Re: [PATCH v2 04/26] mm: allow VM_FAULT_RETRY for multiple times

[Peter Xu]

On Wed, Feb 13, 2019 at 11:34:44AM +0800, Peter Xu wrote:
> On Tue, Feb 12, 2019 at 10:56:10AM +0800, Peter Xu wrote:
> 
> [...]
> 
> > @@ -1351,7 +1351,7 @@ EXPORT_SYMBOL_GPL(__lock_page_killable);
> >  int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
> >  			 unsigned int flags)
> >  {
> > -	if (flags & FAULT_FLAG_ALLOW_RETRY) {
> > +	if (!flags & FAULT_FLAG_TRIED) {
> 
> Sorry, this should be:
> 
>         if (!(flags & FAULT_FLAG_TRIED))

Ok this is problematic too...  Because we for sure allow the page
fault flags to be both !ALLOW_RETRY and !TRIED (e.g., when doing GUP
and when __get_user_pages() is with locked==NULL and !FOLL_NOWAIT).
So current code will fall through the if condition and call
up_read(mmap_sem) even if above condition happened (while we shouldn't
because the GUP caller would assume the mmap_sem should be still
held).  So the correct check should be:

  if ((flags & FAULT_FLAG_ALLOW_RETRY) && !(flags & FAULT_FLAG_TRIED))

To make things easier, I'll just repost this single patch later.
Sorry for the noise.

Regards,

-- 
Peter Xu