linux-kernel - Re: [PATCH] mm, memory_hotplug: Don't bail out in do_migrate

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Date:   Tue, 11 Dec 2018 13:22:27 +0100
From:   osalvador@...e.de
To:     Michal Hocko <mhocko@...nel.org>
Cc:     akpm@...ux-foundation.org, david@...hat.com,
        pasha.tatashin@...een.com, dan.j.williams@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm, memory_hotplug: Don't bail out in do_migrate_range
 prematurely

On 2018-12-11 11:18, Michal Hocko wrote:
>> Currently, if we fail to isolate a single page, we put all already
>> isolated pages back to their LRU and we bail out from the function.
>> This is quite suboptimal, as this will force us to start over again
>> because scan_movable_pages will give us the same range.
>> If there is no chance that we can isolate that page, we will loop here
>> forever.
> 
> This is true but reorganizing the code will not help the underlying
> issue. Because the permanently failing page will be still there for
> scan_movable_pages to encounter.

Well, it would only help in case the page is neither LRU nor
non-movable page, then we would fail to isolate it in do_migrate_range
and we will start over.
Letting do_migrate_range do some work, would mean that at some point
the permanently failing page will not be within a range but the first 
one
of a range, and so scan_movable_pages will skip it.


> 
>> Issue debugged in 4d0c7db96 ("hwpoison, memory_hotplug: allow 
>> hwpoisoned
>> pages to be offlined") has proved that.
> 
> I assume that 4d0c7db96 is a sha1 from the linux-next. Please note that
> this is not going to be the case when merged upstream. So I would use a
> link.

I will replace the sha1 with the link in the next version.

>> Although this patch has proved to be useful when dealing with
>> 4d0c7db96 because it allows us to move forward as long as the
>> page is not in LRU, we still need 4d0c7db96
>> ("hwpoison, memory_hotplug: allow hwpoisoned pages to be offlined")
>> to handle the LRU case and the unmapping of the page if needed.
>> So, this is just a follow-up cleanup.
> 
> I suspect the above paragraph is adding more confusion than necessary. 
> I
> would just drop it.

Fair enough, I will drop it.

> The main question here is. Do we want to migrate as much as possible or
> do we want to be conservative and bail out early. The later could be an
> advantage if the next attempt could fail the whole operation because 
> the
> impact of the failed operation would be somehow reduced. The former
> should be better for throughput because easily done stuff is done 
> first.
> 
> I would go with the throuput because our failure mode is to bail out
> much earlier - even before we try to migrate. Even though the detection
> is not perfect it works reasonably well for most usecases.

I agree here.
I think it is better to do as much work as possible at once.


> you really want to keep this branch. You just do not want to bail out.
> We want to know about pages which fail to isolate and you definitely do
> not want to keep the reference elevated behind. not_managed stuff can 
> go
> away.

Yeah, I just realized when I sent it.
I will fix it up in v2.

Thanks