Re: [shmem] 7a7256d5f5: WARNING:possible_recursive_locking_detected

[Matthew Wilcox]

On Fri, Oct 21, 2022 at 01:30:41PM -0700, Andrew Morton wrote:
> On Fri, 21 Oct 2022 14:09:16 +0100 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> 
> > On Fri, Oct 21, 2022 at 12:10:17PM +0800, kernel test robot wrote:
> > > FYI, we noticed WARNING:possible_recursive_locking_detected due to commit (built with gcc-11):
> > > 
> > > commit: 7a7256d5f512b6c17957df7f59cf5e281b3ddba3 ("shmem: convert shmem_mfill_atomic_pte() to use a folio")
> > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > 
> > Ummm.  Looks to me like this now occurs because of this part of the
> > change:
> > 
> >                 if (!zeropage) {        /* COPY */
> > -                       page_kaddr = kmap_atomic(page);
> > +                       page_kaddr = kmap_local_folio(folio, 0);
> >                         ret = copy_from_user(page_kaddr,
> >                                              (const void __user *)src_addr,
> >                                              PAGE_SIZE);
> > -                       kunmap_atomic(page_kaddr);
> > +                       kunmap_local(page_kaddr);
> > 
> > Should I be using __copy_from_user_inatomic() here?
> 
> Caller __mcopy_atomic() is holding mmap_read_lock(dst_mm) and this
> copy_from_user() calls
> might_fault()->might_lock_read(current->mm->mmap_lock).
> 
> And I guess might_lock_read() gets upset because we're holding another
> mm's mmap_lock.  Which sounds OK to me, unless a concurrent
> mmap_write_lock() could jam things up.

Well, are we sure that dst_mm and current->mm are necessarily different?
If so, we could tell lockdep that.

> But I cannot see why your patch would suddenly trigger this warning -
> kmap_local_folio() and kmap_atomic() are basically the same thing.

Except for the important call in kmap_atomic_prot() to
pagefault_disable().  I mean, we could open-code that in the uffd code?