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@xxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Alex Sierra <alex.sierra@xxxxxxx>
---
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.
