On Thu, Dec 21, 2023 at 12:13 AM Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> wrote: > > 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 I guess, what you mean is that other CPUs could temporarily increase/decrease refcount outside of IOMMU management, do you have an example of why that would happen? I could remove the above warning, and in the worst case we would miss an opportunity to free a page table during unmap, not a big deal, it can be freed during another map/unmap event. Still better than today, where we never free them during unmaps. Pasha
