Linus Torvalds wrote:
> On Fri, Jan 19, 2018 at 4:49 AM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >
> > + if (pfn < page_to_pfn(pvmw->page))
> > + return false;
> > +
> > + /* THP can be referenced by any subpage */
> > + if (pfn - page_to_pfn(pvmw->page) >= hpage_nr_pages(pvmw->page))
> > + return false;
> > +
>
> Is gcc actually clever enough to merge these? The "page_to_pfn()"
> logic can be pretty expensive (exactly for the sparsemem case, but
> per-node DISCOTIGMEM has some complexity too.
As far as I tested, using helper function made no difference. Unless I
explicitly insert barriers like cpu_relax() or smp_mb() between these,
the object side does not change.
>
> So I'd prefer to make that explicit, perhaps by having a helper
> function that does this something like
>
> static inline bool pfn_in_hpage(unsigned long pfn, struct page *hpage)
> {
> unsigned long hpage_pfn = page_to_pfn(hpage);
>
> return pfn >= hpage_pfn && pfn - hpage_pfn < hpage_nr_pages(hpage);
> }
>
> and then just use
>
> return pfn_in_hpage(pfn, pvmw->page);
>
> in that caller. Hmm? Wouldn't that be more legible, and avoid the
> repeated pvmw->page and page_to_pfn() cases?
>
> Even if maybe gcc can do the CSE and turn it all into the same thing
> in the end..
You can apply with
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
added.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
