RE: [hugepage] Fix unmap_and_free_vma backout path

Chen, Kenneth W
2006-11-13T17:38:38Z

Hugh Dickins wrote on Monday, November 13, 2006 9:00 AM
> On Sun, 12 Nov 2006, Chen, Kenneth W wrote:
> > David Gibson wrote on Sunday, November 12, 2006 10:23 PM
> > > >
> > > > Something like this? I haven't tested it yet. But looks plausible
> > > > because we already have if is_file_hugepages() in the generic path.
> > >
> > > Um.. if you're going to test pgoff here, you should also test the
> > > address.
> >
> > prepare_hugepage_range() should catch misaligned memory address, right?
> > What more does get_unmapped_area() need to test?
>
> David made that remark, I see now, because PowerPC alone omits to
> check address and length alignment in its prepare_hugepage_range:
> it should be checking those as ia64 and generic do.

Ah, I see. I only looked at the generic and ia64 version of
prepare_hugepage_range() and wasn't aware that powerpc doesn't check
address alignment in there :-P

> > > Oh, and that point is too late to catch MAP_FIXED mappings.
> >
> > I don't understand what you mean by that.
> > In do_mmap_pgoff(), very early in the code it tries to get an valid
> > virtual address:
> >
> > addr = get_unmapped_area(file, addr, len, pgoff, flags);
> > if (addr & ~PAGE_MASK)
> > return addr;
> >
> > We don't even have a vma at this point, there is no error to recover.
> > If get_unmapped_area() tests the validity of pgoff and return an error
> > code, the immediate two lines of code will catch that and everything
> > stops there. I don't see where the unmap gets called here. Did I
> > miss something?
>
> I agree with Ken on that. I agree with just about everything said by
> people so far. But I think the check looks nicer tucked away with the
> other alignment checks in prepare_hugepage_range: how about this version?

Looks nice with all the extra clean up.

> (Perhaps, in another mood, I've have chosen BUG_ONs instead of just
> deleting all the redundant tests - another deleted in the ppc case.)
>
>
> [PATCH] hugetlb: prepare_hugepage_range check offset too
>
> prepare_hugepage_range should check file offset alignment when it checks
> virtual address and length, to stop MAP_FIXED with a bad huge offset from
> unmapping before it fails further down. PowerPC should apply the same
> prepare_hugepage_range alignment checks as ia64 and all the others do.
>
> Then none of the alignment checks in hugetlbfs_file_mmap are required
> (nor is the check for too small a mapping); but even so, move up setting
> of VM_HUGETLB and add a comment to warn of what David Gibson discovered -
> if hugetlbfs_file_mmap fails before setting it, do_mmap_pgoff's unmap_region
> when unwinding from error will go the non-huge way, which may cause bad
> behaviour on architectures (powerpc and ia64) which segregate their huge
> mappings into a separate region of the address space.
>
> Signed-off-by: Hugh Dickins <hugh@veritas.com>
> ---
>
> arch/ia64/mm/hugetlbpage.c | 4 +++-
> arch/powerpc/mm/hugetlbpage.c | 8 ++++++--
> fs/hugetlbfs/inode.c | 21 ++++++++-------------
> include/linux/hugetlb.h | 10 +++++++---
> mm/mmap.c | 2 +-
> 5 files changed, 25 insertions(+), 20 deletions(-)