On Wed, Sep 16, 2020 at 5:29 PM Ralph Campbell <rcampbell@xxxxxxxxxx> wrote: > > > On 9/15/20 10:36 PM, Christoph Hellwig wrote: > > On Tue, Sep 15, 2020 at 09:39:47AM -0700, Ralph Campbell wrote: > >>> I don't think any of the three ->page_free instances even cares about > >>> the page refcount. > >>> > >> Not true. The page_free() callback records the page is free by setting > >> a bit or putting the page on a free list but when it allocates a free > >> device private struct page to be used with migrate_vma_setup(), it needs to > >> increment the refcount. > >> > >> For the ZONE_DEVICE MEMORY_DEVICE_GENERIC and MEMORY_DEVICE_PCI_P2PDMA > >> struct pages, I think you are correct because they don't define page_free() > >> and from what I can see, don't decrement the page refcount to zero. > > > > Umm, the whole point of ZONE_DEVICE is to have a struct page for > > something that is not system memory. For both the ppc kvm case (magic > > hypervisor pool) and Noveau (device internal) memory that clear is the > > case. But looks like test_hmm uses normal pages to fake this up, so > > I was wrong about the third caller. But I think we can just call > > set_page_count just before freeing the page there with a comment > > explaining what is goin on. > > Dan Williams thought that having the ZONE_DEVICE struct pages > be on a free list with a refcount of one was a bit strange and > that the driver should handle the zero to one transition. > But, that would mean a bit more invasive change to the 3 drivers > to set the reference count to zero after calling memremap_pages() > and setting the reference count to one when allocating a struct > page. What you are suggesting is what I also proposed in v1. IIUC, isn't what Christoph recommending is that drivers handle set_page_count() directly rather than the core since some are prepared for it to be zero on entry?
