lkml.org 
[lkml]   [2019]   [May]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH v2 2/7] mm: Extend copy_vma()
    From
    Date
    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 <ktkhai@virtuozzo.com>
    >> ---
    >> 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

    \
     
     \ /
      Last update: 2019-05-21 10:49    [W:5.316 / U:0.016 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site