On 6/17/21 8:16 AM, Alex Sierra wrote:
From: Ralph Campbell rcampbell@nvidia.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
Signed-off-by: Ralph Campbell rcampbell@nvidia.com Signed-off-by: Alex Sierra alex.sierra@amd.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 | 44 ----------------- 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, 45 insertions(+), 147 deletions(-)
I think it is great that you are picking this up and trying to revive it.
However, I have a number of concerns about how it affects existing ZONE_DEVICE MEMORY_DEVICE_GENERIC and MEMORY_DEVICE_FS_DAX users and I don't see this addressing them. For example, dev_dax_probe() allocates MEMORY_DEVICE_GENERIC struct pages and then: dev_dax_fault() dev_dax_huge_fault() __dev_dax_pte_fault() vmf_insert_mixed() which just inserts the PFN into the CPU page tables without increasing the page refcount so it is zero (whereas it was one before). But using get_page() will trigger VM_BUG_ON_PAGE() if it is enabled. There isn't any current notion of free verses allocated for these struct pages. I suppose init_page_count() could be called on all the struct pages in dev_dax_probe() to fix that though.
I'm even less clear about how to fix MEMORY_DEVICE_FS_DAX. File systems have clear allocate and free states for backing storage but there are the complications with the page cache references, etc. to consider. The >1 to 1 reference count seems to be used to tell when a page is idle (no I/O, reclaim scanners) rather than free (not allocated to any file) but I'm not 100% sure about that since I don't really understand all the issues around why a file system needs to have a DAX mount option besides knowing that the storage block size has to be a multiple of the page size.