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

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Message-ID: <20201203141729.GS5487@ziepe.ca>
Date:   Thu, 3 Dec 2020 10:17:29 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     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>
Subject: Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone

On Wed, Dec 02, 2020 at 08:34:32PM -0500, Pavel Tatashin wrote:

> > Either here or perhaps even lower down the call chain when the page is
> > captured, similar to how GUP fast would detect it. (how is that done
> > anyhow?)
> 
> Ah, thank you for pointing this out. I think I need to address it here:
> 
> https://soleen.com/source/xref/linux/mm/gup.c?r=96e1fac1#94
> 
> static __maybe_unused struct page *try_grab_compound_head()
>               if (unlikely(flags & FOLL_LONGTERM) &&  is_migrate_cma_page(page))
>                    return NULL;
> 
> I need to change is_migrate_cma_page() to all migratable pages. Will
> study, and send an update with this fix.

Yes, missing the two flows is a common error :(

Looking at this code some more.. How is it even correct?

1633  				if (!isolate_lru_page(head)) {
1634  					list_add_tail(&head->lru, &cma_page_list);

Here we are only running under the read side of the mmap sem so multiple
GUPs can be calling that sequence in parallel. I don't see any
obvious exclusion that will prevent corruption of head->lru. The first
GUP thread to do isolate_lru_page() will ClearPageLRU() and the second
GUP thread will be a NOP for isolate_lru_page().

They will both race list_add_tail and other list ops. That is not OK.

> What I meant is the users of the interface do it incrementally not in
> large chunks. For example:
> 
> vfio_pin_pages_remote
>    vaddr_get_pfn
>         ret = pin_user_pages_remote(mm, vaddr, 1, flags |
> FOLL_LONGTERM, page, NULL, NULL);
> 1 -> pin only one pages at a time

I don't know why vfio does this, it is why it so ridiculously slow at
least.

Jason