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Date: Thu, 29 Sep 2022 13:18:15 -0700 From: Dan Williams <dan.j.williams@...el.com> To: Alistair Popple <apopple@...dia.com>, Jason Gunthorpe <jgg@...dia.com> CC: <linux-mm@...ck.org>, Andrew Morton <akpm@...ux-foundation.org>, "Michael Ellerman" <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, "Felix Kuehling" <Felix.Kuehling@....com>, Alex Deucher <alexander.deucher@....com>, Christian König <christian.koenig@....com>, "Pan, Xinhui" <Xinhui.Pan@....com>, David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>, Ben Skeggs <bskeggs@...hat.com>, Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>, Ralph Campbell <rcampbell@...dia.com>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, Alex Sierra <alex.sierra@....com>, John Hubbard <jhubbard@...dia.com>, <linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>, <amd-gfx@...ts.freedesktop.org>, <nouveau@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>, Dan Williams <dan.j.williams@...el.com> Subject: Re: [PATCH 2/7] mm: Free device private pages have zero refcount Alistair Popple wrote: > > Jason Gunthorpe <jgg@...dia.com> 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@...dia.com> > >> --- > >> 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@dwillia2-xfh.jf.intel.com/ Mind if I steal it into my series?
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