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://lore.kernel.org/linux-mm/4f6dd918-d79b-1aa7-3a4c-caa67ddc29bc@xxxxxxxxxx/
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().
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://github.com/RadeonOpenCompute/ROCK-Kernel-Driver.git)
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.
I still think this patch needs an ACK from a FS/DAX maintainer.
