Hi, Kirill, On 21.05.2019 11:18, Kirill A. Shutemov wrote: > On Mon, May 20, 2019 at 05:00:12PM +0300, Kirill Tkhai wrote: >> This prepares the function to copy a vma between >> two processes. Two new arguments are introduced. > > This kind of changes requires a lot more explanation in commit message, > describing all possible corner cases> For instance, I would really like to > see a story on why logic around > need_rmap_locks is safe after the change.
Let me fast answer on the below question firstly, and later I'll write wide explanations, since this requires much more time. >> >> Signed-off-by: Kirill Tkhai <[email protected]> >> --- >> include/linux/mm.h | 4 ++-- >> mm/mmap.c | 33 ++++++++++++++++++++++++--------- >> mm/mremap.c | 4 ++-- >> 3 files changed, 28 insertions(+), 13 deletions(-) >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index 0e8834ac32b7..afe07e4a76f8 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -2329,8 +2329,8 @@ extern void __vma_link_rb(struct mm_struct *, struct >> vm_area_struct *, >> struct rb_node **, struct rb_node *); >> extern void unlink_file_vma(struct vm_area_struct *); >> extern struct vm_area_struct *copy_vma(struct vm_area_struct **, >> - unsigned long addr, unsigned long len, pgoff_t pgoff, >> - bool *need_rmap_locks); >> + struct mm_struct *, unsigned long addr, unsigned long len, >> + pgoff_t pgoff, bool *need_rmap_locks, bool clear_flags_ctx); >> extern void exit_mmap(struct mm_struct *); >> >> static inline int check_data_rlimit(unsigned long rlim, >> diff --git a/mm/mmap.c b/mm/mmap.c >> index 57803a0a3a5c..99778e724ad1 100644 >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -3195,19 +3195,21 @@ int insert_vm_struct(struct mm_struct *mm, struct >> vm_area_struct *vma) >> } >> >> /* >> - * Copy the vma structure to a new location in the same mm, >> - * prior to moving page table entries, to effect an mremap move. >> + * Copy the vma structure to new location in the same vma >> + * prior to moving page table entries, to effect an mremap move; >> */ >> struct vm_area_struct *copy_vma(struct vm_area_struct **vmap, >> - unsigned long addr, unsigned long len, pgoff_t pgoff, >> - bool *need_rmap_locks) >> + struct mm_struct *mm, unsigned long addr, >> + unsigned long len, pgoff_t pgoff, >> + bool *need_rmap_locks, bool clear_flags_ctx) >> { >> struct vm_area_struct *vma = *vmap; >> unsigned long vma_start = vma->vm_start; >> - struct mm_struct *mm = vma->vm_mm; >> + struct vm_userfaultfd_ctx uctx; >> struct vm_area_struct *new_vma, *prev; >> struct rb_node **rb_link, *rb_parent; >> bool faulted_in_anon_vma = true; >> + unsigned long flags; >> >> /* >> * If anonymous vma has not yet been faulted, update new pgoff >> @@ -3220,15 +3222,25 @@ struct vm_area_struct *copy_vma(struct >> vm_area_struct **vmap, >> >> if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent)) >> return NULL; /* should never get here */ >> - new_vma = vma_merge(mm, prev, addr, addr + len, vma->vm_flags, >> - vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma), >> - vma->vm_userfaultfd_ctx); >> + >> + uctx = vma->vm_userfaultfd_ctx; >> + flags = vma->vm_flags; >> + if (clear_flags_ctx) { >> + uctx = NULL_VM_UFFD_CTX; >> + flags &= ~(VM_UFFD_MISSING | VM_UFFD_WP | VM_MERGEABLE | >> + VM_LOCKED | VM_LOCKONFAULT | VM_WIPEONFORK | >> + VM_DONTCOPY); >> + } > > Why is the new logic required? No justification given. Ditto. >> + >> + new_vma = vma_merge(mm, prev, addr, addr + len, flags, vma->anon_vma, >> + vma->vm_file, pgoff, vma_policy(vma), uctx); >> if (new_vma) { >> /* >> * Source vma may have been merged into new_vma >> */ >> if (unlikely(vma_start >= new_vma->vm_start && >> - vma_start < new_vma->vm_end)) { >> + vma_start < new_vma->vm_end) && >> + vma->vm_mm == mm) { > > How can vma_merge() succeed if vma->vm_mm != mm? We don't use vma as an argument of vma_merge(). We use vma as a source of vma->anon_vma, vma->vm_file and vma_policy(). We search some new_vma in mm with the same characteristics as vma has in vma->vm_mm. In case of success vma_merge() returns it for us. For example, it may success, when vma->vm_mm is mm_struct of forked process, while mm is mm_struct of its parent. [...] Kirill
