Alistair Popple wrote: > > Dan Williams <dan.j.williams@xxxxxxxxx> writes: > > > Alistair Popple wrote: > >> > >> Jason Gunthorpe <jgg@xxxxxxxxxx> writes: > >> > >> > On Mon, Sep 26, 2022 at 04:03:06PM +1000, Alistair Popple wrote: > >> >> Since 27674ef6c73f ("mm: remove the extra ZONE_DEVICE struct page > >> >> refcount") device private pages have no longer had an extra reference > >> >> count when the page is in use. However before handing them back to the > >> >> owning device driver we add an extra reference count such that free > >> >> pages have a reference count of one. > >> >> > >> >> This makes it difficult to tell if a page is free or not because both > >> >> free and in use pages will have a non-zero refcount. Instead we should > >> >> return pages to the drivers page allocator with a zero reference count. > >> >> Kernel code can then safely use kernel functions such as > >> >> get_page_unless_zero(). > >> >> > >> >> Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx> > >> >> --- > >> >> arch/powerpc/kvm/book3s_hv_uvmem.c | 1 + > >> >> drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 1 + > >> >> drivers/gpu/drm/nouveau/nouveau_dmem.c | 1 + > >> >> lib/test_hmm.c | 1 + > >> >> mm/memremap.c | 5 ----- > >> >> mm/page_alloc.c | 6 ++++++ > >> >> 6 files changed, 10 insertions(+), 5 deletions(-) > >> > > >> > I think this is a great idea, but I'm surprised no dax stuff is > >> > touched here? > >> > >> free_zone_device_page() shouldn't be called for pgmap->type == > >> MEMORY_DEVICE_FS_DAX so I don't think we should have to worry about DAX > >> there. Except that the folio code looks like it might have introduced a > >> bug. AFAICT put_page() always calls > >> put_devmap_managed_page(&folio->page) but folio_put() does not (although > >> folios_put() does!). So it seems folio_put() won't end up calling > >> __put_devmap_managed_page_refs() as I think it should. > >> > >> I think you're right about the change to __init_zone_device_page() - I > >> should limit it to DEVICE_PRIVATE/COHERENT pages only. But I need to > >> look at Dan's patch series more closely as I suspect it might be better > >> to rebase this patch on top of that. > > > > Apologies for the delay I was travelling the past few days. Yes, I think > > this patch slots in nicely to avoid the introduction of an init_mode > > [1]: > > > > https://lore.kernel.org/nvdimm/166329940343.2786261.6047770378829215962.stgit@xxxxxxxxxxxxxxxxxxxxxxxxx/ > > > > Mind if I steal it into my series? > > No problem, although I notice Andrew has already merged it into > mm-unstable. If you end up rebasing your series on top of mine I think > all that's needed is a patch somewhere in your series to drop the > various `if (pgmap->type == MEMORY_DEVICE_*)` I added to (hopefully) > avoid breaking DAX. Assuming DAX takes a pagemap reference on struct > page allocation something like below. Yeah, I'll go that route and rebase on top of -mm. Thanks again.