From 718b13861d2256ac95d65b892953282a63faf240 Mon Sep 17 00:00:00 2001 From: Qi Zheng Date: Wed, 4 Dec 2024 19:09:50 +0800 Subject: x86: mm: free page table pages by RCU instead of semi RCU Now, if CONFIG_MMU_GATHER_RCU_TABLE_FREE is selected, the page table pages will be freed by semi RCU, that is: - batch table freeing: asynchronous free by RCU - single table freeing: IPI + synchronous free In this way, the page table can be lockless traversed by disabling IRQ in paths such as fast GUP. But this is not enough to free the empty PTE page table pages in paths other that munmap and exit_mmap path, because IPI cannot be synchronized with rcu_read_lock() in pte_offset_map{_lock}(). In preparation for supporting empty PTE page table pages reclaimation, let single table also be freed by RCU like batch table freeing. Then we can also use pte_offset_map() etc to prevent PTE page from being freed. Like pte_free_defer(), we can also safely use ptdesc->pt_rcu_head to free the page table pages: - The pt_rcu_head is unioned with pt_list and pmd_huge_pte. - For pt_list, it is used to manage the PGD page in x86. Fortunately tlb_remove_table() will not be used for free PGD pages, so it is safe to use pt_rcu_head. - For pmd_huge_pte, it is used for THPs, so it is safe. After applying this patch, if CONFIG_PT_RECLAIM is enabled, the function call of free_pte() is as follows: free_pte pte_free_tlb __pte_free_tlb ___pte_free_tlb paravirt_tlb_remove_table tlb_remove_table [!CONFIG_PARAVIRT, Xen PV, Hyper-V, KVM] [no-free-memory slowpath:] tlb_table_invalidate tlb_remove_table_one __tlb_remove_table_one [frees via RCU] [fastpath:] tlb_table_flush tlb_remove_table_free [frees via RCU] native_tlb_remove_table [CONFIG_PARAVIRT on native] tlb_remove_table [see above] Link: https://lkml.kernel.org/r/0287d442a973150b0e1019cc406e6322d148277a.1733305182.git.zhengqi.arch@bytedance.com Signed-off-by: Qi Zheng Cc: Dave Hansen Cc: Andy Lutomirski Cc: Peter Zijlstra Cc: Catalin Marinas Cc: David Hildenbrand Cc: David Rientjes Cc: Hugh Dickins Cc: Jann Horn Cc: Lorenzo Stoakes Cc: Matthew Wilcox Cc: Mel Gorman Cc: Muchun Song Cc: Peter Xu Cc: Will Deacon Cc: Zach O'Keefe Cc: Dan Carpenter Signed-off-by: Andrew Morton --- include/linux/mm_types.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'include/linux') diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 332cee285662..7490d84af310 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -438,7 +438,9 @@ FOLIO_MATCH(compound_head, _head_2a); * struct ptdesc - Memory descriptor for page tables. * @__page_flags: Same as page flags. Powerpc only. * @pt_rcu_head: For freeing page table pages. - * @pt_list: List of used page tables. Used for s390 and x86. + * @pt_list: List of used page tables. Used for s390 gmap shadow pages + * (which are not linked into the user page tables) and x86 + * pgds. * @_pt_pad_1: Padding that aliases with page's compound head. * @pmd_huge_pte: Protected by ptdesc->ptl, used for THPs. * @__page_mapping: Aliases with page->mapping. Unused for page tables. -- cgit v1.2.3