Here is v3 of the series of patches to mm (and a few architectures), based on v6.5-rc1 which includes the preceding two series (thank you!): in which khugepaged takes advantage of pte_offset_map[_lock]() allowing for pmd transitions. Differences from v1 and v2 are noted patch by patch below. This replaces the v2 "mm: free retracted page table by RCU" https://lore.kernel.org/linux-mm/54cb04f-3762-987f-8294-91dafd8ebfb0@xxxxxxxxxx/ series of 12 posted on 2023-06-20. What is it all about? Some mmap_lock avoidance i.e. latency reduction. Initially just for the case of collapsing shmem or file pages to THPs: the usefulness of MADV_COLLAPSE on shmem is being limited by that mmap_write_lock it currently requires. Likely to be relied upon later in other contexts e.g. freeing of empty page tables (but that's not work I'm doing). mmap_write_lock avoidance when collapsing to anon THPs? Perhaps, but again that's not work I've done: a quick attempt was not as easy as the shmem/file case. These changes (though of course not these exact patches) have been in Google's data centre kernel for three years now: we do rely upon them. Based on v6.5-rc1; and almost good on current mm-unstable or current linux-next - just one patch conflicts, the 12/13: I'll reply to that one with its mm-unstable or linux-next equivalent (vma_assert_locked() has been added next to where vma_try_start_write() is being removed). 01/13 mm/pgtable: add rcu_read_lock() and rcu_read_unlock()s v3: same as v1 02/13 mm/pgtable: add PAE safety to __pte_offset_map() v3: same as v2 v2: rename to pmdp_get_lockless_start/end() per Matthew; so use inlines without _irq_save(flags) macro oddity; add pmdp_get_lockless_sync() for use later in 09/13. 03/13 arm: adjust_pte() use pte_offset_map_nolock() v3: same as v1 04/13 powerpc: assert_pte_locked() use pte_offset_map_nolock() v3: same as v1 05/13 powerpc: add pte_free_defer() for pgtables sharing page v3: much simpler version, following suggestion by Jason v2: fix rcu_head usage to cope with concurrent deferrals; add para to commit message explaining rcu_head issue. 06/13 sparc: add pte_free_defer() for pte_t *pgtable_t v3: same as v2 v2: use page_address() instead of less common page_to_virt(); add para to commit message explaining simple conversion; changed title since sparc64 pgtables do not share page. 07/13 s390: add pte_free_defer() for pgtables sharing page v3: much simpler version, following suggestion by Gerald v2: complete rewrite, integrated with s390's existing pgtable management; temporarily using a global mm_pgtable_list_lock, to be restored to per-mm spinlock in a later followup patch. 08/13 mm/pgtable: add pte_free_defer() for pgtable as page v3: same as v2 v2: add comment on rcu_head to "Page table pages", per JannH 09/13 mm/khugepaged: retract_page_tables() without mmap or vma lock v3: same as v2 v2: repeat checks under ptl because UFFD, per PeterX and JannH; bring back mmu_notifier calls for PMD, per JannH and Jason; pmdp_get_lockless_sync() to issue missing interrupt if PAE. 10/13 mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock() v3: updated to using ptent instead of *pte v2: first check VMA, in case page tables torn down, per JannH; pmdp_get_lockless_sync() to issue missing interrupt if PAE; moved mmu_notifier after step 1, reworked final goto labels. 11/13 mm/khugepaged: delete khugepaged_collapse_pte_mapped_thps() v3: rediffed v2: same as v1 12/13 mm: delete mmap_write_trylock() and vma_try_start_write() v3: rediffed (different diff needed for mm-unstable or linux-next) v2: same as v1 13/13 mm/pgtable: notes on pte_offset_map[_lock]() v3: new: JannH asked for more helpful comment, this is my attempt; could be moved to be the first in the series. arch/arm/mm/fault-armv.c | 3 +- arch/powerpc/include/asm/pgalloc.h | 4 + arch/powerpc/mm/pgtable-frag.c | 29 +- arch/powerpc/mm/pgtable.c | 16 +- arch/s390/include/asm/pgalloc.h | 4 + arch/s390/mm/pgalloc.c | 80 ++++- arch/sparc/include/asm/pgalloc_64.h | 4 + arch/sparc/mm/init_64.c | 16 + include/linux/mm.h | 17 -- include/linux/mm_types.h | 4 + include/linux/mmap_lock.h | 10 - include/linux/pgtable.h | 10 +- mm/khugepaged.c | 481 +++++++++++------------------- mm/pgtable-generic.c | 97 +++++- 14 files changed, 404 insertions(+), 371 deletions(-) Hugh
