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

Peter Xu
2019-02-20T11:49:04Z

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