On Wed, Dec 20, 2023 at 11:16 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Thu, Dec 21, 2023 at 03:19:12AM +0000, Pasha Tatashin wrote: > > This series frees empty page tables on unmaps. It intends to be a > > low overhead feature. > > > > The read-writer lock is used to synchronize page table, but most of > > time the lock is held is reader. It is held as a writer for short > > period of time when unmapping a page that is bigger than the current > > iova request. For all other cases this lock is read-only. > > > > page->refcount is used in order to track number of entries at each page > > table. > > Have I not put enough DANGER signs up around the page refcount? > > * If you want to use the refcount field, it must be used in such a way > * that other CPUs temporarily incrementing and then decrementing the > * refcount does not cause problems. On receiving the page from > * alloc_pages(), the refcount will be positive. > > You can't use refcount for your purpose, and honestly I'm shocked you > haven't seen any of your WARNings trigger. Hi Matthew, Thank you for looking at this. Could you please explain exactly why refcount can't be used like this? After alloc_page() refcount is set to 1, we never reduce it to 0, every new entry in a page table adds 1, so we get up-to 513, that is why I added warn like this: WARN_ON_ONCE(rc > 513 || rc < 2); to dma_set_pte() macro. When refcount == 1, we know that the page table is empty, and can be added to a freelist for a delayed freeing. What is wrong with using refcount for a scalable way of keeping the track of number of entries in a iommu page table? Is there a better way that I should use? Thank you, Pasha
