EINTR;
>  				reason = "signal backoff";
> -- 
> 2.41.0

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

[David Hildenbrand]

On 11.07.23 22:47, Michal Hocko wrote:
> On Tue 11-07-23 19:40:50, David Hildenbrand wrote:
>> Let's update the documentation that any signal is sufficient, and
>> add a comment that not only checking for fatal signals is historical
>> baggage: changing it now could break existing user space. although
>> unlikely.
>>
>> For example, when an app provides a custom SIGALRM handler and triggers
>> memory offlining, the timeout cmd would no longer stop memory offlining,
>> because SIGALRM would no longer be considered a fatal signal.
> 
> Yes, and it is likely goot to mention here that this is an antipattern
> for many other kernel operations like IO (e.g. write) but it is a long
> term behavior that somebody might depend on and it is safer to reflect
> the documentation to the realitity rather than other way around (which
> would be imho better).
> 

You mean adding something like

"Note that using signal_pending() instead of fatal_signal_pending() is 
an anti-pattern, but slowly deprecating that behavior to eventually 
change it in the far future is probably not worth the effort. If this 
ever becomes relevant for user-space, we might want to rethink."


Thanks!

-- 
Cheers,

David / dhildenb

Re: [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

[Michal Hocko]

On Wed 12-07-23 21:09:25, David Hildenbrand wrote:
> On 11.07.23 22:47, Michal Hocko wrote:
> > On Tue 11-07-23 19:40:50, David Hildenbrand wrote:
> > > Let's update the documentation that any signal is sufficient, and
> > > add a comment that not only checking for fatal signals is historical
> > > baggage: changing it now could break existing user space. although
> > > unlikely.
> > > 
> > > For example, when an app provides a custom SIGALRM handler and triggers
> > > memory offlining, the timeout cmd would no longer stop memory offlining,
> > > because SIGALRM would no longer be considered a fatal signal.
> > 
> > Yes, and it is likely goot to mention here that this is an antipattern
> > for many other kernel operations like IO (e.g. write) but it is a long
> > term behavior that somebody might depend on and it is safer to reflect
> > the documentation to the realitity rather than other way around (which
> > would be imho better).
> > 
> 
> You mean adding something like
> 
> "Note that using signal_pending() instead of fatal_signal_pending() is an
> anti-pattern, but slowly deprecating that behavior to eventually change it
> in the far future is probably not worth the effort. If this ever becomes
> relevant for user-space, we might want to rethink."

Yes, something like that. Thanks!

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

[Anshuman Khandual]

On 7/11/23 23:10, David Hildenbrand wrote:
> Let's update the documentation that any signal is sufficient, and
> add a comment that not only checking for fatal signals is historical
> baggage: changing it now could break existing user space. although
> unlikely.
> 
> For example, when an app provides a custom SIGALRM handler and triggers
> memory offlining, the timeout cmd would no longer stop memory offlining,
> because SIGALRM would no longer be considered a fatal signal.
> 
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  Documentation/admin-guide/mm/memory-hotplug.rst | 2 +-
>  mm/memory_hotplug.c                             | 5 +++++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> index 1b02fe5807cc..bd77841041af 100644
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> @@ -669,7 +669,7 @@ when still encountering permanently unmovable pages within ZONE_MOVABLE
>  (-> BUG), memory offlining will keep retrying until it eventually succeeds.
>  
>  When offlining is triggered from user space, the offlining context can be
> -terminated by sending a fatal signal. A timeout based offlining can easily be
> +terminated by sending a signal. A timeout based offlining can easily be
>  implemented via::
>  
>  	% timeout $TIMEOUT offline_block | failure_handling
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 3f231cf1b410..7cfd13c91568 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1843,6 +1843,11 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>  	do {
>  		pfn = start_pfn;
>  		do {
> +			/*
> +			 * Historically we always checked for any signal and
> +			 * can't limit it to fatal signals without eventually
> +			 * breaking user space.> +			 */

Just curious, could 'signal type' to stop memory offline process be considered
an ABI and cannot be changed in kernel ever if required ? Just wondering if an
additional '!fatal_signal_pending()' check be introduced to warn about support
being deprecated, before finally replacing it with fatal_signal_pending().

>  			if (signal_pending(current)) {
>  				ret = -EINTR;
>  				reason = "signal backoff";

Re: [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

[David Hildenbrand]

On 12.07.23 08:47, Anshuman Khandual wrote:
> 
> 
> On 7/11/23 23:10, David Hildenbrand wrote:
>> Let's update the documentation that any signal is sufficient, and
>> add a comment that not only checking for fatal signals is historical
>> baggage: changing it now could break existing user space. although
>> unlikely.
>>
>> For example, when an app provides a custom SIGALRM handler and triggers
>> memory offlining, the timeout cmd would no longer stop memory offlining,
>> because SIGALRM would no longer be considered a fatal signal.
>>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Jonathan Corbet <corbet@lwn.net>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   Documentation/admin-guide/mm/memory-hotplug.rst | 2 +-
>>   mm/memory_hotplug.c                             | 5 +++++
>>   2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
>> index 1b02fe5807cc..bd77841041af 100644
>> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
>> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
>> @@ -669,7 +669,7 @@ when still encountering permanently unmovable pages within ZONE_MOVABLE
>>   (-> BUG), memory offlining will keep retrying until it eventually succeeds.
>>   
>>   When offlining is triggered from user space, the offlining context can be
>> -terminated by sending a fatal signal. A timeout based offlining can easily be
>> +terminated by sending a signal. A timeout based offlining can easily be
>>   implemented via::
>>   
>>   	% timeout $TIMEOUT offline_block | failure_handling
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 3f231cf1b410..7cfd13c91568 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1843,6 +1843,11 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>>   	do {
>>   		pfn = start_pfn;
>>   		do {
>> +			/*
>> +			 * Historically we always checked for any signal and
>> +			 * can't limit it to fatal signals without eventually
>> +			 * breaking user space.> +			 */
> 
> Just curious, could 'signal type' to stop memory offline process be considered
> an ABI and cannot be changed in kernel ever if required ? Just wondering if an
> additional '!fatal_signal_pending()' check be introduced to warn about support
> being deprecated, before finally replacing it with fatal_signal_pending().

See my reply to Michal, while that would be doable it is probably not 
worth the effort, and we'd still have to stick with the existing 
handling for quite a while.

Thanks!

-- 
Cheers,

David / dhildenb