[CRIU] [PATCH 2/5] mm: map grow-down VMA-s with guard pages

[Andrey Wagin]

2013/8/28 Pavel Emelyanov <xemul at parallels.com>

> On 08/26/2013 05:15 PM, Andrey Vagin wrote:
> > In /proc/pid/maps grow-down VMA-s are shown without guard pages, but
> > sometime these "guard" pages can contain usefull data. For example if
> > a real guard page has been remmaped by another VMA. Let's call such
> > pages as fake guard pages.
> >
> > So when a grow-down VMA is mmaped on restore, it should be mapped with
> > one more guard page to restore content of the fake guard page.
> >
> > https://bugzilla.openvz.org/show_bug.cgi?id=2715
> > Signed-off-by: Andrey Vagin <avagin at openvz.org>
> > ---
> >  cr-restore.c | 60
> +++++++++++++++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 53 insertions(+), 7 deletions(-)
> >
> > diff --git a/cr-restore.c b/cr-restore.c
> > index 14bf300..8fea9ee 100644
> > --- a/cr-restore.c
> > +++ b/cr-restore.c
> > @@ -207,7 +207,7 @@ static int map_private_vma(pid_t pid, struct
> vma_area *vma, void *tgt_addr,
> >  {
> >       int ret;
> >       void *addr, *paddr = NULL;
> > -     unsigned long nr_pages;
> > +     unsigned long nr_pages, size;
> >       struct vma_area *p = *pvma;
> >
> >       if (vma_entry_is(&vma->vma, VMA_FILE_PRIVATE)) {
> > @@ -242,6 +242,17 @@ static int map_private_vma(pid_t pid, struct
> vma_area *vma, void *tgt_addr,
> >
> >       *pvma = p;
> >
> > +     /*
> > +      * A grow-down VMA has a guard page, which protect a VMA below it.
> > +      * So one more page is mapped here to restore content of the first
> page
> > +      */
> > +     if (vma->vma.flags & MAP_GROWSDOWN) {
> > +             vma->vma.start -= PAGE_SIZE;
> > +             if (paddr)
> > +                     paddr -= PAGE_SIZE;
> > +     }
> > +
> > +     size = vma_entry_len(&vma->vma);
> >       if (paddr == NULL) {
> >               /*
> >                * The respective memory area was NOT found in the parent.
> > @@ -250,7 +261,7 @@ static int map_private_vma(pid_t pid, struct
> vma_area *vma, void *tgt_addr,
> >               pr_info("Map 0x%016"PRIx64"-0x%016"PRIx64" 0x%016"PRIx64"
> vma\n",
> >                       vma->vma.start, vma->vma.end, vma->vma.pgoff);
> >
> > -             addr = mmap(tgt_addr, vma_entry_len(&vma->vma),
> > +             addr = mmap(tgt_addr, size,
> >                               vma->vma.prot | PROT_WRITE,
> >                               vma->vma.flags | MAP_FIXED,
> >                               vma->vma.fd, vma->vma.pgoff);
> > @@ -266,7 +277,7 @@ static int map_private_vma(pid_t pid, struct
> vma_area *vma, void *tgt_addr,
> >                */
> >               vma->ppage_bitmap = p->page_bitmap;
> >
> > -             addr = mremap(paddr, vma_area_len(vma), vma_area_len(vma),
> > +             addr = mremap(paddr, size, size,
> >                               MREMAP_FIXED | MREMAP_MAYMOVE, tgt_addr);
> >               if (addr != tgt_addr) {
> >                       pr_perror("Unable to remap a private vma");
> > @@ -279,10 +290,15 @@ static int map_private_vma(pid_t pid, struct
> vma_area *vma, void *tgt_addr,
> >       pr_debug("\tpremap 0x%016"PRIx64"-0x%016"PRIx64" -> %016lx\n",
> >               vma->vma.start, vma->vma.end, (unsigned long)addr);
> >
> > +     if (vma->vma.flags & MAP_GROWSDOWN) { /* Skip gurad page */
> > +             vma->vma.start += PAGE_SIZE;
> > +             vma_premmaped_start(&vma->vma) += PAGE_SIZE;
> > +     }
> > +
> >       if (vma_entry_is(&vma->vma, VMA_FILE_PRIVATE))
> >               close(vma->vma.fd);
> >
> > -     return 0;
> > +     return size;
> >  }
> >
> >  static int restore_priv_vma_content(pid_t pid)
> > @@ -475,8 +491,11 @@ static int prepare_mappings(int pid)
> >                       break;
> >               }
> >
> > -             if (vma_priv(&vma->vma))
> > +             if (vma_priv(&vma->vma)) {
> >                       rst_vmas.priv_size += vma_area_len(vma);
> > +                     if (vma->vma.flags & MAP_GROWSDOWN)
> > +                             rst_vmas.priv_size += PAGE_SIZE;
> > +             }
> >       }
> >       close(fd);
> >
> > @@ -512,10 +531,10 @@ static int prepare_mappings(int pid)
> >               if (ret < 0)
> >                       break;
> >
> > -             addr += vma_area_len(vma);
> > +             addr += ret;
> >       }
> >
> > -     if (ret == 0)
> > +     if (ret >= 0)
> >               ret = restore_priv_vma_content(pid);
> >
> >  out:
> > @@ -536,6 +555,31 @@ out:
> >       return ret;
> >  }
> >
> > +/*
> > + * A gard page must be unmapped after restoring content and
> > + * forking children to restore COW memory.
> > + */
> > +int unmap_guard_pages()
> > +{
> > +     struct vma_area *vma;
> > +
> > +     list_for_each_entry(vma, &rst_vmas.h, list) {
>
> Let's unmap them "in place", instead of one more vmas list scan.
>

We can't do that, because children are not forked in this moment.


>
> > +             if (!vma_priv(&vma->vma))
> > +                     continue;
> > +
> > +             if (vma->vma.flags & MAP_GROWSDOWN) {
> > +                     void *addr = (void *)
> vma_premmaped_start(&vma->vma);
> > +
> > +                     if (munmap(addr - PAGE_SIZE, PAGE_SIZE)) {
> > +                             pr_perror("Can't unmap guard page\n");
> > +                             return -1;
> > +                     }
> > +             }
> > +     }
> > +
> > +     return 0;
> > +}
> > +
> >  static int open_vmas(int pid)
> >  {
> >       struct vma_area *vma;
> > @@ -1184,6 +1228,8 @@ static int restore_task_with_children(void *_arg)
> >       if (create_children_and_session())
> >               exit(1);
> >
> > +     if (unmap_guard_pages())
> > +             exit(1);
> >       /*
> >        * Unlike sessions, process groups (a.k.a. pgids) can be joined
> >        * by any task, provided the task with pid == pgid (group leader)
> >
>
>
>
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