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

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Date:   Thu, 17 Feb 2022 20:19:35 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Felix Kuehling <felix.kuehling@....com>
Cc:     David Hildenbrand <david@...hat.com>,
        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

On Thu, Feb 17, 2022 at 04:12:20PM -0500, Felix Kuehling wrote:

> 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.

I would say the expectation is that only 'page cache and anon' pages
are returned - ie the first union in struct page

This is because the first file in your list I looked at:

static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
				unsigned long addr, unsigned long end,
				struct mm_walk *walk)

{
		page = vm_normal_page(vma, addr, ptent);
[..]
		if (pageout) {
			if (!isolate_lru_page(page)) {

Uses the LRU field, so this is incompatible with all the other page
types.

One mitigation of this might be to formally make vm_normal_page() ==
'pte to page cache and anon page' and add a new function that is 'pte
to any struct page'

Then go through and sort callers as appropriate.

The 'pte to page cache and anon page' can detect ZONE_DEVICE by
calling is_zone_device_page() insted of pte_devmap() and then continue
to return NULL. This same trick will fix GUP_fast.

Jason