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Date: Thu, 19 Aug 2021 13:00:34 -0500 From: "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com> To: Ralph Campbell <rcampbell@...dia.com>, Felix Kuehling <felix.kuehling@....com>, akpm@...ux-foundation.org, linux-mm@...ck.org, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org Cc: amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org, hch@....de, jgg@...dia.com, jglisse@...hat.com Subject: Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page refcount On 8/18/2021 2:28 PM, Ralph Campbell wrote: > On 8/17/21 5:35 PM, Felix Kuehling wrote: >> Am 2021-08-17 um 8:01 p.m. schrieb Ralph Campbell: >>> On 8/12/21 11:31 PM, Alex Sierra wrote: >>>> From: Ralph Campbell <rcampbell@...dia.com> >>>> >>>> ZONE_DEVICE struct pages have an extra reference count that >>>> complicates the >>>> code for put_page() and several places in the kernel that need to >>>> check the >>>> reference count to see that a page is not being used (gup, compaction, >>>> migration, etc.). Clean up the code so the reference count doesn't >>>> need to >>>> be treated specially for ZONE_DEVICE. >>>> >>>> v2: >>>> AS: merged this patch in linux 5.11 version >>>> >>>> v5: >>>> AS: add condition at try_grab_page to check for the zone device type, >>>> while >>>> page ref counter is checked less/equal to zero. In case of device >>>> zone, pages >>>> ref counter are initialized to zero. >>>> >>>> Signed-off-by: Ralph Campbell <rcampbell@...dia.com> >>>> Signed-off-by: Alex Sierra <alex.sierra@....com> >>>> --- >>>> arch/powerpc/kvm/book3s_hv_uvmem.c | 2 +- >>>> drivers/gpu/drm/nouveau/nouveau_dmem.c | 2 +- >>>> fs/dax.c | 4 +- >>>> include/linux/dax.h | 2 +- >>>> include/linux/memremap.h | 7 +-- >>>> include/linux/mm.h | 13 +---- >>>> lib/test_hmm.c | 2 +- >>>> mm/internal.h | 8 +++ >>>> mm/memremap.c | 68 >>>> +++++++------------------- >>>> mm/migrate.c | 5 -- >>>> mm/page_alloc.c | 3 ++ >>>> mm/swap.c | 45 ++--------------- >>>> 12 files changed, 46 insertions(+), 115 deletions(-) >>>> >>> I haven't seen a response to the issues I raised back at v3 of this >>> series. >>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flinux-mm%2F4f6dd918-d79b-1aa7-3a4c-caa67ddc29bc%40nvidia.com%2F&data=04%7C01%7Calex.sierra%40amd.com%7Cd2bd2d4fbf764528540908d9627e5dcd%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637649117156919772%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=P7FxYm%2BkJaCkMFa3OHtuKrPOn7SvytFRmYQdIzq7rN4%3D&reserved=0 >>> >>> >>> >>> Did I miss something? >> I think part of the response was that we did more testing. Alex added >> support for DEVICE_GENERIC pages to test_hmm and he ran DAX tests >> recommended by Theodore Tso. In that testing he ran into a WARN_ON_ONCE >> about a zero page refcount in try_get_page. The fix is in the latest >> version of patch 2. But it's already obsolete because John Hubbard is >> about to remove that function altogether. >> >> I think the issues you raised were more uncertainty than known bugs. It >> seems the fact that you can have DAX pages with 0 refcount is a feature >> more than a bug. >> >> Regards, >> Felix > > Did you test on a system without CONFIG_ARCH_HAS_PTE_SPECIAL defined? > In that case, mmap() of a DAX device will call insert_page() which calls > get_page() which would trigger VM_BUG_ON_PAGE(). > > I can believe it is OK for PTE_SPECIAL page table entries to have no > struct page or that MEMORY_DEVICE_GENERIC struct pages be mapped with > a zero reference count using insert_pfn(). Hi Ralph, We have tried the DAX tests with and without CONFIG_ARCH_HAS_PTE_SPECIAL defined. Apparently none of the tests touches that condition for a DAX device. Of course, that doesn't mean it could happen. Regards, Alex S. > > > I find it hard to believe that other MM developers don't see an issue > with a struct page with refcount == 0 and mapcount == 1. > > I don't see where init_page_count() is being called for the > MEMORY_DEVICE_GENERIC or MEMORY_DEVICE_PRIVATE struct pages the AMD > driver allocates and passes to migrate_vma_setup(). > Looks like svm_migrate_get_vram_page() needs to call init_page_count() > instead of get_page(). (I'm looking at branch > origin/alexsierrag/device_generic > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FRadeonOpenCompute%2FROCK-Kernel-Driver.git&data=04%7C01%7Calex.sierra%40amd.com%7Cd2bd2d4fbf764528540908d9627e5dcd%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637649117156919772%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=IXe8HP2s8x5OdJdERBkGOYJCQk3iqCu5AYkwpDL8zec%3D&reserved=0) Yes, you're right. My bad. Thanks for catching this up. I didn't realize I was missing to define CONFIG_DEBUG_VM on my build. Therefore this BUG was never caught. It worked after I replaced get_pages by init_page_count at svm_migrate_get_vram_page. However, I don't think this is the best way to fix it. Ideally, get_pages call should work for device pages with ref count equal to 0 too. Otherwise, we could overwrite refcounter if someone else is grabbing the page concurrently. I was thinking to add a special condition in get_pages for dev pages. This could also fix the insert_page -> get_page call from a DAX device. Regards, Alex S. > > > Also, what about the other places where is_device_private_page() is > called? > Don't they need to be updated to call is_device_page() instead? > One of my goals for this patch was to remove special casing reference > counts > for ZONE_DEVICE pages in rmap.c, etc. Correct, is_device_private_page is still used in rmap, memcontrol and migrate.c files Looks like rmap and memcontrol should be replaced by is_device_page function. However, I still need test to validate this. For migrate.c is used in remove_migration_pte and migrate_vma_insert_page, however these are specific conditions for private device type Thanks for raise these questions, I think we're getting close. Regards, Alex S. > > I still think this patch needs an ACK from a FS/DAX maintainer. >
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