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move_ptes() explains when rmap locks can be skipped, when reading the code setting pmc.need_rmap_locks it is not immediately obvious when need_rmap_locks can be false. Add a brief explanation in copy_vma() and relocate_vma_down(), and add a pointer to the comment in move_ptes(). Meanwhile, fix and improve the comment in move_ptes(). Signed-off-by: Harry Yoo --- mm/mremap.c | 4 +++- mm/vma.c | 7 +++++++ mm/vma_exec.c | 5 +++++ 3 files changed, 15 insertions(+), 1 deletion(-) diff --git a/mm/mremap.c b/mm/mremap.c index e618a706aff5..86adb872bea0 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -218,8 +218,10 @@ static int move_ptes(struct pagetable_move_control *pmc, * When need_rmap_locks is false, we use other ways to avoid * such races: * - * - During exec() shift_arg_pages(), we use a specially tagged vma + * - During exec() relocate_vma_down(), we use a specially tagged vma * which rmap call sites look for using vma_is_temporary_stack(). + * Folios mapped in the temporary stack vma cannot be migrated until + * the relocation is complete. * * - During mremap(), new_vma is often known to be placed after vma * in rmap traversal order. This ensures rmap will always observe diff --git a/mm/vma.c b/mm/vma.c index 3b12c7579831..3da49f79e9ba 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -1842,6 +1842,11 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap, vmg.next = vma_iter_next_rewind(&vmi, NULL); new_vma = vma_merge_new_range(&vmg); + /* + * rmap locks can be skipped as long as new_vma is traversed + * after vma during rmap walk (new_vma->vm_pgoff >= vma->vm_pgoff). + * See the comment in move_ptes(). + */ if (new_vma) { /* * Source vma may have been merged into new_vma @@ -1879,6 +1884,8 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap, new_vma->vm_ops->open(new_vma); if (vma_link(mm, new_vma)) goto out_vma_link; + + /* new_vma->pg_off is always >= vma->pg_off if not merged */ *need_rmap_locks = false; } return new_vma; diff --git a/mm/vma_exec.c b/mm/vma_exec.c index 922ee51747a6..a895dd39ac46 100644 --- a/mm/vma_exec.c +++ b/mm/vma_exec.c @@ -63,6 +63,11 @@ int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift) * process cleanup to remove whatever mess we made. */ pmc.for_stack = true; + /* + * pmc.need_rmap_locks is false since rmap locks can be safely skipped + * because migration is disabled for this vma during relocation. + * See the comment in move_ptes(). + */ if (length != move_page_tables(&pmc)) return -ENOMEM; -- 2.43.0