Jason Gunthorpe wrote: > On Tue, Sep 06, 2022 at 11:37:36AM -0700, Dan Williams wrote: > > Jason Gunthorpe wrote: > > > On Tue, Sep 06, 2022 at 10:23:41AM -0700, Dan Williams wrote: > > > > > > > > Can we continue to have the weird page->refcount behavior and still > > > > > change the other things? > > > > > > > > No at a minimum the pgmap vs page->refcount problem needs to be solved > > > > first. > > > > > > So who will do the put page after the PTE/PMD's are cleared out? In > > > the normal case the tlb flusher does it integrated into zap.. > > > > AFAICS the zap manages the _mapcount not _refcount. Are you talking > > about page_remove_rmap() or some other reference count drop? > > No, page refcount. > > __tlb_remove_page() eventually causes a put_page() via > tlb_batch_pages_flush() calling free_pages_and_swap_cache() > > Eg: > > * MMU_GATHER_NO_GATHER > * > * If the option is set the mmu_gather will not track individual pages for > * delayed page free anymore. A platform that enables the option needs to > * provide its own implementation of the __tlb_remove_page_size() function to > * free pages. Ok, yes, that is a vm_normal_page() mechanism which I was going to defer since it is incremental to the _refcount handling fix and maintain that DAX pages are still !vm_normal_page() in this set. > > > Can we safely have the put page in the fsdax side after the zap? > > > > The _refcount is managed from the lifetime insert_page() to > > truncate_inode_pages(), where for DAX those are managed from > > dax_insert_dentry() to dax_delete_mapping_entry(). > > As long as we all understand the page doesn't become re-allocatable > until the refcount reaches 0 and the free op is called it may be OK! Yes, but this does mean that page_maybe_dma_pinned() is not sufficient for when the filesystem can safely reuse the page, it really needs to wait for the reference count to drop to 0 similar to how it waits for the page-idle condition today.
