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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=J+xKZ2vH; dmarc=pass (policy=quarantine) header.from=nvidia.com; spf=none (imf05.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.243.48) smtp.mailfrom=jhubbard@nvidia.com X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 5231F5082D45 X-Stat-Signature: 9xz8xxxotdra8seeqzgferafu38anaon X-HE-Tag: 1634539648-243561 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 10/17/21 21:52, Alistair Popple wrote: > 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. s/migarted/migrated/ > > 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(). > > Signed-off-by: Alistair Popple > Reported-by: Ralph Campbell > --- > mm/rmap.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) Looks very clearly correct to me. Reviewed-by: John Hubbard thanks, -- John Hubbard NVIDIA > > diff --git a/mm/rmap.c b/mm/rmap.c > index b9eb5c12f3fe..271de8118cdd 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1804,6 +1804,7 @@ 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; > > @@ -1812,8 +1813,11 @@ 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); > > /* >