Re: [PATCH] mm/gup.c: Handling ERR within unpin_user_pages()

[Souptick Joarder]

On Sun, Sep 13, 2020 at 8:25 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Sun, Sep 13, 2020 at 08:02:35PM +0530, Souptick Joarder wrote:
> > It is possible that a buggy caller of unpin_user_pages()
> > (specially in error handling path) may end up calling it with
> > npages < 0 which is unnecessary.
> > @@ -328,6 +328,9 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
> >  {
> >       unsigned long index;
> >
> > +     if (WARN_ON_ONCE(npages < 0))
> > +             return;
>
> But npages is unsigned long.  So it can't be less than zero.

Sorry, I missed it.

Then, it means if npages is assigned with -ERRNO by caller, unpin_user_pages()
may end up calling a big loop, which is unnecessary.