Messages in this thread Patch in this message | | | Subject | Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page | From | Robbie Ko <> | Date | Mon, 13 Jul 2020 09:57:23 +0800 |
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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@synology.com> >> >> 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 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
>> 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@synology.com> >> --- >> 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) >> >
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