On Tue 03-09-19 05:11:55, Matthew Wilcox wrote:
> On Tue, Sep 03, 2019 at 01:57:48PM +0200, Michal Hocko wrote:
> > On Mon 02-09-19 03:23:40, William Kucharski wrote:
> > > Add an 'order' argument to __page_cache_alloc() and
> > > do_read_cache_page(). Ensure the allocated pages are compound pages.
> >
> > Why do we need to touch all the existing callers and change them to use
> > order 0 when none is actually converted to a different order? This just
> > seem to add a lot of code churn without a good reason. If anything I
> > would simply add __page_cache_alloc_order and make __page_cache_alloc
> > call it with order 0 argument.
>
> Patch 2/2 uses a non-zero order.
It is a new caller and it can use a new function right?
> I agree it's a lot of churn without
> good reason; that's why I tried to add GFP_ORDER flags a few months ago.
> Unfortunately, you didn't like that approach either.
Is there any future plan that all/most __page_cache_alloc will get a
non-zero order argument?
> > Also is it so much to ask callers to provide __GFP_COMP explicitly?
>
> Yes, it's an unreasonable burden on the callers.
Care to exaplain why? __GFP_COMP tends to be used in the kernel quite
extensively.
> Those that pass 0 will
> have the test optimised away by the compiler (for the non-NUMA case).
> For the NUMA case, passing zero is going to be only a couple of extra
> instructions to not set the GFP_COMP flag.
>
> > > #ifdef CONFIG_NUMA
> > > -extern struct page *__page_cache_alloc(gfp_t gfp);
> > > +extern struct page *__page_cache_alloc(gfp_t gfp, unsigned int order);
> > > #else
> > > -static inline struct page *__page_cache_alloc(gfp_t gfp)
> > > +static inline struct page *__page_cache_alloc(gfp_t gfp, unsigned int order)
> > > {
> > > - return alloc_pages(gfp, 0);
> > > + if (order > 0)
> > > + gfp |= __GFP_COMP;
> > > + return alloc_pages(gfp, order);
> > > }
> > > #endif
--
Michal Hocko
SUSE Labs
