Re: [PATCH v1 2/6] mm/page_isolation: don't dump_page(NULL) in set_migratetype_isolate()

2020-07-29 Thread David Hildenbrand


> Am 29.07.2020 um 20:36 schrieb Mike Kravetz :
> 
> ´╗┐On 7/29/20 11:08 AM, David Hildenbrand wrote:
>> I have no clue what you mean with "reintroducing this abandoning of
>> pageblocks". All this patch is changing is not doing the dump_page() -
>> or am I missing something important?
> 
> My apologies!!!
> 

No worries, thanks for reviewing!!

> I got confused when I saw 'Return -EBUSY' removed from the comment and
> assumed the code would not return an error code.  The code now more
> explicitly does return -EBUSY.  My concern was when I incorrectly thought
> you were removing the error return code.  Sorry for the noise.
> 
> Acked-by: Mike Kravetz 
> -- 
> Mike Kravetz
> 

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Re: [PATCH v1 2/6] mm/page_isolation: don't dump_page(NULL) in set_migratetype_isolate()

2020-07-29 Thread David Hildenbrand
On 29.07.20 19:31, Mike Kravetz wrote:
> On 6/30/20 7:26 AM, David Hildenbrand wrote:
>> Right now, if we have two isolations racing, we might trigger the
>> WARN_ON_ONCE() and to dump_page(NULL), dereferencing NULL. Let's just
>> return directly.
> 
> Just curious, what call path has the WARN_ON_ONCE()/dump_page(NULL)?

See below, two set_migratetype_isolate() caller racing.

> 
>>
>> In the future, we might want to report -EAGAIN to the caller instead, as
>> this could indicate a temporary isolation failure only.
>>
>> Cc: Andrew Morton 
>> Cc: Michal Hocko 
>> Cc: Michael S. Tsirkin 
>> Signed-off-by: David Hildenbrand 
> 
> Hi David,
> 
> That 'return -EAGAIN' was added as a sort of synchronization mechanism.
> See commit message for 2c7452a075d4d.  Before adding the 'return -EAGAIN',
> I could create races which would abandon isolated pageblocks.  Repeating
> those races over and over would result in a good chunk of system memory
> being isolated and unusable.

It's actually -EBUSY, it should maybe later be changed to -EAGAIN (see
comment), so caller can decide to retry immediately. Other discussion.

> 
> Admittedly, these races are rare and I had to work really hard to produce
> them.  I'll try to find my testing mechanism.  My concern is reintroducing
> this abandoning of pageblocks.  I have not looked further in your series
> to see if this potentially addressed later.  If not, then we should not
> remove the return code.
> 

Memory offlining could race with alloc_contig_range(), e.g., called when
allocating gigantic pages, or when virtio-mem tries to unplug memory.
The latter two could also race.

We are getting more alloc_contig_range() users, which is why these races
will become more relevant.

I have no clue what you mean with "reintroducing this abandoning of
pageblocks". All this patch is changing is not doing the dump_page() -
or am I missing something important?


-- 
Thanks,

David / dhildenb

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