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. >>> >>> The page ditry bit is cleared, but the dirty bit of the pte still exists, >>> so if mmap continues to write, it will result in data loss. >> Curious, did you observe this data loss? What filesystem? If yes, it seems >> serious enough to >> CC stable and determine a Fixes: tag? > > Yes, there is data loss. > I'm using a btrfs environment, but not the following patch And the kernel is otherwise upstream? Which version? Anyway we better let btrfs guys know (+CC) even if the fix is in MM code. > btrfs: implement migratepage callback for data pages > https://git.kernel.org/pub/scm/linux/kernel > /git/torvalds/linux.git/commit/?h=v5.8-rc5& > id=f8e6608180a31cc72a23b74969da428da236dbd1 That's a new commit, so if this is really affecting upstream btrfs pre-5.8 we should either backport that commit, or your fix (after review). >>> We fix the by first remove the migration entry and then clearing >>> the dirty bits of the page, which also clears the pte's dirty bits. >>> >>> Signed-off-by: Robbie Ko <robbieko@xxxxxxxxxxxx> >>> --- >>> mm/migrate.c | 8 ++++---- >>> 1 file changed, 4 insertions(+), 4 deletions(-) >>> >>> diff --git a/mm/migrate.c b/mm/migrate.c >>> index f37729673558..5c407434b9ba 100644 >>> --- a/mm/migrate.c >>> +++ b/mm/migrate.c >>> @@ -875,10 +875,6 @@ static int writeout(struct address_space *mapping, struct page *page) >>> /* No write method for the address space */ >>> return -EINVAL; >>> >>> - if (!clear_page_dirty_for_io(page)) >>> - /* Someone else already triggered a write */ >>> - return -EAGAIN; >>> - >>> /* >>> * A dirty page may imply that the underlying filesystem has >>> * the page on some queue. So the page must be clean for >>> @@ -889,6 +885,10 @@ static int writeout(struct address_space *mapping, struct page *page) >>> */ >>> remove_migration_ptes(page, page, false); >>> >>> + if (!clear_page_dirty_for_io(page)) >>> + /* Someone else already triggered a write */ >>> + return -EAGAIN; >>> + >>> rc = mapping->a_ops->writepage(page, &wbc); >>> >>> if (rc != AOP_WRITEPAGE_ACTIVATE) >>> >> >
