linux-kernel - Re: [PATCH 6/7] nouveau/dmem: Evict device private memory during release

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Date:   Tue, 27 Sep 2022 09:45:35 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Lyude Paul <lyude@...hat.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Ben Skeggs <bskeggs@...hat.com>,
        Karol Herbst <kherbst@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Alex Sierra <alex.sierra@....com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, Jason Gunthorpe <jgg@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 6/7] nouveau/dmem: Evict device private memory during
 release


John Hubbard <jhubbard@...dia.com> writes:

> On 9/26/22 14:35, Lyude Paul wrote:
>>> +	for (i = 0; i < npages; i++) {
>>> +		if (src_pfns[i] & MIGRATE_PFN_MIGRATE) {
>>> +			struct page *dpage;
>>> +
>>> +			/*
>>> +			 * _GFP_NOFAIL because the GPU is going away and there
>>> +			 * is nothing sensible we can do if we can't copy the
>>> +			 * data back.
>>> +			 */
>>
>> You'll have to excuse me for a moment since this area of nouveau isn't one of
>> my strongpoints, but are we sure about this? IIRC __GFP_NOFAIL means infinite
>> retry, in the case of a GPU hotplug event I would assume we would rather just
>> stop trying to migrate things to the GPU and just drop the data instead of
>> hanging on infinite retries.
>>

No problem, thanks for taking a look!

> Hi Lyude!
>
> Actually, I really think it's better in this case to keep trying
> (presumably not necessarily infinitely, but only until memory becomes
> available), rather than failing out and corrupting data.
>
> That's because I'm not sure it's completely clear that this memory is
> discardable. And at some point, we're going to make this all work with
> file-backed memory, which will *definitely* not be discardable--I
> realize that we're not there yet, of course.
>
> But here, it's reasonable to commit to just retrying indefinitely,
> really. Memory should eventually show up. And if it doesn't, then
> restarting the machine is better than corrupting data, generally.

The memory is definitely not discardable here if the migration failed
because that implies it is still mapped into some userspace process.

We could avoid restarting the machine by doing something similar to what
happens during memory failure and killing every process that maps the
page(s). But overall I think it's better to retry until memory is
available, because that allows things like reclaim to work and in the
worst case allows the OOM killer to select an appropriate task to kill.
It also won't cause data corruption if/when we have file-backed memory.

> thanks,