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mm/rmap.c: avoid double faults migrating device private pages

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During migration special page table entries are installed for each page
being migrated.  These entries store the pfn and associated permissions
of ptes mapping the page being migarted.

Device-private pages use special swap pte entries to distinguish
read-only vs.  writeable pages which the migration code checks when
creating migration entries.  Normally this follows a fast path in
migrate_vma_collect_pmd() which correctly copies the permissions of
device-private pages over to migration entries when migrating pages back
to the CPU.

However the slow-path falls back to using try_to_migrate() which
unconditionally creates read-only migration entries for device-private
pages.  This leads to unnecessary double faults on the CPU as the new
pages are always mapped read-only even when they could be mapped
writeable.  Fix this by correctly copying device-private permissions in
try_to_migrate_one().

Link: https://lkml.kernel.org/r/20211018045247.3128058-1-apopple@nvidia.com
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reported-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Alistair Popple 2021-11-05 13:45:00 -07:00 committed by Linus Torvalds
parent 32befe9e27
commit 3d88705c10
1 changed files with 6 additions and 2 deletions
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8
mm/rmap.c
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@ -1807,6 +1807,7 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
update_hiwater_rss(mm);
if (is_zone_device_page(page)) {
unsigned long pfn = page_to_pfn(page);
swp_entry_t entry;
pte_t swp_pte;
@ -1815,8 +1816,11 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
* pte. do_swap_page() will wait until the migration
* pte is removed and then restart fault handling.
*/
entry = make_readable_migration_entry(
page_to_pfn(page));
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);
/*