Matthew Wilcox, Ira Weiny, and others have complained that ZONE_DEVICE struct page reference counting is ugly because they are "free" when the reference count is one instead of zero. This leads to explicit checks for ZONE_DEVICE pages in places like put_page(), GUP, THP splitting, and page migration which have to adjust the expected reference count when determining if the page is isolated or idle. This is my attempt to make ZONE_DEVICE pages be free when the reference count is zero and removing the special cases. I'm only sending this out as a RFC since I'm not that familiar with the DAX, PMEM, XEN, and other uses of ZONE_DEVICE struct pages allocated with devm_memremap_pages() or memremap_pages() but my best reading of the code looks like it might be OK. I could use help testing these configurations. I have been able to successfully run xfstests on ext4 with the memmap kernel boot option to simulate pmem. One of the big changes in v2 is that devm_memremap_pages() and memremap_pages() now return the struct pages' reference count set to zero instead of one. Normally, get_page() will VM_BUG_ON_PAGE() if page->_refcount is zero. I didn't see any such warnings running the xfstests with dax/pmem but I'm not clear how the zero to one reference count is handled. Other changes in v2: Rebased to Linux-5.9.0-rc6 to include pmem fixes. I added patch 1 to introduce a page refcount helper for ext4 and xfs as suggested by Christoph Hellwig. I also applied Christoph Hellwig's other suggested changes for removing the devmap_managed_key, etc. Ralph Campbell (2): ext4/xfs: add page refcount helper mm: remove extra ZONE_DEVICE struct page refcount arch/powerpc/kvm/book3s_hv_uvmem.c | 2 +- drivers/gpu/drm/nouveau/nouveau_dmem.c | 2 +- fs/dax.c | 8 +-- fs/ext4/inode.c | 2 +- fs/xfs/xfs_file.c | 2 +- include/linux/dax.h | 5 ++ include/linux/memremap.h | 7 ++- include/linux/mm.h | 44 -------------- lib/test_hmm.c | 2 +- mm/gup.c | 44 -------------- mm/internal.h | 8 +++ mm/memremap.c | 82 ++++++-------------------- mm/migrate.c | 5 -- mm/page_alloc.c | 3 + mm/swap.c | 46 +++------------ 15 files changed, 54 insertions(+), 208 deletions(-) -- 2.20.1
