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Message-Id: <447A41F3-EB94-4DA4-8B98-038B127774A5@redhat.com>
Date: Fri, 11 Dec 2020 22:53:00 +0100
From: David Hildenbrand <david@...hat.com>
To: Pavel Tatashin <pasha.tatashin@...een.com>
Cc: David Hildenbrand <david@...hat.com>,
Jason Gunthorpe <jgg@...pe.ca>,
LKML <linux-kernel@...r.kernel.org>,
linux-mm <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>,
Michal Hocko <mhocko@...e.com>,
Oscar Salvador <osalvador@...e.de>,
Dan Williams <dan.j.williams@...el.com>,
Sasha Levin <sashal@...nel.org>,
Tyler Hicks <tyhicks@...ux.microsoft.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
Steven Rostedt <rostedt@...dmis.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Mel Gorman <mgorman@...e.de>,
Matthew Wilcox <willy@...radead.org>,
David Rientjes <rientjes@...gle.com>,
John Hubbard <jhubbard@...dia.com>,
Linux Doc Mailing List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone
> Am 11.12.2020 um 22:36 schrieb Pavel Tatashin <pasha.tatashin@...een.com>:
>
> On Fri, Dec 11, 2020 at 4:29 PM David Hildenbrand <david@...hat.com> wrote:
>>
>>
>>>> Am 11.12.2020 um 22:09 schrieb Pavel Tatashin <pasha.tatashin@...een.com>:
>>>
>>> On Fri, Dec 11, 2020 at 3:46 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>>>>
>>>>> On Fri, Dec 11, 2020 at 03:40:57PM -0500, Pavel Tatashin wrote:
>>>>> On Fri, Dec 11, 2020 at 3:23 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>>>>>>
>>>>>> On Fri, Dec 11, 2020 at 03:21:39PM -0500, Pavel Tatashin wrote:
>>>>>>> @@ -1593,7 +1592,7 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
>>>>>>> }
>>>>>>>
>>>>>>> if (!isolate_lru_page(head)) {
>>>>>>> - list_add_tail(&head->lru, &cma_page_list);
>>>>>>> + list_add_tail(&head->lru, &movable_page_list);
>>>>>>> mod_node_page_state(page_pgdat(head),
>>>>>>> NR_ISOLATED_ANON +
>>>>>>> page_is_file_lru(head),
>>>>>>> @@ -1605,7 +1604,7 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
>>>>>>> i += step;
>>>>>>> }
>>>>>>>
>>>>>>> - if (!list_empty(&cma_page_list)) {
>>>>>>> + if (!list_empty(&movable_page_list)) {
>>>>>>
>>>>>> You didn't answer my earlier question, is it OK that ZONE_MOVABLE
>>>>>> pages leak out here if ioslate_lru_page() fails but the
>>>>>> moval_page_list is empty?
>>>>>>
>>>>>> I think the answer is no, right?
>>>>> In my opinion it is OK. We are doing our best to not pin movable
>>>>> pages, but if isolate_lru_page() fails because pages are currently
>>>>> locked by someone else, we will end up long-term pinning them.
>>>>> See comment in this patch:
>>>>> + * 1. Pinned pages: (long-term) pinning of movable pages is avoided
>>>>> + * when pages are pinned and faulted, but it is still possible that
>>>>> + * address space already has pages in ZONE_MOVABLE at the time when
>>>>> + * pages are pinned (i.e. user has touches that memory before
>>>>> + * pinning). In such case we try to migrate them to a different zone,
>>>>> + * but if migration fails the pages can still end-up pinned in
>>>>> + * ZONE_MOVABLE. In such case, memory offlining might retry a long
>>>>> + * time and will only succeed once user application unpins pages.
>>>>
>>>> It is not "retry a long time" it is "might never complete" because
>>>> userspace will hold the DMA pin indefinitely.
>>>>
>>>> Confused what the point of all this is then ??
>>>>
>>>> I thought to goal here is to make memory unplug reliable, if you leave
>>>> a hole like this then any hostile userspace can block it forever.
>>>
>>> You are right, I used a wording from the previous comment, and it
>>> should be made clear that pin may be forever. Without these patches it
>>> is guaranteed that hot-remove will fail if there are pinned pages as
>>> ZONE_MOVABLE is actually the first to be searched. Now, it will fail
>>> only due to exceptions listed in ZONE_MOVABLE comment:
>>>
>>> 1. pin + migration/isolation failure
>>
>> Not sure what that really means. We have short-term pinnings (although we might have a better term for „pinning“ here) for example, when a process dies (IIRC). There is a period where pages cannot get migrated and offlining code has to retry (which might take a while). This still applies after your change - are you referring to that?
>>
>>> 2. memblock allocation due to limited amount of space for kernelcore
>>> 3. memory holes
>>> 4. hwpoison
>>> 5. Unmovable PG_offline pages (? need to study why this is a scenario).
>>
>> Virtio-mem is the primary user in this context.
>>
>>> Do you think we should unconditionally unpin pages, and return error
>>> when isolation/migration fails?
>>
>> I‘m not sure what you mean here. Who’s supposed to unpin which pages?
>
> Hi David,
>
> When check_and_migrate_movable_pages() is called, the pages are
> already pinned. If some of those pages are in movable zone, and we
> fail to migrate or isolate them what should we do: proceed, and keep
> it as exception of when movable zone can actually have pinned pages or
> unpin all pages in the array, and return an error, or unpin only pages
> in movable zone, and return an error?
>
I guess revert what we did (unpin) and return an error. The interesting question is what can make migration/isolation fail
a) out of memory: smells like a zone setup issue. Failures are acceptable I guess.
b) short term pinnings: process dying - not relevant I guess. Other cases? (Fork?)
c) ?
Once we clarified that, we actually know how likely it will be to return an error (and making vfio pinnings fail etc).
> Pasha
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