LinuxLists.cc - [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On Sat, Jan 18, 2020 at 01:26:43PM +0800, Yang Shi wrote:
>The do_move_pages_to_node() might return > 0 value, the number of pages
>that are not migrated, then the value will be returned to userspace
>directly. But, move_pages() syscall would just return 0 or errno. So,
>we need reset the return value to 0 for such case as what pre-v4.17 did.
>
>Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
>Cc: Michal Hocko <[email protected]>
>Cc: Wei Yang <[email protected]>
>Cc: <[email protected]> [4.17+]
>Signed-off-by: Yang Shi <[email protected]>
>---
> mm/migrate.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
>diff --git a/mm/migrate.c b/mm/migrate.c
>index 86873b6..3e75432 100644
>--- a/mm/migrate.c
>+++ b/mm/migrate.c
>@@ -1659,8 +1659,11 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> goto out_flush;
>
> err = do_move_pages_to_node(mm, &pagelist, current_node);
>- if (err)
>+ if (err) {
>+ if (err > 0)
>+ err = 0;
> goto out;
>+ }
> if (i > start) {
> err = store_status(status, start, current_node, i - start);
> if (err)
>--
>1.8.3.1

Hey, I am afraid you missed something. There are three calls of
do_move_pages_to_node() in do_pages_move(). Why you just handle one return
value? How about the other two?

--
Wei Yang
Help you, Help me

2020-01-19 02:58:31

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On Sun, Jan 19, 2020 at 10:37:20AM +0800, Wei Yang wrote:
>On Sat, Jan 18, 2020 at 01:26:43PM +0800, Yang Shi wrote:
>>The do_move_pages_to_node() might return > 0 value, the number of pages
>>that are not migrated, then the value will be returned to userspace
>>directly. But, move_pages() syscall would just return 0 or errno. So,
>>we need reset the return value to 0 for such case as what pre-v4.17 did.
>>
>>Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
>>Cc: Michal Hocko <[email protected]>
>>Cc: Wei Yang <[email protected]>
>>Cc: <[email protected]> [4.17+]
>>Signed-off-by: Yang Shi <[email protected]>
>>---
>> mm/migrate.c | 5 ++++-
>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>
>>diff --git a/mm/migrate.c b/mm/migrate.c
>>index 86873b6..3e75432 100644
>>--- a/mm/migrate.c
>>+++ b/mm/migrate.c
>>@@ -1659,8 +1659,11 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>> goto out_flush;
>>
>> err = do_move_pages_to_node(mm, &pagelist, current_node);
>>- if (err)
>>+ if (err) {
>>+ if (err > 0)
>>+ err = 0;
>> goto out;
>>+ }
>> if (i > start) {
>> err = store_status(status, start, current_node, i - start);
>> if (err)
>>--
>>1.8.3.1
>
>
>Hey, I am afraid you missed something. There are three calls of
>do_move_pages_to_node() in do_pages_move(). Why you just handle one return
>value? How about the other two?
>

Well, current logic in do_pages_move() is a little complicated to read.

I did a cleanup to make it easy to read and also friendly to do this fix.

If they look good to you, you could rebase your fix on top of them.

>--
>Wei Yang
>Help you, Help me

--
Wei Yang
Help you, Help me

2020-01-19 05:45:47

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On 1/18/20 6:37 PM, Wei Yang wrote:
> On Sat, Jan 18, 2020 at 01:26:43PM +0800, Yang Shi wrote:
>> The do_move_pages_to_node() might return > 0 value, the number of pages
>> that are not migrated, then the value will be returned to userspace
>> directly. But, move_pages() syscall would just return 0 or errno. So,
>> we need reset the return value to 0 for such case as what pre-v4.17 did.
>>
>> Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
>> Cc: Michal Hocko <[email protected]>
>> Cc: Wei Yang <[email protected]>
>> Cc: <[email protected]> [4.17+]
>> Signed-off-by: Yang Shi <[email protected]>
>> ---
>> mm/migrate.c | 5 ++++-
>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 86873b6..3e75432 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1659,8 +1659,11 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>> goto out_flush;
>>
>> err = do_move_pages_to_node(mm, &pagelist, current_node);
>> - if (err)
>> + if (err) {
>> + if (err > 0)
>> + err = 0;
>> goto out;
>> + }
>> if (i > start) {
>> err = store_status(status, start, current_node, i - start);
>> if (err)
>> --
>> 1.8.3.1
>
> Hey, I am afraid you missed something. There are three calls of
> do_move_pages_to_node() in do_pages_move(). Why you just handle one return
> value? How about the other two?

A late night patch... I just simply missed those two. Thanks for
catching it, will resolve in v2.

>

2020-01-19 05:48:36

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On 1/18/20 6:57 PM, Wei Yang wrote:
> On Sun, Jan 19, 2020 at 10:37:20AM +0800, Wei Yang wrote:
>> On Sat, Jan 18, 2020 at 01:26:43PM +0800, Yang Shi wrote:
>>> The do_move_pages_to_node() might return > 0 value, the number of pages
>>> that are not migrated, then the value will be returned to userspace
>>> directly. But, move_pages() syscall would just return 0 or errno. So,
>>> we need reset the return value to 0 for such case as what pre-v4.17 did.
>>>
>>> Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
>>> Cc: Michal Hocko <[email protected]>
>>> Cc: Wei Yang <[email protected]>
>>> Cc: <[email protected]> [4.17+]
>>> Signed-off-by: Yang Shi <[email protected]>
>>> ---
>>> mm/migrate.c | 5 ++++-
>>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 86873b6..3e75432 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1659,8 +1659,11 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>>> goto out_flush;
>>>
>>> err = do_move_pages_to_node(mm, &pagelist, current_node);
>>> - if (err)
>>> + if (err) {
>>> + if (err > 0)
>>> + err = 0;
>>> goto out;
>>> + }
>>> if (i > start) {
>>> err = store_status(status, start, current_node, i - start);
>>> if (err)
>>> --
>>> 1.8.3.1
>>
>> Hey, I am afraid you missed something. There are three calls of
>> do_move_pages_to_node() in do_pages_move(). Why you just handle one return
>> value? How about the other two?
>>
> Well, current logic in do_pages_move() is a little complicated to read.
>
> I did a cleanup to make it easy to read and also friendly to do this fix.
>
> If they look good to you, you could rebase your fix on top of them.

Regression fix typically has higher priority. Since we already spotted
the regressions and proposed fixes, I'd suggest we fix them in 5.5-rc,
then aim any cleanup for 5.6 or 5.7. This should also make review easier
IMHO.

>
>> --
>> Wei Yang
>> Help you, Help me

2020-01-20 13:07:50

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On Sat 18-01-20 13:26:43, Yang Shi wrote:
> The do_move_pages_to_node() might return > 0 value, the number of pages
> that are not migrated, then the value will be returned to userspace
> directly. But, move_pages() syscall would just return 0 or errno. So,
> we need reset the return value to 0 for such case as what pre-v4.17 did.

The patch is wrong. migrate_pages returns the number of pages it
_hasn't_ migrated or -errno. Yeah that semantic sucks but...
So err != 0 is always an error. Except err > 0 doesn't really provide
any useful information to the userspace. I cannot really remember what
was the actual behavior before my rework because there were some gotchas
hidden there.

If you want to fix this properly then you have to query node status of
each page unmigrated when migrate_pages fails with > 0. This would be
easier if the fix is done on the latest cleanup posted to the list which
consolidates all do_move_pages_to_node and store_status calls to a
single function.

> Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
> Cc: Michal Hocko <[email protected]>
> Cc: Wei Yang <[email protected]>
> Cc: <[email protected]> [4.17+]
> Signed-off-by: Yang Shi <[email protected]>
> ---
> mm/migrate.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 86873b6..3e75432 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1659,8 +1659,11 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> goto out_flush;
>
> err = do_move_pages_to_node(mm, &pagelist, current_node);
> - if (err)
> + if (err) {
> + if (err > 0)
> + err = 0;
> goto out;
> + }
> if (i > start) {
> err = store_status(status, start, current_node, i - start);
> if (err)
> --
> 1.8.3.1

--
Michal Hocko
SUSE Labs

2020-01-20 13:19:06

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On Mon 20-01-20 14:06:26, Michal Hocko wrote:
> On Sat 18-01-20 13:26:43, Yang Shi wrote:
> > The do_move_pages_to_node() might return > 0 value, the number of pages
> > that are not migrated, then the value will be returned to userspace
> > directly. But, move_pages() syscall would just return 0 or errno. So,
> > we need reset the return value to 0 for such case as what pre-v4.17 did.
>
> The patch is wrong. migrate_pages returns the number of pages it
> _hasn't_ migrated or -errno. Yeah that semantic sucks but...
> So err != 0 is always an error. Except err > 0 doesn't really provide
> any useful information to the userspace. I cannot really remember what
> was the actual behavior before my rework because there were some gotchas
> hidden there.

OK, so I've double checked. do_move_page_to_node_array would carry the
error code over to do_pages_move and it would store the status stored
in the pm array. It contains page_to_nid(page) so the resulting code
indeed behaves properly before my change and this is a regression. I
have a very vague recollection that this has been brought up already.
<...looks in notes...>
Found it! The report is
http://lkml.kernel.org/r/0329efa0984b9b0252ef166abb4498c0795fab36.1535113317.git.jstancek@redhat.com
and my proposed workaround was http://lkml.kernel.org/r/[email protected]

> If you want to fix this properly then you have to query node status of
> each page unmigrated when migrate_pages fails with > 0. This would be
> easier if the fix is done on the latest cleanup posted to the list which
> consolidates all do_move_pages_to_node and store_status calls to a
> single function.

Sorry forgot to put a reference to the patch: http://lkml.kernel.org/r/[email protected]

--
Michal Hocko
SUSE Labs

2020-01-21 01:45:19

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On Mon, Jan 20, 2020 at 02:17:44PM +0100, Michal Hocko wrote:
>On Mon 20-01-20 14:06:26, Michal Hocko wrote:
>> On Sat 18-01-20 13:26:43, Yang Shi wrote:
>> > The do_move_pages_to_node() might return > 0 value, the number of pages
>> > that are not migrated, then the value will be returned to userspace
>> > directly. But, move_pages() syscall would just return 0 or errno. So,
>> > we need reset the return value to 0 for such case as what pre-v4.17 did.
>>
>> The patch is wrong. migrate_pages returns the number of pages it
>> _hasn't_ migrated or -errno. Yeah that semantic sucks but...
>> So err != 0 is always an error. Except err > 0 doesn't really provide
>> any useful information to the userspace. I cannot really remember what
>> was the actual behavior before my rework because there were some gotchas
>> hidden there.
>
>OK, so I've double checked. do_move_page_to_node_array would carry the
>error code over to do_pages_move and it would store the status stored
>in the pm array. It contains page_to_nid(page) so the resulting code
>indeed behaves properly before my change and this is a regression. I

Thanks, I see the change.

>have a very vague recollection that this has been brought up already.
><...looks in notes...>
>Found it! The report is
>http://lkml.kernel.org/r/0329efa0984b9b0252ef166abb4498c0795fab36.1535113317.git.jstancek@redhat.com
>and my proposed workaround was http://lkml.kernel.org/r/[email protected]

Well, the above two links return 404.

>
>> If you want to fix this properly then you have to query node status of
>> each page unmigrated when migrate_pages fails with > 0. This would be
>> easier if the fix is done on the latest cleanup posted to the list which
>> consolidates all do_move_pages_to_node and store_status calls to a
>> single function.
>
>Sorry forgot to put a reference to the patch: http://lkml.kernel.org/r/[email protected]
>
>--
>Michal Hocko
>SUSE Labs

--
Wei Yang
Help you, Help me

2020-01-21 08:41:54

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On Tue 21-01-20 09:44:16, Wei Yang wrote:
> On Mon, Jan 20, 2020 at 02:17:44PM +0100, Michal Hocko wrote:
> >On Mon 20-01-20 14:06:26, Michal Hocko wrote:
> >> On Sat 18-01-20 13:26:43, Yang Shi wrote:
> >> > The do_move_pages_to_node() might return > 0 value, the number of pages
> >> > that are not migrated, then the value will be returned to userspace
> >> > directly. But, move_pages() syscall would just return 0 or errno. So,
> >> > we need reset the return value to 0 for such case as what pre-v4.17 did.
> >>
> >> The patch is wrong. migrate_pages returns the number of pages it
> >> _hasn't_ migrated or -errno. Yeah that semantic sucks but...
> >> So err != 0 is always an error. Except err > 0 doesn't really provide
> >> any useful information to the userspace. I cannot really remember what
> >> was the actual behavior before my rework because there were some gotchas
> >> hidden there.
> >
> >OK, so I've double checked. do_move_page_to_node_array would carry the
> >error code over to do_pages_move and it would store the status stored
> >in the pm array. It contains page_to_nid(page) so the resulting code
> >indeed behaves properly before my change and this is a regression. I
>
> Thanks, I see the change.
>
> >have a very vague recollection that this has been brought up already.
> ><...looks in notes...>
> >Found it! The report is
> >http://lkml.kernel.org/r/0329efa0984b9b0252ef166abb4498c0795fab36.1535113317.git.jstancek@redhat.com
> >and my proposed workaround was http://lkml.kernel.org/r/[email protected]
>
> Well, the above two links return 404.

You are right. They are not archived for some reason. Anyway, the patch
I was proposing back then is below:

commit cfb88c266b645197135cde2905c2bfc82f6d82a9
Author: Michal Hocko <[email protected]>
Date: Wed Nov 14 12:19:09 2018 +0100

mm: fix do_pages_move error reporting

a49bd4d71637 ("mm, numa: rework do_pages_move") has changed the way how
we report error to layers above. As the changelog mentioned the semantic
was quite unclear previously because the return 0 could mean both
success and failure.

The above mentioned commit didn't get all the way down to fix this
completely because it doesn't report pages that we even haven't
attempted to migrate and therefore we cannot simply say that the
semantic is:
- err < 0 - errno
- err >= 0 number of non-migrated pages.

Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
Signed-off-by: Michal Hocko <[email protected]>

diff --git a/mm/migrate.c b/mm/migrate.c
index f7e4bfdc13b7..aa53ebc523eb 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1615,8 +1615,16 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
goto out_flush;

err = do_move_pages_to_node(mm, &pagelist, current_node);
- if (err)
+ if (err) {
+ /*
+ * Possitive err means the number of failed pages to
+ * migrate. Make sure to report the rest of the
+ * nr_pages is not migrated as well.
+ */
+ if (err > 0)
+ err += nr_pages - i - 1;
goto out;
+ }
if (i > start) {
err = store_status(status, start, current_node, i - start);
if (err)
--
Michal Hocko
SUSE Labs

2020-01-21 19:03:00

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Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On 1/21/20 12:40 AM, Michal Hocko wrote:
> On Tue 21-01-20 09:44:16, Wei Yang wrote:
>> On Mon, Jan 20, 2020 at 02:17:44PM +0100, Michal Hocko wrote:
>>> On Mon 20-01-20 14:06:26, Michal Hocko wrote:
>>>> On Sat 18-01-20 13:26:43, Yang Shi wrote:
>>>>> The do_move_pages_to_node() might return > 0 value, the number of pages
>>>>> that are not migrated, then the value will be returned to userspace
>>>>> directly. But, move_pages() syscall would just return 0 or errno. So,
>>>>> we need reset the return value to 0 for such case as what pre-v4.17 did.
>>>> The patch is wrong. migrate_pages returns the number of pages it
>>>> _hasn't_ migrated or -errno. Yeah that semantic sucks but...
>>>> So err != 0 is always an error. Except err > 0 doesn't really provide
>>>> any useful information to the userspace. I cannot really remember what
>>>> was the actual behavior before my rework because there were some gotchas
>>>> hidden there.
>>> OK, so I've double checked. do_move_page_to_node_array would carry the
>>> error code over to do_pages_move and it would store the status stored
>>> in the pm array. It contains page_to_nid(page) so the resulting code
>>> indeed behaves properly before my change and this is a regression. I
>> Thanks, I see the change.
>>
>>> have a very vague recollection that this has been brought up already.
>>> <...looks in notes...>
>>> Found it! The report is
>>> http://lkml.kernel.org/r/0329efa0984b9b0252ef166abb4498c0795fab36.1535113317.git.jstancek@redhat.com
>>> and my proposed workaround was http://lkml.kernel.org/r/[email protected]
>> Well, the above two links return 404.
> You are right. They are not archived for some reason. Anyway, the patch
> I was proposing back then is below:
>
> commit cfb88c266b645197135cde2905c2bfc82f6d82a9
> Author: Michal Hocko <[email protected]>
> Date: Wed Nov 14 12:19:09 2018 +0100
>
> mm: fix do_pages_move error reporting
>
> a49bd4d71637 ("mm, numa: rework do_pages_move") has changed the way how
> we report error to layers above. As the changelog mentioned the semantic
> was quite unclear previously because the return 0 could mean both
> success and failure.
>
> The above mentioned commit didn't get all the way down to fix this
> completely because it doesn't report pages that we even haven't
> attempted to migrate and therefore we cannot simply say that the
> semantic is:
> - err < 0 - errno
> - err >= 0 number of non-migrated pages.
>
> Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
> Signed-off-by: Michal Hocko <[email protected]>

Thanks, Michal. But, it looks this patch still could return > 0 value
(the total number of non-migrated pages, including not even attempted
pages) too, but the problem we are trying to fix is to make
do_pages_move() return <= 0 value only since the man page of
move_pages() doesn't allow return > 0 value.

And, by looking into the old code (v4.16), I spotted another problem.
The migrate_pages() would store the migration failure error code into
page_to_node->status. So, When do_move_page_to_node_array() returns > 0
value, the return value would be reset to 0 and the migration error
codes for non-migrated pages would be stored into status to return to
userspace. But, the rework removed this.

I didn't dig into the intention of the rework, is it expected?

>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index f7e4bfdc13b7..aa53ebc523eb 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1615,8 +1615,16 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> goto out_flush;
>
> err = do_move_pages_to_node(mm, &pagelist, current_node);
> - if (err)
> + if (err) {
> + /*
> + * Possitive err means the number of failed pages to
> + * migrate. Make sure to report the rest of the
> + * nr_pages is not migrated as well.
> + */
> + if (err > 0)
> + err += nr_pages - i - 1;
> goto out;
> + }
> if (i > start) {
> err = store_status(status, start, current_node, i - start);
> if (err)

2020-01-22 08:09:08

[permalink] [raw]

Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On Tue 21-01-20 11:01:30, Yang Shi wrote:
>
>
> On 1/21/20 12:40 AM, Michal Hocko wrote:
> > On Tue 21-01-20 09:44:16, Wei Yang wrote:
> > > On Mon, Jan 20, 2020 at 02:17:44PM +0100, Michal Hocko wrote:
> > > > On Mon 20-01-20 14:06:26, Michal Hocko wrote:
> > > > > On Sat 18-01-20 13:26:43, Yang Shi wrote:
> > > > > > The do_move_pages_to_node() might return > 0 value, the number of pages
> > > > > > that are not migrated, then the value will be returned to userspace
> > > > > > directly. But, move_pages() syscall would just return 0 or errno. So,
> > > > > > we need reset the return value to 0 for such case as what pre-v4.17 did.
> > > > > The patch is wrong. migrate_pages returns the number of pages it
> > > > > _hasn't_ migrated or -errno. Yeah that semantic sucks but...
> > > > > So err != 0 is always an error. Except err > 0 doesn't really provide
> > > > > any useful information to the userspace. I cannot really remember what
> > > > > was the actual behavior before my rework because there were some gotchas
> > > > > hidden there.
> > > > OK, so I've double checked. do_move_page_to_node_array would carry the
> > > > error code over to do_pages_move and it would store the status stored
> > > > in the pm array. It contains page_to_nid(page) so the resulting code
> > > > indeed behaves properly before my change and this is a regression. I
> > > Thanks, I see the change.
> > >
> > > > have a very vague recollection that this has been brought up already.
> > > > <...looks in notes...>
> > > > Found it! The report is
> > > > http://lkml.kernel.org/r/0329efa0984b9b0252ef166abb4498c0795fab36.1535113317.git.jstancek@redhat.com
> > > > and my proposed workaround was http://lkml.kernel.org/r/[email protected]
> > > Well, the above two links return 404.
> > You are right. They are not archived for some reason. Anyway, the patch
> > I was proposing back then is below:
> >
> > commit cfb88c266b645197135cde2905c2bfc82f6d82a9
> > Author: Michal Hocko <[email protected]>
> > Date: Wed Nov 14 12:19:09 2018 +0100
> >
> > mm: fix do_pages_move error reporting
> > a49bd4d71637 ("mm, numa: rework do_pages_move") has changed the way how
> > we report error to layers above. As the changelog mentioned the semantic
> > was quite unclear previously because the return 0 could mean both
> > success and failure.
> > The above mentioned commit didn't get all the way down to fix this
> > completely because it doesn't report pages that we even haven't
> > attempted to migrate and therefore we cannot simply say that the
> > semantic is:
> > - err < 0 - errno
> > - err >= 0 number of non-migrated pages.
> > Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
> > Signed-off-by: Michal Hocko <[email protected]>
>
> Thanks, Michal. But, it looks this patch still could return > 0 value (the
> total number of non-migrated pages, including not even attempted pages) too,
> but the problem we are trying to fix is to make do_pages_move() return <= 0
> value only since the man page of move_pages() doesn't allow return > 0
> value.

Yes this patch just lives with the changed semantic and tries to make it
sensible. So if some page cannot be migrated then we just stop and
return the number of non migrated pages at the tail of the given array.
This would make error handling slightly easier because you know that
count - ret pages of the array can be skipped if ret >= 0.

> And, by looking into the old code (v4.16), I spotted another problem. The
> migrate_pages() would store the migration failure error code into
> page_to_node->status. So, When do_move_page_to_node_array() returns > 0
> value, the return value would be reset to 0 and the migration error codes
> for non-migrated pages would be stored into status to return to userspace.
> But, the rework removed this.
>
> I didn't dig into the intention of the rework, is it expected?

I have tried to preserve the original semantic as possible. As explained
in the changelog there were quite some discrepancies even before. This
new one was not really intentional. We have effectively two options
here. Either somebody really depend on the former semantic and we have
to fix this or we can relax the semantic as the above patch attempts.

I would be more inclined for the second option as nobody has complained
about the new semantic except for few ltp tests which do not represent
real workload. If you have a real usecase then speak up please.
--
Michal Hocko
SUSE Labs

2020-01-22 17:28:19

[permalink] [raw]

Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On 1/22/20 12:06 AM, Michal Hocko wrote:
> On Tue 21-01-20 11:01:30, Yang Shi wrote:
>>
>> On 1/21/20 12:40 AM, Michal Hocko wrote:
>>> On Tue 21-01-20 09:44:16, Wei Yang wrote:
>>>> On Mon, Jan 20, 2020 at 02:17:44PM +0100, Michal Hocko wrote:
>>>>> On Mon 20-01-20 14:06:26, Michal Hocko wrote:
>>>>>> On Sat 18-01-20 13:26:43, Yang Shi wrote:
>>>>>>> The do_move_pages_to_node() might return > 0 value, the number of pages
>>>>>>> that are not migrated, then the value will be returned to userspace
>>>>>>> directly. But, move_pages() syscall would just return 0 or errno. So,
>>>>>>> we need reset the return value to 0 for such case as what pre-v4.17 did.
>>>>>> The patch is wrong. migrate_pages returns the number of pages it
>>>>>> _hasn't_ migrated or -errno. Yeah that semantic sucks but...
>>>>>> So err != 0 is always an error. Except err > 0 doesn't really provide
>>>>>> any useful information to the userspace. I cannot really remember what
>>>>>> was the actual behavior before my rework because there were some gotchas
>>>>>> hidden there.
>>>>> OK, so I've double checked. do_move_page_to_node_array would carry the
>>>>> error code over to do_pages_move and it would store the status stored
>>>>> in the pm array. It contains page_to_nid(page) so the resulting code
>>>>> indeed behaves properly before my change and this is a regression. I
>>>> Thanks, I see the change.
>>>>
>>>>> have a very vague recollection that this has been brought up already.
>>>>> <...looks in notes...>
>>>>> Found it! The report is
>>>>> http://lkml.kernel.org/r/0329efa0984b9b0252ef166abb4498c0795fab36.1535113317.git.jstancek@redhat.com
>>>>> and my proposed workaround was http://lkml.kernel.org/r/[email protected]
>>>> Well, the above two links return 404.
>>> You are right. They are not archived for some reason. Anyway, the patch
>>> I was proposing back then is below:
>>>
>>> commit cfb88c266b645197135cde2905c2bfc82f6d82a9
>>> Author: Michal Hocko <[email protected]>
>>> Date: Wed Nov 14 12:19:09 2018 +0100
>>>
>>> mm: fix do_pages_move error reporting
>>> a49bd4d71637 ("mm, numa: rework do_pages_move") has changed the way how
>>> we report error to layers above. As the changelog mentioned the semantic
>>> was quite unclear previously because the return 0 could mean both
>>> success and failure.
>>> The above mentioned commit didn't get all the way down to fix this
>>> completely because it doesn't report pages that we even haven't
>>> attempted to migrate and therefore we cannot simply say that the
>>> semantic is:
>>> - err < 0 - errno
>>> - err >= 0 number of non-migrated pages.
>>> Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
>>> Signed-off-by: Michal Hocko <[email protected]>
>> Thanks, Michal. But, it looks this patch still could return > 0 value (the
>> total number of non-migrated pages, including not even attempted pages) too,
>> but the problem we are trying to fix is to make do_pages_move() return <= 0
>> value only since the man page of move_pages() doesn't allow return > 0
>> value.
> Yes this patch just lives with the changed semantic and tries to make it
> sensible. So if some page cannot be migrated then we just stop and
> return the number of non migrated pages at the tail of the given array.
> This would make error handling slightly easier because you know that
> count - ret pages of the array can be skipped if ret >= 0.

OK, I see. Returning > 0 value sounds more straightforward for userspace
error handling.

BTW, we should update manpage to reflect the semantic change to indicate
> 0 return value as an error case.

>
>> And, by looking into the old code (v4.16), I spotted another problem. The
>> migrate_pages() would store the migration failure error code into
>> page_to_node->status. So, When do_move_page_to_node_array() returns > 0
>> value, the return value would be reset to 0 and the migration error codes
>> for non-migrated pages would be stored into status to return to userspace.
>> But, the rework removed this.
>>
>> I didn't dig into the intention of the rework, is it expected?
> I have tried to preserve the original semantic as possible. As explained
> in the changelog there were quite some discrepancies even before. This
> new one was not really intentional. We have effectively two options
> here. Either somebody really depend on the former semantic and we have
> to fix this or we can relax the semantic as the above patch attempts.
>
> I would be more inclined for the second option as nobody has complained
> about the new semantic except for few ltp tests which do not represent
> real workload. If you have a real usecase then speak up please.

No, I don't have any real usecase. And, I tend to agree the most users
may not care the reason of migration failure at all. Returning the
number of non-migrated pages seems more straightforward.

I agree we could stick with the new semantic and fix the return value as
what your patch did. I'm going to rebase your patch on top of Wei Yang's
cleanup if you don't mind.

2020-01-22 17:37:45

[permalink] [raw]

Subject: Re: [PATCH] mm: move_pages: fix the return value if there are not-migrated pages

On Wed 22-01-20 09:26:48, Yang Shi wrote:
>
>
> On 1/22/20 12:06 AM, Michal Hocko wrote:
> > On Tue 21-01-20 11:01:30, Yang Shi wrote:
> > >
> > > On 1/21/20 12:40 AM, Michal Hocko wrote:
> > > > On Tue 21-01-20 09:44:16, Wei Yang wrote:
> > > > > On Mon, Jan 20, 2020 at 02:17:44PM +0100, Michal Hocko wrote:
> > > > > > On Mon 20-01-20 14:06:26, Michal Hocko wrote:
> > > > > > > On Sat 18-01-20 13:26:43, Yang Shi wrote:
> > > > > > > > The do_move_pages_to_node() might return > 0 value, the number of pages
> > > > > > > > that are not migrated, then the value will be returned to userspace
> > > > > > > > directly. But, move_pages() syscall would just return 0 or errno. So,
> > > > > > > > we need reset the return value to 0 for such case as what pre-v4.17 did.
> > > > > > > The patch is wrong. migrate_pages returns the number of pages it
> > > > > > > _hasn't_ migrated or -errno. Yeah that semantic sucks but...
> > > > > > > So err != 0 is always an error. Except err > 0 doesn't really provide
> > > > > > > any useful information to the userspace. I cannot really remember what
> > > > > > > was the actual behavior before my rework because there were some gotchas
> > > > > > > hidden there.
> > > > > > OK, so I've double checked. do_move_page_to_node_array would carry the
> > > > > > error code over to do_pages_move and it would store the status stored
> > > > > > in the pm array. It contains page_to_nid(page) so the resulting code
> > > > > > indeed behaves properly before my change and this is a regression. I
> > > > > Thanks, I see the change.
> > > > >
> > > > > > have a very vague recollection that this has been brought up already.
> > > > > > <...looks in notes...>
> > > > > > Found it! The report is
> > > > > > http://lkml.kernel.org/r/0329efa0984b9b0252ef166abb4498c0795fab36.1535113317.git.jstancek@redhat.com
> > > > > > and my proposed workaround was http://lkml.kernel.org/r/[email protected]
> > > > > Well, the above two links return 404.
> > > > You are right. They are not archived for some reason. Anyway, the patch
> > > > I was proposing back then is below:
> > > >
> > > > commit cfb88c266b645197135cde2905c2bfc82f6d82a9
> > > > Author: Michal Hocko <[email protected]>
> > > > Date: Wed Nov 14 12:19:09 2018 +0100
> > > >
> > > > mm: fix do_pages_move error reporting
> > > > a49bd4d71637 ("mm, numa: rework do_pages_move") has changed the way how
> > > > we report error to layers above. As the changelog mentioned the semantic
> > > > was quite unclear previously because the return 0 could mean both
> > > > success and failure.
> > > > The above mentioned commit didn't get all the way down to fix this
> > > > completely because it doesn't report pages that we even haven't
> > > > attempted to migrate and therefore we cannot simply say that the
> > > > semantic is:
> > > > - err < 0 - errno
> > > > - err >= 0 number of non-migrated pages.
> > > > Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
> > > > Signed-off-by: Michal Hocko <[email protected]>
> > > Thanks, Michal. But, it looks this patch still could return > 0 value (the
> > > total number of non-migrated pages, including not even attempted pages) too,
> > > but the problem we are trying to fix is to make do_pages_move() return <= 0
> > > value only since the man page of move_pages() doesn't allow return > 0
> > > value.
> > Yes this patch just lives with the changed semantic and tries to make it
> > sensible. So if some page cannot be migrated then we just stop and
> > return the number of non migrated pages at the tail of the given array.
> > This would make error handling slightly easier because you know that
> > count - ret pages of the array can be skipped if ret >= 0.
>
> OK, I see. Returning > 0 value sounds more straightforward for userspace
> error handling.
>
> BTW, we should update manpage to reflect the semantic change to indicate > 0
> return value as an error case.

Absolutely.

> > > And, by looking into the old code (v4.16), I spotted another problem. The
> > > migrate_pages() would store the migration failure error code into
> > > page_to_node->status. So, When do_move_page_to_node_array() returns > 0
> > > value, the return value would be reset to 0 and the migration error codes
> > > for non-migrated pages would be stored into status to return to userspace.
> > > But, the rework removed this.
> > >
> > > I didn't dig into the intention of the rework, is it expected?
> > I have tried to preserve the original semantic as possible. As explained
> > in the changelog there were quite some discrepancies even before. This
> > new one was not really intentional. We have effectively two options
> > here. Either somebody really depend on the former semantic and we have
> > to fix this or we can relax the semantic as the above patch attempts.
> >
> > I would be more inclined for the second option as nobody has complained
> > about the new semantic except for few ltp tests which do not represent
> > real workload. If you have a real usecase then speak up please.
>
> No, I don't have any real usecase. And, I tend to agree the most users may
> not care the reason of migration failure at all. Returning the number of
> non-migrated pages seems more straightforward.
>
> I agree we could stick with the new semantic and fix the return value as
> what your patch did. I'm going to rebase your patch on top of Wei Yang's
> cleanup if you don't mind.

Go ahead. Thanks a lot!

--
Michal Hocko
SUSE Labs

2020-01-23 03:30:12