[PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[David Hildenbrand]

We see multiple issues with the implementation/interface to compute
whether a memory block can be offlined (exposed via
/sys/devices/system/memory/memoryX/removable) and would like to simplify
it (remove the implementation).

1. It runs basically lockless. While this might be good for performance,
   we see possible races with memory offlining/unplug that will require
   at least some sort of locking to fix.

2. Nowadays, more false positives are possible. No arch-specific checks
   are performed that validate if memory offlining will not be denied
   right away (and such check will require locking). For example, arm64
   won't allow to offline any memory block that was added during boot -
   which will imply a very high error rate. Other archs have other
   constraints.

3. The interface is inherently racy. E.g., if a memory block is
   detected to be removable (and was not a false positive at that time),
   there is still no guarantee that offlining will actually succeed. So
   any caller already has to deal with false positives.

4. It is unclear which performance benefit this interface actually
   provides. The introducing commit 5c755e9fd813 ("memory-hotplug: add
   sysfs removable attribute for hotplug memory remove") mentioned
	"A user-level agent must be able to identify which sections of
	 memory are likely to be removable before attempting the
	 potentially expensive operation."
   However, no actual performance comparison was included.

Known users:
- lsmem: Will group memory blocks based on the "removable" property. [1]
- chmem: Indirect user. It has a RANGE mode where one can specify
	 removable ranges identified via lsmem to be offlined. However, it
	 also has a "SIZE" mode, which allows a sysadmin to skip the manual
	 "identify removable blocks" step. [2]
- powerpc-utils: Uses the "removable" attribute to skip some memory
		 blocks right away when trying to find some to
		 offline+remove. However, with ballooning enabled, it
		 already skips this information completely (because it
		 once resulted in many false negatives). Therefore, the
		 implementation can deal with false positives properly
		 already. [3]

With CONFIG_MEMORY_HOTREMOVE, always indicating "removable" should not
break any user space tool. We implement a very bad heuristic now. (in
contrast: always returning "not removable" would at least affect
powerpc-utils)

Without CONFIG_MEMORY_HOTREMOVE we cannot offline anything, so report
"not removable" as before.

Original discussion can be found in [4] ("[PATCH RFC v1] mm:
is_mem_section_removable() overhaul").

Other users of is_mem_section_removable() will be removed next, so that
we can remove is_mem_section_removable() completely.

[1] http://man7.org/linux/man-pages/man1/lsmem.1.html
[2] http://man7.org/linux/man-pages/man8/chmem.8.html
[3] https://github.com/ibm-power-utilities/powerpc-utils
[4] https://lkml.kernel.org/r/20200117105759.27905-1-david@redhat.com

Suggested-by: Michal Hocko <mhocko@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: powerpc-utils-devel@googlegroups.com
Cc: util-linux@vger.kernel.org
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Karel Zak <kzak@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---

1. Are there any use cases that really require this interface to keep
   producing "more reliable" results?

2. Is there any real performance advantage when using this interface to
   identify memory blocks to offline?

---
 drivers/base/memory.c | 27 +++++++--------------------
 1 file changed, 7 insertions(+), 20 deletions(-)

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 6503f5d0b749..d78a92f09984 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -105,30 +105,17 @@ static ssize_t phys_index_show(struct device *dev,
 }
 
 /*
- * Show whether the memory block is likely to be offlineable (or is already
- * offline). Once offline, the memory block could be removed. The return
- * value does, however, not indicate that there is a way to remove the
- * memory block.
+ * Legacy interface that we cannot remove. Always indicate "removable"
+ * with CONFIG_MEMORY_HOTREMOVE - bad heuristic.
  */
 static ssize_t removable_show(struct device *dev, struct device_attribute *attr,
 			      char *buf)
 {
-	struct memory_block *mem = to_memory_block(dev);
-	unsigned long pfn;
-	int ret = 1, i;
-
-	if (mem->state != MEM_ONLINE)
-		goto out;
-
-	for (i = 0; i < sections_per_block; i++) {
-		if (!present_section_nr(mem->start_section_nr + i))
-			continue;
-		pfn = section_nr_to_pfn(mem->start_section_nr + i);
-		ret &= is_mem_section_removable(pfn, PAGES_PER_SECTION);
-	}
-
-out:
-	return sprintf(buf, "%d\n", ret);
+#ifdef CONFIG_MEMORY_HOTREMOVE
+	return sprintf(buf, "1\n");
+#else
+	return sprintf(buf, "0\n");
+#endif
 }
 
 /*
-- 
2.24.1

Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[Fontenot, Nathan]

It's been awhile since I've looked at the powerpc-utils drmgr command and
pseries DLPAR code but a quick scan makes and it appears that it hasn't changed
too much. Given that, some thoughts.

The sysfs 'removable' file was a great help when memory DLPAR was driven
from userspace in the powerpc-utils drmgr command. Having this check did improve
performance though I can't point to any numbers.

Currently, memory DLPAR is done completely in the kernel. The request is
initiated from drmgr writing to /sys/kernel/dlpar (for pHyp partitions)
or from a hotplug interrupt (for guests). I don't believe the 'removable'
sysfs file is used in either of these paths by drmgr. The only time it is
used is on older kernels that do not support in-kernel memory DLPAR.

Given this, I don't think removing the 'removable' sysfs file would cause any
issues for the drmgr command. The only scenario I can think of is using an old
version of drmgr that does not support in-kernel memory DLPAR on a new kernel
where the 'removable' sysfs file has been removed. This doesn't seem likely
though and drmgr could be updated to detect this.

-Nathan Fontenot

On 1/24/2020 9:53 AM, David Hildenbrand wrote:
> We see multiple issues with the implementation/interface to compute
> whether a memory block can be offlined (exposed via
> /sys/devices/system/memory/memoryX/removable) and would like to simplify
> it (remove the implementation).
> 
> 1. It runs basically lockless. While this might be good for performance,
>    we see possible races with memory offlining/unplug that will require
>    at least some sort of locking to fix.
> 
> 2. Nowadays, more false positives are possible. No arch-specific checks
>    are performed that validate if memory offlining will not be denied
>    right away (and such check will require locking). For example, arm64
>    won't allow to offline any memory block that was added during boot -
>    which will imply a very high error rate. Other archs have other
>    constraints.
> 
> 3. The interface is inherently racy. E.g., if a memory block is
>    detected to be removable (and was not a false positive at that time),
>    there is still no guarantee that offlining will actually succeed. So
>    any caller already has to deal with false positives.
> 
> 4. It is unclear which performance benefit this interface actually
>    provides. The introducing commit 5c755e9fd813 ("memory-hotplug: add
>    sysfs removable attribute for hotplug memory remove") mentioned
> 	"A user-level agent must be able to identify which sections of
> 	 memory are likely to be removable before attempting the
> 	 potentially expensive operation."
>    However, no actual performance comparison was included.

> 
> Known users:
> - lsmem: Will group memory blocks based on the "removable" property. [1]
> - chmem: Indirect user. It has a RANGE mode where one can specify
> 	 removable ranges identified via lsmem to be offlined. However, it
> 	 also has a "SIZE" mode, which allows a sysadmin to skip the manual
> 	 "identify removable blocks" step. [2]
> - powerpc-utils: Uses the "removable" attribute to skip some memory
> 		 blocks right away when trying to find some to
> 		 offline+remove. However, with ballooning enabled, it
> 		 already skips this information completely (because it
> 		 once resulted in many false negatives). Therefore, the
> 		 implementation can deal with false positives properly
> 		 already. [3]
> 
> With CONFIG_MEMORY_HOTREMOVE, always indicating "removable" should not
> break any user space tool. We implement a very bad heuristic now. (in
> contrast: always returning "not removable" would at least affect
> powerpc-utils)
> 
> Without CONFIG_MEMORY_HOTREMOVE we cannot offline anything, so report
> "not removable" as before.
> 
> Original discussion can be found in [4] ("[PATCH RFC v1] mm:
> is_mem_section_removable() overhaul").
> 
> Other users of is_mem_section_removable() will be removed next, so that
> we can remove is_mem_section_removable() completely.
> 
> [1] http://man7.org/linux/man-pages/man1/lsmem.1.html
> [2] http://man7.org/linux/man-pages/man8/chmem.8.html
> [3] https://github.com/ibm-power-utilities/powerpc-utils
> [4] https://lkml.kernel.org/r/20200117105759.27905-1-david@redhat.com
> 
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: powerpc-utils-devel@googlegroups.com
> Cc: util-linux@vger.kernel.org
> Cc: Badari Pulavarty <pbadari@us.ibm.com>
> Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
> Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Karel Zak <kzak@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> 
> 1. Are there any use cases that really require this interface to keep
>    producing "more reliable" results?
> 
> 2. Is there any real performance advantage when using this interface to
>    identify memory blocks to offline?
> 
> ---
>  drivers/base/memory.c | 27 +++++++--------------------
>  1 file changed, 7 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index 6503f5d0b749..d78a92f09984 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -105,30 +105,17 @@ static ssize_t phys_index_show(struct device *dev,
>  }
>  
>  /*
> - * Show whether the memory block is likely to be offlineable (or is already
> - * offline). Once offline, the memory block could be removed. The return
> - * value does, however, not indicate that there is a way to remove the
> - * memory block.
> + * Legacy interface that we cannot remove. Always indicate "removable"
> + * with CONFIG_MEMORY_HOTREMOVE - bad heuristic.
>   */
>  static ssize_t removable_show(struct device *dev, struct device_attribute *attr,
>  			      char *buf)
>  {
> -	struct memory_block *mem = to_memory_block(dev);
> -	unsigned long pfn;
> -	int ret = 1, i;
> -
> -	if (mem->state != MEM_ONLINE)
> -		goto out;
> -
> -	for (i = 0; i < sections_per_block; i++) {
> -		if (!present_section_nr(mem->start_section_nr + i))
> -			continue;
> -		pfn = section_nr_to_pfn(mem->start_section_nr + i);
> -		ret &= is_mem_section_removable(pfn, PAGES_PER_SECTION);
> -	}
> -
> -out:
> -	return sprintf(buf, "%d\n", ret);
> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +	return sprintf(buf, "1\n");
> +#else
> +	return sprintf(buf, "0\n");
> +#endif
>  }
>  
>  /*
> 

Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[Michal Hocko]

On Fri 24-01-20 13:10:22, Fontenot, Nathan wrote:
> It's been awhile since I've looked at the powerpc-utils drmgr command and
> pseries DLPAR code but a quick scan makes and it appears that it hasn't changed
> too much. Given that, some thoughts.
> 
> The sysfs 'removable' file was a great help when memory DLPAR was driven
> from userspace in the powerpc-utils drmgr command. Having this check did improve
> performance though I can't point to any numbers.

Do you still have an access to the HW to give it a try?

> Currently, memory DLPAR is done completely in the kernel. The request is
> initiated from drmgr writing to /sys/kernel/dlpar (for pHyp partitions)
> or from a hotplug interrupt (for guests). I don't believe the 'removable'
> sysfs file is used in either of these paths by drmgr. The only time it is
> used is on older kernels that do not support in-kernel memory DLPAR.
> 
> Given this, I don't think removing the 'removable' sysfs file would cause any
> issues for the drmgr command. The only scenario I can think of is using an old
> version of drmgr that does not support in-kernel memory DLPAR on a new kernel
> where the 'removable' sysfs file has been removed. This doesn't seem likely
> though and drmgr could be updated to detect this.

Thanks for the information!
-- 
Michal Hocko
SUSE Labs

Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[David Hildenbrand]

On 27.01.20 10:23, Michal Hocko wrote:
> On Fri 24-01-20 13:10:22, Fontenot, Nathan wrote:
>> It's been awhile since I've looked at the powerpc-utils drmgr command and
>> pseries DLPAR code but a quick scan makes and it appears that it hasn't changed
>> too much. Given that, some thoughts.
>>
>> The sysfs 'removable' file was a great help when memory DLPAR was driven
>> from userspace in the powerpc-utils drmgr command. Having this check did improve
>> performance though I can't point to any numbers.
> 
> Do you still have an access to the HW to give it a try?
> 
>> Currently, memory DLPAR is done completely in the kernel. The request is
>> initiated from drmgr writing to /sys/kernel/dlpar (for pHyp partitions)
>> or from a hotplug interrupt (for guests). I don't believe the 'removable'
>> sysfs file is used in either of these paths by drmgr. The only time it is
>> used is on older kernels that do not support in-kernel memory DLPAR.
>>
>> Given this, I don't think removing the 'removable' sysfs file would cause any
>> issues for the drmgr command. The only scenario I can think of is using an old
>> version of drmgr that does not support in-kernel memory DLPAR on a new kernel
>> where the 'removable' sysfs file has been removed. This doesn't seem likely
>> though and drmgr could be updated to detect this.
> 
> Thanks for the information!
> 

(weird, I never received the mail from Nathan - mail deliver issues
brighten my Mondays :) )

Thanks for the information! Looks like powerpc indeed can live without
the interface (old userspace on shiny new kernel would in the worst case
simply be slower).

Of course, the alternative to returning always "removable" would be to
drop the attribute completely. So, if the "removable" attribute is not
present

- powerpc-utils will fallback to "removable"
- lsmem will fallback to "not removable". Could be because it assumes
  "old kernel with lacking offlining capability".

I don't know how likely it is that this could break custom scripts that
used the returned value for any purpose (e.g., use it as an indicator if
memory offlining is supported at all etc.).

-- 
Thanks,

David / dhildenb

Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[Michal Hocko]

On Mon 27-01-20 10:33:55, David Hildenbrand wrote:
> On 27.01.20 10:23, Michal Hocko wrote:
> > On Fri 24-01-20 13:10:22, Fontenot, Nathan wrote:
> >> It's been awhile since I've looked at the powerpc-utils drmgr command and
> >> pseries DLPAR code but a quick scan makes and it appears that it hasn't changed
> >> too much. Given that, some thoughts.
> >>
> >> The sysfs 'removable' file was a great help when memory DLPAR was driven
> >> from userspace in the powerpc-utils drmgr command. Having this check did improve
> >> performance though I can't point to any numbers.
> > 
> > Do you still have an access to the HW to give it a try?
> > 
> >> Currently, memory DLPAR is done completely in the kernel. The request is
> >> initiated from drmgr writing to /sys/kernel/dlpar (for pHyp partitions)
> >> or from a hotplug interrupt (for guests). I don't believe the 'removable'
> >> sysfs file is used in either of these paths by drmgr. The only time it is
> >> used is on older kernels that do not support in-kernel memory DLPAR.
> >>
> >> Given this, I don't think removing the 'removable' sysfs file would cause any
> >> issues for the drmgr command. The only scenario I can think of is using an old
> >> version of drmgr that does not support in-kernel memory DLPAR on a new kernel
> >> where the 'removable' sysfs file has been removed. This doesn't seem likely
> >> though and drmgr could be updated to detect this.
> > 
> > Thanks for the information!
> > 
> 
> (weird, I never received the mail from Nathan - mail deliver issues
> brighten my Mondays :) )
> 
> Thanks for the information! Looks like powerpc indeed can live without
> the interface (old userspace on shiny new kernel would in the worst case
> simply be slower).
> 
> Of course, the alternative to returning always "removable" would be to
> drop the attribute completely. So, if the "removable" attribute is not
> present
> 
> - powerpc-utils will fallback to "removable"
> - lsmem will fallback to "not removable". Could be because it assumes
>   "old kernel with lacking offlining capability".
> 
> I don't know how likely it is that this could break custom scripts that
> used the returned value for any purpose (e.g., use it as an indicator if
> memory offlining is supported at all etc.).

Our long term tradition with user visible knobs is to keep them in place
and simply fake the answer. This seems to be a safer option and less
likely to lead to failures.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[Fontenot, Nathan]

On 1/27/2020 3:33 AM, David Hildenbrand wrote:
> On 27.01.20 10:23, Michal Hocko wrote:
>> On Fri 24-01-20 13:10:22, Fontenot, Nathan wrote:
>>> It's been awhile since I've looked at the powerpc-utils drmgr command and
>>> pseries DLPAR code but a quick scan makes and it appears that it hasn't changed
>>> too much. Given that, some thoughts.
>>>
>>> The sysfs 'removable' file was a great help when memory DLPAR was driven
>>> from userspace in the powerpc-utils drmgr command. Having this check did improve
>>> performance though I can't point to any numbers.
>>
>> Do you still have an access to the HW to give it a try?

No, I no longer have access to Power hardware.

-Nathan

>>
>>> Currently, memory DLPAR is done completely in the kernel. The request is
>>> initiated from drmgr writing to /sys/kernel/dlpar (for pHyp partitions)
>>> or from a hotplug interrupt (for guests). I don't believe the 'removable'
>>> sysfs file is used in either of these paths by drmgr. The only time it is
>>> used is on older kernels that do not support in-kernel memory DLPAR.
>>>
>>> Given this, I don't think removing the 'removable' sysfs file would cause any
>>> issues for the drmgr command. The only scenario I can think of is using an old
>>> version of drmgr that does not support in-kernel memory DLPAR on a new kernel
>>> where the 'removable' sysfs file has been removed. This doesn't seem likely
>>> though and drmgr could be updated to detect this.
>>
>> Thanks for the information!
>>
> 
> (weird, I never received the mail from Nathan - mail deliver issues
> brighten my Mondays :) )
> 
> Thanks for the information! Looks like powerpc indeed can live without
> the interface (old userspace on shiny new kernel would in the worst case
> simply be slower).
> 
> Of course, the alternative to returning always "removable" would be to
> drop the attribute completely. So, if the "removable" attribute is not
> present
> 
> - powerpc-utils will fallback to "removable"
> - lsmem will fallback to "not removable". Could be because it assumes
>   "old kernel with lacking offlining capability".
> 
> I don't know how likely it is that this could break custom scripts that
> used the returned value for any purpose (e.g., use it as an indicator if
> memory offlining is supported at all etc.).
> 

Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[Michal Hocko]

On Fri 24-01-20 16:53:36, David Hildenbrand wrote:
> We see multiple issues with the implementation/interface to compute
> whether a memory block can be offlined (exposed via
> /sys/devices/system/memory/memoryX/removable) and would like to simplify
> it (remove the implementation).
> 
> 1. It runs basically lockless. While this might be good for performance,
>    we see possible races with memory offlining/unplug that will require
>    at least some sort of locking to fix.
> 
> 2. Nowadays, more false positives are possible. No arch-specific checks
>    are performed that validate if memory offlining will not be denied
>    right away (and such check will require locking). For example, arm64
>    won't allow to offline any memory block that was added during boot -
>    which will imply a very high error rate. Other archs have other
>    constraints.
> 
> 3. The interface is inherently racy. E.g., if a memory block is
>    detected to be removable (and was not a false positive at that time),
>    there is still no guarantee that offlining will actually succeed. So
>    any caller already has to deal with false positives.
> 
> 4. It is unclear which performance benefit this interface actually
>    provides. The introducing commit 5c755e9fd813 ("memory-hotplug: add
>    sysfs removable attribute for hotplug memory remove") mentioned
> 	"A user-level agent must be able to identify which sections of
> 	 memory are likely to be removable before attempting the
> 	 potentially expensive operation."
>    However, no actual performance comparison was included.
> 
> Known users:
> - lsmem: Will group memory blocks based on the "removable" property. [1]
> - chmem: Indirect user. It has a RANGE mode where one can specify
> 	 removable ranges identified via lsmem to be offlined. However, it
> 	 also has a "SIZE" mode, which allows a sysadmin to skip the manual
> 	 "identify removable blocks" step. [2]
> - powerpc-utils: Uses the "removable" attribute to skip some memory
> 		 blocks right away when trying to find some to
> 		 offline+remove. However, with ballooning enabled, it
> 		 already skips this information completely (because it
> 		 once resulted in many false negatives). Therefore, the
> 		 implementation can deal with false positives properly
> 		 already. [3]
> 
> With CONFIG_MEMORY_HOTREMOVE, always indicating "removable" should not
> break any user space tool. We implement a very bad heuristic now. (in
> contrast: always returning "not removable" would at least affect
> powerpc-utils)
> 
> Without CONFIG_MEMORY_HOTREMOVE we cannot offline anything, so report
> "not removable" as before.
> 
> Original discussion can be found in [4] ("[PATCH RFC v1] mm:
> is_mem_section_removable() overhaul").
> 
> Other users of is_mem_section_removable() will be removed next, so that
> we can remove is_mem_section_removable() completely.
> 
> [1] http://man7.org/linux/man-pages/man1/lsmem.1.html
> [2] http://man7.org/linux/man-pages/man8/chmem.8.html
> [3] https://github.com/ibm-power-utilities/powerpc-utils
> [4] https://lkml.kernel.org/r/20200117105759.27905-1-david@redhat.com
> 
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: powerpc-utils-devel@googlegroups.com
> Cc: util-linux@vger.kernel.org
> Cc: Badari Pulavarty <pbadari@us.ibm.com>
> Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
> Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Karel Zak <kzak@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Please add information provided by Nathan.
Acked-by: Michal Hocko <mhocko@suse.com>

Minor nit below.

> +#ifdef CONFIG_MEMORY_HOTREMOVE
> +	return sprintf(buf, "1\n");
> +#else
> +	return sprintf(buf, "0\n");
> +#endif
	int ret = IS_ENABLED(CONFIG_MEMORY_HOTREMOVE);

	return sprintf(buf, "%d\n", ret)

would be slightly nicer than explicit ifdefs.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[David Hildenbrand]

On 27.01.20 14:29, Michal Hocko wrote:
> On Fri 24-01-20 16:53:36, David Hildenbrand wrote:
>> We see multiple issues with the implementation/interface to compute
>> whether a memory block can be offlined (exposed via
>> /sys/devices/system/memory/memoryX/removable) and would like to simplify
>> it (remove the implementation).
>>
>> 1. It runs basically lockless. While this might be good for performance,
>>    we see possible races with memory offlining/unplug that will require
>>    at least some sort of locking to fix.
>>
>> 2. Nowadays, more false positives are possible. No arch-specific checks
>>    are performed that validate if memory offlining will not be denied
>>    right away (and such check will require locking). For example, arm64
>>    won't allow to offline any memory block that was added during boot -
>>    which will imply a very high error rate. Other archs have other
>>    constraints.
>>
>> 3. The interface is inherently racy. E.g., if a memory block is
>>    detected to be removable (and was not a false positive at that time),
>>    there is still no guarantee that offlining will actually succeed. So
>>    any caller already has to deal with false positives.
>>
>> 4. It is unclear which performance benefit this interface actually
>>    provides. The introducing commit 5c755e9fd813 ("memory-hotplug: add
>>    sysfs removable attribute for hotplug memory remove") mentioned
>> 	"A user-level agent must be able to identify which sections of
>> 	 memory are likely to be removable before attempting the
>> 	 potentially expensive operation."
>>    However, no actual performance comparison was included.
>>
>> Known users:
>> - lsmem: Will group memory blocks based on the "removable" property. [1]
>> - chmem: Indirect user. It has a RANGE mode where one can specify
>> 	 removable ranges identified via lsmem to be offlined. However, it
>> 	 also has a "SIZE" mode, which allows a sysadmin to skip the manual
>> 	 "identify removable blocks" step. [2]
>> - powerpc-utils: Uses the "removable" attribute to skip some memory
>> 		 blocks right away when trying to find some to
>> 		 offline+remove. However, with ballooning enabled, it
>> 		 already skips this information completely (because it
>> 		 once resulted in many false negatives). Therefore, the
>> 		 implementation can deal with false positives properly
>> 		 already. [3]
>>
>> With CONFIG_MEMORY_HOTREMOVE, always indicating "removable" should not
>> break any user space tool. We implement a very bad heuristic now. (in
>> contrast: always returning "not removable" would at least affect
>> powerpc-utils)
>>
>> Without CONFIG_MEMORY_HOTREMOVE we cannot offline anything, so report
>> "not removable" as before.
>>
>> Original discussion can be found in [4] ("[PATCH RFC v1] mm:
>> is_mem_section_removable() overhaul").
>>
>> Other users of is_mem_section_removable() will be removed next, so that
>> we can remove is_mem_section_removable() completely.
>>
>> [1] http://man7.org/linux/man-pages/man1/lsmem.1.html
>> [2] http://man7.org/linux/man-pages/man8/chmem.8.html
>> [3] https://github.com/ibm-power-utilities/powerpc-utils
>> [4] https://lkml.kernel.org/r/20200117105759.27905-1-david@redhat.com
>>
>> Suggested-by: Michal Hocko <mhocko@kernel.org>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: powerpc-utils-devel@googlegroups.com
>> Cc: util-linux@vger.kernel.org
>> Cc: Badari Pulavarty <pbadari@us.ibm.com>
>> Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
>> Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
>> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
>> Cc: Karel Zak <kzak@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> Please add information provided by Nathan.
> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Minor nit below.
> 
>> +#ifdef CONFIG_MEMORY_HOTREMOVE
>> +	return sprintf(buf, "1\n");
>> +#else
>> +	return sprintf(buf, "0\n");
>> +#endif
> 	int ret = IS_ENABLED(CONFIG_MEMORY_HOTREMOVE);
> 
> 	return sprintf(buf, "%d\n", ret)
> 
> would be slightly nicer than explicit ifdefs.
> 

Indeed, thanks!

-- 
Thanks,

David / dhildenb