linux-kernel - Re: [PATCH v1 09/10] mm/memory_hotplug: teach offline

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Message-ID: <20180524143953.GK20441@dhcp22.suse.cz>
Date:   Thu, 24 May 2018 16:39:53 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rashmica Gupta <rashmica.g@...il.com>,
        Balbir Singh <bsingharora@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v1 09/10] mm/memory_hotplug: teach offline_pages() to not
 try forever

[I didn't really go through other patch but this one caught my eyes just
 because of the similar request proposed yesterday]

On Wed 23-05-18 17:11:50, David Hildenbrand wrote:
[...]
> @@ -1686,6 +1686,10 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  	pfn = scan_movable_pages(start_pfn, end_pfn);
>  	if (pfn) { /* We have movable pages */
>  		ret = do_migrate_range(pfn, end_pfn);
> +		if (ret && !retry_forever) {
> +			ret = -EBUSY;
> +			goto failed_removal;
> +		}
>  		goto repeat;
>  	}
>  

Btw. this will not work in practice. Even a single temporary pin on a page
will fail way too easily. If you really need to control this then make
it a retry counter with default -1UL.

We really do need a better error reporting from do_migrate_range and
distinguish transient from permanent failures. In general we shouldn't
even get here for pages which are not migrateable...
-- 
Michal Hocko
SUSE Labs