Re: [RFC v1 2/2] mm/gup/writeback: add callbacks for inaccessible pages

[John Hubbard]

On 2/29/20 2:49 AM, Claudio Imbrenda wrote:
> > ...
> > > > @@ -458,7 +457,6 @@ static struct page *follow_page_pte(struct
> > > > vm_area_struct *vma, }
> > > >   
> > > >   	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
> > > > -		int ret;
> > > >   		get_page(page);
> > > >   		pte_unmap_unlock(ptep, ptl);
> > > >   		lock_page(page);
> > > > @@ -475,6 +473,14 @@ static struct page *follow_page_pte(struct
> > > > vm_area_struct *vma, page = ERR_PTR(-ENOMEM);
> > > >   		goto out;
> > > >   	}
> > > > +	if (flags & FOLL_PIN) {
> >
> >
> > What about FOLL_GET? Unless your calling code has some sort of
> > BUG_ON(flags & FOLL_GET), I'm not sure it's a good idea to leave that
> > case unhandled.
>
> if I understood the semantics of FOLL_PIN correctly, then we don't need
> to make the page accessible for FOLL_GET. FOLL_PIN indicates intent to
> access the content of the page, whereas FOLL_GET is only for the struct
> page.
>
> if we are not touching the content of the page, there is no need to
> make it accessible


OK, I hope I'm not overlooking anything, but that sounds correct to me.


thanks,
--
John Hubbard
NVIDIA