linux-ext4 - Re: [PATCH v1 01/15] mm: add zone device coherent type memory support

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Date:   Thu, 12 May 2022 12:58:00 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Alex Sierra <alex.sierra@....com>
Cc:     jgg@...dia.com, david@...hat.com, Felix.Kuehling@....com,
        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,
        hch@....de, jglisse@...hat.com, willy@...radead.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v1 01/15] mm: add zone device coherent type memory support


Alex Sierra <alex.sierra@....com> writes:

[...]

> diff --git a/mm/rmap.c b/mm/rmap.c
> index fedb82371efe..d57102cd4b43 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1995,7 +1995,8 @@ void try_to_migrate(struct folio *folio, enum ttu_flags flags)
>  					TTU_SYNC)))
>  		return;
>
> -	if (folio_is_zone_device(folio) && !folio_is_device_private(folio))
> +	if (folio_is_zone_device(folio) &&
> +	    (!folio_is_device_private(folio) && !folio_is_device_coherent(folio)))
>  		return;
>
>  	/*

I vaguely recall commenting on this previously, or at least intending
to. In try_to_migrate_one() we have this:

		if (folio_is_zone_device(folio)) {
			unsigned long pfn = folio_pfn(folio);
			swp_entry_t entry;
			pte_t swp_pte;

			/*
			 * Store the pfn of the page in a special migration
			 * pte. do_swap_page() will wait until the migration
			 * pte is removed and then restart fault handling.
			 */
			entry = pte_to_swp_entry(pteval);
			if (is_writable_device_private_entry(entry))
				entry = make_writable_migration_entry(pfn);
			else
				entry = make_readable_migration_entry(pfn);
			swp_pte = swp_entry_to_pte(entry);

The check in try_to_migrate() guarantees that if folio_is_zone_device()
is true this must be a DEVICE_PRIVATE page and it treats it as such by
assuming there is a special device private swap entry there.

Relying on that assumption seems bad, and I have no idea why I didn't
just use is_device_private_page() originally but I think the fix is just
to change this to:

		if (folio_is_device_private(folio))

And let DEVICE_COHERENT pages fall through to normal page processing.

 - Alistair