Re: [PATCH v1 2/9] mm: add device coherent vma selection for memory migration

On Tuesday, 16 November 2021 6:30:19 AM AEDT Alex Sierra wrote:

This case is used to migrate pages from device memory, back to system memory. Device coherent type memory is cache coherent from device and CPU point of view.

Signed-off-by: Alex Sierra alex.sierra@amd.com

v2: condition added when migrations from device coherent pages.

---

include/linux/migrate.h | 1 + mm/migrate.c | 9 +++++++-- 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h index c8077e936691..e74bb0978f6f 100644 --- a/include/linux/migrate.h +++ b/include/linux/migrate.h @@ -138,6 +138,7 @@ static inline unsigned long migrate_pfn(unsigned long pfn) enum migrate_vma_direction { MIGRATE_VMA_SELECT_SYSTEM = 1 << 0, MIGRATE_VMA_SELECT_DEVICE_PRIVATE = 1 << 1,

• MIGRATE_VMA_SELECT_DEVICE_COHERENT = 1 << 2,

};

struct migrate_vma { diff --git a/mm/migrate.c b/mm/migrate.c index f74422a42192..166bfec7d85e 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -2340,8 +2340,6 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, if (is_writable_device_private_entry(entry)) mpfn |= MIGRATE_PFN_WRITE; } else {

• 	if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
  

• 		goto next;
  pfn = pte_pfn(pte);
  if (is_zero_pfn(pfn)) {
  	mpfn = MIGRATE_PFN_MIGRATE;
  

@@ -2349,6 +2347,13 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, goto next; } page = vm_normal_page(migrate->vma, addr, pte);

• 	if (!is_zone_device_page(page) &&
  

• 	    !(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
  

• 		goto next;
  

• 	if (is_zone_device_page(page) &&
  

• 	    (!(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_COHERENT) ||
  

• 	     page->pgmap->owner != migrate->pgmap_owner))
  

• 		goto next;
  

Thanks Alex, couple of comments on this:

1. vm_normal_page() can return NULL so you need to add a check for page == NULL otherwise the call to is_zone_device_page(NULL) will crash. 2. The check for a coherent device page is too indirect. Being explicit and using is_zone_device_coherent_page() instead would make it more direct and obvious, particularly for developers who may not immediately realise that device-private pages should never have pte_present() entries.

	mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
	mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
}