Kirill A. Shutemov 於 2020/7/15 下午4:11 寫道:
On Wed, Jul 15, 2020 at 10:45:39AM +0800, Robbie Ko wrote:
Kirill A. Shutemov 於 2020/7/14 下午6:19 寫道:
On Tue, Jul 14, 2020 at 11:46:12AM +0200, Vlastimil Babka wrote:
On 7/13/20 3:57 AM, Robbie Ko wrote:
Vlastimil Babka 於 2020/7/10 下午11:31 寫道:
On 7/9/20 4:48 AM, robbieko wrote:
From: Robbie Ko <robbieko@xxxxxxxxxxxx>
When a migrate page occurs, we first create a migration entry
to replace the original pte, and then go to fallback_migrate_page
to execute a writeout if the migratepage is not supported.
In the writeout, we will clear the dirty bit of the page and use
page_mkclean to clear the dirty bit along with the corresponding pte,
but page_mkclean does not support migration entry.
I don't follow the scenario.
When we establish migration entries with try_to_unmap(), it transfers
dirty bit from PTE to the page.
Sorry, I mean is _PAGE_RW with pte_write
When we establish migration entries with try_to_unmap(),
we create a migration entry, and if pte_write we set it to SWP_MIGRATION_WRITE,
which will replace the migration entry with the original pte.
When migratepage, we go to fallback_migrate_page to execute a writeout
if the migratepage is not supported.
In the writeout, we call clear_page_dirty_for_io to clear the dirty bit of the page
and use page_mkclean to clear pte _PAGE_RW with pte_wrprotect in page_mkclean_one.
However, page_mkclean_one does not support migration entries, so the
migration entry is still SWP_MIGRATION_WRITE.
In writeout, then we call remove_migration_ptes to remove the migration entry,
because it is still SWP_MIGRATION_WRITE so set _PAGE_RW to pte via pte_mkwrite.
Therefore, subsequent mmap wirte will not trigger page_mkwrite to cause data loss.
Hm, okay.
Folks, is there any good reason why try_to_unmap(TTU_MIGRATION) should not
clear PTE (make the PTE none) for file page?
This, I'm not sure.
But I think that for the fs that support migratepage, when migratepage
is finished,
the page should still be dirty, and the pte should still have _PAGE_RW,
when the next mmap write occurs, we don't need to trigger the
page_mkwrite again.
