It was observed at [1] and [2] that the current kernel behaviour of shattering a hugezeropage is inconsistent and suboptimal. For a VMA with a THP allowable order, when we write-fault on it, the kernel installs a PMD-mapped THP. On the other hand, if we first get a read fault, we get a PMD pointing to the hugezeropage; subsequent write will trigger a write-protection fault, shattering the hugezeropage into one writable page, and all the other PTEs write-protected. The conclusion being, as compared to the case of a single write-fault, applications have to suffer 512 extra page faults if they were to use the VMA as such, plus we get the overhead of khugepaged trying to replace that area with a THP anyway. Instead, replace the hugezeropage with a THP on wp-fault. [1]: https://lore.kernel.org/all/3743d7e1-0b79-4eaf-82d5-d1ca29fe347d@xxxxxxx/ [2]: https://lore.kernel.org/all/1cfae0c0-96a2-4308-9c62-f7a640520242@xxxxxxx/ The patchset has been rebased on the mm-unstable branch. v5->v6: - More goto ommissions, remove build warning for !CONFIG_NUMA v4->v5: - Directly return VM_FAULT_FALLBACK in case of !folio v3->v4: - Renames: pmd_thp_fault_alloc -> vma_alloc_anon_folio_pmd, map_pmd_thp -> map_anon_folio_pmd - Instead of passing around, compute haddr at various places, similar with gfp flags - Pass haddr to update_mmu_cache_pmd() instead of unaligned address - Do not pass vmf to map_anon_folio_pmd - Do declarations in reverse xmas tree order - Drop a new line which was introduced accidentally - Call __pmd_thp_fault_success_stats from map_anon_folio_pmd - Correctly return NULL from vma_alloc_anon_folio_pmd - Initialize pgtable to NULL in __do_huge_pmd_anonymous_page, to prevent freeing pgtable when not even allocated - Drop if conditions from map_anon_folio_pmd, let the caller handle that v2->v3: - Drop foliop and order parameters, prefix the thp functions with pmd_ - First allocate THP, then pgtable, not vice-versa - Move pgtable_trans_huge_deposit() from map_pmd_thp() to caller - Drop exposing functions in include/linux/huge_mm.h - Open code do_huge_zero_wp_pmd_locked() - Release folio in case of pmd change after taking the lock, or check_stable_address_space() returning VM_FAULT_SIGBUS - Drop uffd-wp preservation. Looking at page_table_check_pmd_flags(), preserving uffd-wp on a writable entry is invalid. Looking at mfill_atomic(), uffd_copy() is a null operation when pmd is marked uffd-wp. v1->v2: - Wrap do_huge_zero_wp_pmd_locked() around lock and unlock - Call thp_fault_alloc() before do_huge_zero_wp_pmd_locked() to avoid - calling sleeping function from spinlock context Dev Jain (2): mm: Abstract THP allocation mm: Allocate THP on hugezeropage wp-fault mm/huge_memory.c | 139 +++++++++++++++++++++++++++++++++-------------- 1 file changed, 97 insertions(+), 42 deletions(-) -- 2.30.2
