linux-kernel - Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone

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Message-Id: <10F682D5-0654-4C42-9989-F999D4434295@redhat.com>
Date:   Fri, 11 Dec 2020 22:29:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     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>,
        David Hildenbrand <david@...hat.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 22:09 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?

> 
> Pasha
> 
>> 
>> Jason
>