Re: [PATCH v3 1/7] mm: Remove BUG_ON mmap_sem not held from xxx_trans_huge_lock()

[Thomas HellstrÃm (VMware)]

Hi, Kirill,

On 10/3/19 1:02 PM, Kirill A. Shutemov wrote:
> On Wed, Oct 02, 2019 at 03:47:24PM +0200, Thomas HellstrÃm (VMware) wrote:
> > From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> >
> > The caller needs to make sure that the vma is not torn down during the
> > lock operation and can also use the i_mmap_rwsem for file-backed vmas.
> > Remove the BUG_ON. We could, as an alternative, add a test that either
> > vma->vm_mm->mmap_sem or vma->vm_file->f_mapping->i_mmap_rwsem are held.
> >
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> > Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> The patch looks good to me:
>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> But I looked at usage at pagewalk.c and it is inconsitent.  The walker
> takes ptl before calling ->pud_entry(), but not for ->pmd_entry().
>
> It should be fixed: do not take the lock before ->pud_entry(). The
> callback must take care of it.
>
> Looks like we have single ->pud_entry() implementation the whole kernel.
> It should be trivial to fix.
>
> Could you do this?
I could probably fix that. There are some comments in the patch 
introducing that code as to why it was done that way, though, but I 
don't remember offhand what the arguments were.

But there seems to be more races WRT puds. See my next email. Perhaps 
this should be fixed as part of a larger audit of the huge_pud code?

/Thomas