Messages in this thread | | | Date | Tue, 11 Dec 2018 13:22:27 +0100 | From | osalvador@suse ... | Subject | Re: [PATCH] mm, memory_hotplug: Don't bail out in do_migrate_range prematurely |
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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
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