Re: [PATCH 3/7] mm/rmap: map_pte() was not handling private ZONE_DEVICE page properly v2

[Balbir Singh]

On Thu, Aug 30, 2018 at 10:41:56AM -0400, jglisse@xxxxxxxxxx wrote:
> From: Ralph Campbell <rcampbell@xxxxxxxxxx>
> 
> Private ZONE_DEVICE pages use a special pte entry and thus are not
> present. Properly handle this case in map_pte(), it is already handled
> in check_pte(), the map_pte() part was lost in some rebase most probably.
> 
> Without this patch the slow migration path can not migrate back private
> ZONE_DEVICE memory to regular memory. This was found after stress
> testing migration back to system memory. This ultimatly can lead the
> CPU to an infinite page fault loop on the special swap entry.
> 
> Changes since v1:
>     - properly lock pte directory in map_pte()
> 
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Balbir Singh <bsingharora@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  mm/page_vma_mapped.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index ae3c2a35d61b..bd67e23dce33 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -21,7 +21,14 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw)
>  			if (!is_swap_pte(*pvmw->pte))
>  				return false;
>  		} else {
> -			if (!pte_present(*pvmw->pte))
> +			if (is_swap_pte(*pvmw->pte)) {
> +				swp_entry_t entry;
> +
> +				/* Handle un-addressable ZONE_DEVICE memory */
> +				entry = pte_to_swp_entry(*pvmw->pte);
> +				if (!is_device_private_entry(entry))
> +					return false;

OK, so we skip this pte from unmap since it's already unmapped? This prevents
try_to_unmap from unmapping it and it gets restored with MIGRATE_PFN_MIGRATE
flag cleared?

Sounds like the right thing, if I understand it correctly

Acked-by: Balbir Singh <bsingharora@xxxxxxxxx>

Balbir Singh.