Re: [PATCH] mm/hmm: populate PFNs from PMD swap entry

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Sep 08, 2025 at 11:10:52AM +0200, Francois Dugast wrote:
> Once support for THP migration of zone device pages is enabled, device
> private swap entries will be found during the walk not only for PTEs but
> also for PMDs.
> 
> Therefore, it is necessary to extend to PMDs the special handling which is
> already in place for PTEs when device private pages are owned by the
> caller: instead of faulting or skipping the range, the correct behavior is
> to use the swap entry to populate HMM PFNs.
> 
> This change is a prerequisite to make use of device-private THP in drivers
> using drivers/gpu/drm/drm_pagemap, such as xe.
> 
> Even though subsequent PFNs can be inferred when handling large order
> PFNs, the PFN list is still fully populated because this is currently
> expected by HMM users. In case this changes in the future, that is all HMM
> users support a sparsely populated PFN list, the for() loop can be made to
> skip remaining PFNs for the current order. A quick test shows the loop
> takes about 10 ns, roughly 20 times faster than without this optimization.
> 
> Cc: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Leon Romanovsky <leonro@nvidia.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Balbir Singh <balbirs@nvidia.com>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Mika Penttilä <mpenttil@redhat.com>
> Cc: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Francois Dugast <francois.dugast@intel.com>
> ---
>  mm/hmm.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 65 insertions(+), 5 deletions(-)

Please put version numbers on your patches and include a change log.

Jason