On 3/20/24 06:00, Oscar Salvador wrote: > On Tue, Mar 19, 2024 at 06:48:31PM +0000, Matthew Wilcox wrote: >> Is this the right way to fix this problem? I would have thought we'd >> be better off accounting this as migration freeing the old page and >> allocating the new page. If I understand correctly, this is the code >> which says "This page was last allocated by X and freed by Y", and I >> would think that being last freed (or allocated) by the migration code >> would be a very nice hint about where a problem might stem from. > > I hear you, and I had the same thought when I stumbled upon this. > I did not know that the handle was being changed, otherwise it would > have saved me quite a lot of debugging time. > > Checking the history of this, I can see this decision was made in > 2016 in: > > commit d435edca928805074dae005ab9a42d9fa60fc702 > Author: Vlastimil Babka <vbabka@xxxxxxx> > Date: Tue Mar 15 14:56:15 2016 -0700 > > mm, page_owner: copy page owner info during migration > Yeah I think we could keep that logic. But we could also simply subtract the refcount of the old handle (the "allocated for migration") in __folio_copy_owner() no? Then we wouldn't need the extra migrate_handle. Also we might have more issues here. Most page owner code takes care to set everything for all pages within a folio, but __folio_copy_owner() and __set_page_owner_migrate_reason() don't.
