linux-kernel - Re: [RFC][PATCH 08/13] mm/migrate: demote pages during reclaim

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Date:   Tue, 2 Feb 2021 10:34:27 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yang Shi <shy828301@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>,
        Huang Ying <ying.huang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [RFC][PATCH 08/13] mm/migrate: demote pages during reclaim

On 2/2/21 10:22 AM, Yang Shi wrote:
>> +static struct page *alloc_demote_page(struct page *page, unsigned long node)
>> +{
>> +        struct migration_target_control mtc = {
>> +               /*
>> +                * Fail quickly and quietly.  Page will likely
>> +                * just be discarded instead of migrated.
>> +                */
>> +               .gfp_mask = GFP_HIGHUSER | __GFP_NORETRY | __GFP_NOWARN,
>> +               .nid = node
>> +       };
>> +
>> +        return alloc_migration_target(page, (unsigned long)&mtc);
> Other than the gfp flag question raised by Oscar, I'm wondering how we
> guarantee the demotion allocation happens on the designated node. In
> the previous version __GFP_THISNODE is set to guarantee this. In this
> version you switched to use alloc_migration_target() API but without
> having nodemask or __GFP_THISNODE. If nodemask is NULL the allocation
> may fall back to an unexpected node.
> 
> And GFP_HIGHUSER does respect cpuset, so if the demotion target node
> is excluded by the cpuset which the task belongs to, the migration
> would fail. This might be a way to respect cpuset, but it should just
> work for direct reclaimer. So, is this change really expected?

No, that wasn't intended.  I'll restore __GFP_THISNODE.  Thanks for
noting this.