From: Peng Zhang zhangpeng.00@bytedance.com
mainline inclusion from mainline-v6.8-rc1 commit d2406291483775ecddaee929231a39c70c08fda2 category: performance bugzilla: https://gitee.com/openeuler/kernel/issues/I9AYM3 CVE: NA
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
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In dup_mmap(), using __mt_dup() to duplicate the old maple tree and then directly replacing the entries of VMAs in the new maple tree can result in better performance. __mt_dup() uses DFS pre-order to duplicate the maple tree, so it is efficient.
The average time complexity of __mt_dup() is O(n), where n is the number of VMAs. The proof of the time complexity is provided in the commit log that introduces __mt_dup(). After duplicating the maple tree, each element is traversed and replaced (ignoring the cases of deletion, which are rare). Since it is only a replacement operation for each element, this process is also O(n).
Analyzing the exact time complexity of the previous algorithm is challenging because each insertion can involve appending to a node, pushing data to adjacent nodes, or even splitting nodes. The frequency of each action is difficult to calculate. The worst-case scenario for a single insertion is when the tree undergoes splitting at every level. If we consider each insertion as the worst-case scenario, we can determine that the upper bound of the time complexity is O(n*log(n)), although this is a loose upper bound. However, based on the test data, it appears that the actual time complexity is likely to be O(n).
As the entire maple tree is duplicated using __mt_dup(), if dup_mmap() fails, there will be a portion of VMAs that have not been duplicated in the maple tree. To handle this, we mark the failure point with XA_ZERO_ENTRY. In exit_mmap(), if this marker is encountered, stop releasing VMAs that have not been duplicated after this point.
There is a "spawn" in byte-unixbench[1], which can be used to test the performance of fork(). I modified it slightly to make it work with different number of VMAs.
Below are the test results. The first row shows the number of VMAs. The second and third rows show the number of fork() calls per ten seconds, corresponding to next-20231006 and the this patchset, respectively. The test results were obtained with CPU binding to avoid scheduler load balancing that could cause unstable results. There are still some fluctuations in the test results, but at least they are better than the original performance.
21 121 221 421 821 1621 3221 6421 12821 25621 51221 112100 76261 54227 34035 20195 11112 6017 3161 1606 802 393 114558 83067 65008 45824 28751 16072 8922 4747 2436 1233 599 2.19% 8.92% 19.88% 34.64% 42.37% 44.64% 48.28% 50.17% 51.68% 53.74% 52.42%
[1] https://github.com/kdlucas/byte-unixbench/tree/master
Link: https://lkml.kernel.org/r/20231027033845.90608-11-zhangpeng.00@bytedance.com Signed-off-by: Peng Zhang zhangpeng.00@bytedance.com Suggested-by: Liam R. Howlett Liam.Howlett@oracle.com Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com Cc: Christian Brauner brauner@kernel.org Cc: Jonathan Corbet corbet@lwn.net Cc: Mateusz Guzik mjguzik@gmail.com Cc: Mathieu Desnoyers mathieu.desnoyers@efficios.com Cc: Matthew Wilcox willy@infradead.org Cc: Michael S. Tsirkin mst@redhat.com Cc: Mike Christie michael.christie@oracle.com Cc: Nicholas Piggin npiggin@gmail.com Cc: Peter Zijlstra peterz@infradead.org Cc: Suren Baghdasaryan surenb@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: ZhangPeng zhangpeng362@huawei.com --- include/linux/mm.h | 11 +++++++++++ kernel/fork.c | 40 +++++++++++++++++++++++++++++----------- mm/internal.h | 11 ----------- mm/memory.c | 7 ++++++- mm/mmap.c | 9 ++++++--- 5 files changed, 52 insertions(+), 26 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h index 55dbab37adea..ffa00d1cd17d 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1010,6 +1010,17 @@ static inline int vma_iter_bulk_alloc(struct vma_iterator *vmi, return mas_expected_entries(&vmi->mas, count); }
+static inline int vma_iter_clear_gfp(struct vma_iterator *vmi, + unsigned long start, unsigned long end, gfp_t gfp) +{ + __mas_set_range(&vmi->mas, start, end - 1); + mas_store_gfp(&vmi->mas, NULL, gfp); + if (unlikely(mas_is_err(&vmi->mas))) + return -ENOMEM; + + return 0; +} + /* Free any unused preallocations */ static inline void vma_iter_free(struct vma_iterator *vmi) { diff --git a/kernel/fork.c b/kernel/fork.c index caa8a5530883..43c7a00bb935 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -663,7 +663,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, int retval; unsigned long charge = 0; LIST_HEAD(uf); - VMA_ITERATOR(old_vmi, oldmm, 0); VMA_ITERATOR(vmi, mm, 0);
uprobe_start_dup_mmap(); @@ -691,16 +690,22 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, goto out; khugepaged_fork(mm, oldmm);
- retval = vma_iter_bulk_alloc(&vmi, oldmm->map_count); - if (retval) + /* Use __mt_dup() to efficiently build an identical maple tree. */ + retval = __mt_dup(&oldmm->mm_mt, &mm->mm_mt, GFP_KERNEL); + if (unlikely(retval)) goto out;
mt_clear_in_rcu(vmi.mas.tree); - for_each_vma(old_vmi, mpnt) { + for_each_vma(vmi, mpnt) { struct file *file;
vma_start_write(mpnt); if (mpnt->vm_flags & VM_DONTCOPY) { + retval = vma_iter_clear_gfp(&vmi, mpnt->vm_start, + mpnt->vm_end, GFP_KERNEL); + if (retval) + goto loop_out; + vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt)); continue; } @@ -762,9 +767,11 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, if (is_vm_hugetlb_page(tmp)) hugetlb_dup_vma_private(tmp);
- /* Link the vma into the MT */ - if (vma_iter_bulk_store(&vmi, tmp)) - goto fail_nomem_vmi_store; + /* + * Link the vma into the MT. After using __mt_dup(), memory + * allocation is not necessary here, so it cannot fail. + */ + vma_iter_bulk_store(&vmi, tmp);
mm->map_count++; if (!(tmp->vm_flags & VM_WIPEONFORK)) @@ -773,15 +780,28 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, if (tmp->vm_ops && tmp->vm_ops->open) tmp->vm_ops->open(tmp);
- if (retval) + if (retval) { + mpnt = vma_next(&vmi); goto loop_out; + } } /* a new mm has just been created */ retval = arch_dup_mmap(oldmm, mm); loop_out: vma_iter_free(&vmi); - if (!retval) + if (!retval) { mt_set_in_rcu(vmi.mas.tree); + } else if (mpnt) { + /* + * The entire maple tree has already been duplicated. If the + * mmap duplication fails, mark the failure point with + * XA_ZERO_ENTRY. In exit_mmap(), if this marker is encountered, + * stop releasing VMAs that have not been duplicated after this + * point. + */ + mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1); + mas_store(&vmi.mas, XA_ZERO_ENTRY); + } out: mmap_write_unlock(mm); flush_tlb_mm(oldmm); @@ -791,8 +811,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, uprobe_end_dup_mmap(); return retval;
-fail_nomem_vmi_store: - unlink_anon_vmas(tmp); fail_nomem_anon_vma_fork: mpol_put(vma_policy(tmp)); fail_nomem_policy: diff --git a/mm/internal.h b/mm/internal.h index d02db3e9eda9..eab2199d16c8 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -1140,17 +1140,6 @@ static inline void vma_iter_clear(struct vma_iterator *vmi) mas_store_prealloc(&vmi->mas, NULL); }
-static inline int vma_iter_clear_gfp(struct vma_iterator *vmi, - unsigned long start, unsigned long end, gfp_t gfp) -{ - __mas_set_range(&vmi->mas, start, end - 1); - mas_store_gfp(&vmi->mas, NULL, gfp); - if (unlikely(mas_is_err(&vmi->mas))) - return -ENOMEM; - - return 0; -} - static inline struct vm_area_struct *vma_iter_load(struct vma_iterator *vmi) { return mas_walk(&vmi->mas); diff --git a/mm/memory.c b/mm/memory.c index e25f9293eef3..13012c3294b4 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -375,6 +375,8 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas, * be 0. This will underflow and is okay. */ next = mas_find(mas, ceiling - 1); + if (unlikely(xa_is_zero(next))) + next = NULL;
/* * Hide vma from rmap and truncate_pagecache before freeing @@ -396,6 +398,8 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas, && !is_vm_hugetlb_page(next)) { vma = next; next = mas_find(mas, ceiling - 1); + if (unlikely(xa_is_zero(next))) + next = NULL; if (mm_wr_locked) vma_start_write(vma); unlink_anon_vmas(vma); @@ -1752,7 +1756,8 @@ void unmap_vmas(struct mmu_gather *tlb, struct ma_state *mas, unmap_single_vma(tlb, vma, start, end, &details, mm_wr_locked); hugetlb_zap_end(vma, &details); - } while ((vma = mas_find(mas, tree_end - 1)) != NULL); + vma = mas_find(mas, tree_end - 1); + } while (vma && likely(!xa_is_zero(vma))); mmu_notifier_invalidate_range_end(&range); }
diff --git a/mm/mmap.c b/mm/mmap.c index 21d1fc39bf21..8b5295653475 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -3252,10 +3252,11 @@ void exit_mmap(struct mm_struct *mm) arch_exit_mmap(mm);
vma = mas_find(&mas, ULONG_MAX); - if (!vma) { + if (!vma || unlikely(xa_is_zero(vma))) { /* Can happen if dup_mmap() received an OOM */ mmap_read_unlock(mm); - return; + mmap_write_lock(mm); + goto destroy; }
lru_add_drain(); @@ -3290,11 +3291,13 @@ void exit_mmap(struct mm_struct *mm) remove_vma(vma, true); count++; cond_resched(); - } while ((vma = mas_find(&mas, ULONG_MAX)) != NULL); + vma = mas_find(&mas, ULONG_MAX); + } while (vma && likely(!xa_is_zero(vma)));
BUG_ON(count != mm->map_count);
trace_exit_mmap(mm); +destroy: __mt_destroy(&mm->mm_mt); mmap_write_unlock(mm); vm_unacct_memory(nr_accounted);