On Thu, Dec 13, 2018 at 03:23:37PM +0100, Richard Weinberger wrote: > Hello zhangjun, > > thanks a lot for bringing this up! > > Am Mittwoch, 12. Dezember 2018, 15:13:57 CET schrieb zhangjun: > > Because the PagePrivate() in UBIFS is different meanings, > > alloc_cma() will fail when one dirty page cache located in > > the type of MIGRATE_CMA > > > > If not adjust the 'extra_count' for dirty page, > > ubifs_migrate_page() -> migrate_page_move_mapping() will > > always return -EAGAIN for: > > expected_count += page_has_private(page) > > This causes the migration to fail until the page cache is cleaned > > > > In general, PagePrivate() indicates that buff_head is already bound > > to this page, and at the same time page_count() will also increase. That's an invalid assumption. We should not be trying to infer what PagePrivate() means in code that has no business using looking at it i.e. page->private is private information for the owner of the page, and it's life cycle and intent are unknown to anyone other than the page owner. e.g. on XFS, a page cache page's page->private /might/ contain a struct iomap_page, or it might be NULL. Assigning a struct iomap_page to the page does not change the reference count on the page. IOWs, the page needs to be handled exactly the same way by external code regardless of whether there is somethign attached to page->private or not. Hence it looks to me like the migration code is making invalid assumptions about PagePrivate inferring reference counts and so the migration code needs to be fixed. Requiring filesystems to work around invalid assumptions in the migration code is a sure recipe for problems with random filesystems using page->private for their own internal purposes.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
