linux-kernel - Re: [PATCH v5 08/14] mm/gup: do not allow zero page for pinned pages

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Date:   Tue, 19 Jan 2021 15:48:11 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v5 08/14] mm/gup: do not allow zero page for pinned pages

> I was thinking about a use case where userland would pin an address
> without FOLL_WRITE, because the PTE for that address is not going to
> be writable, but some device via DMA will write to it. Now, if we got
> a zero page we have a problem... If this usecase is not valid then the
> fix for movable zero page is make the zero page always come from a
> non-movable zone so we do not need to isolate it during migration, and
> so the memory can be offlined later.

I looked into making zero_page non-movable, and I am confused here.

huge zero page is already not movable:
get_huge_zero_page()
   zero_page = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE, ...

Base zero page can be in a movable zone, which is a bug: if there are
references to zero page, that page cannot be migrated, and we won't be
hot-remove memory area where that page is located. On x86, zero page
should always come from the bottom 4G of physical memory / DMA32 ZONE.

However, I see that sometimes it is not (I reproduce in QEMU emulator):
QEMU instance with 16G of memory and kernelcore=5G

Boot#1:
zero_pfn 48a8d
zero_pfn zone: ZONE_DMA32

Boot#2:
zero_pfn 20168d
zero_pfn zone: ZONE_MOVABLE (???)

The problem is that the x86 zero page comes from the .bss segment:
https://soleen.com/source/xref/linux/arch/x86/kernel/head_64.S?r=31d85460#583

Which, I thought would always be set within the first 4G of physical
memory. What is going on here?

Pasha