linux-ext4 - Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

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Date:   Thu, 17 Feb 2022 16:12:20 -0500
From:   Felix Kuehling <felix.kuehling@....com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        David Hildenbrand <david@...hat.com>
Cc:     Alistair Popple <apopple@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Alex Sierra <alex.sierra@....com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        jglisse@...hat.com, willy@...radead.org
Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

Am 2022-02-16 um 07:26 schrieb Jason Gunthorpe:
> The other place that needs careful audit is all the callers using
> vm_normal_page() - they must all be able to accept a ZONE_DEVICE page
> if we don't set pte_devmap.

How much code are we talking about here? A quick search finds 26 
call-sites in 12 files in current master:

    fs/proc/task_mmu.c
    mm/hmm.c
    mm/gup.c
    mm/huge_memory.c (vm_normal_page_pmd)
    mm/khugepaged.c
    mm/madvise.c
    mm/mempolicy.c
    mm/memory.c
    mm/mlock.c
    mm/migrate.c
    mm/mprotect.c
    mm/memcontrol.c

I'm thinking of a more theoretical approach: Instead of auditing all 
users, I'd ask, what are the invariants that a vm_normal_page should 
have. Then check, whether our DEVICE_COHERENT pages satisfy them. But 
maybe the concept of a vm_normal_page isn't defined clearly enough for that.

That said, I think we (Alex and myself) made an implicit assumption from 
the start, that a DEVICE_COHERENT page should behave a lot like a normal 
page in terms of VMA mappings, even if we didn't know what that means in 
detail.

I can now at least name some differences between DEVICE_COHERENT and 
normal pages: how the memory is allocated, how data is migrated into 
DEVICE_COHERENT pages and that it can't be on any LRU list (because the 
lru list_head in struct page is aliased by pgmap and zone_device_data). 
Maybe I'll find more differences if I keep digging.

Regards,
   Felix

>
> Jason