Re: [PATCH v5 1/2] mm: Allow the page cache to allocate large pages

From: Michal Hocko <mhocko@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: William Kucharski <william.kucharski@oracle.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Song Liu <songliubraving@fb.com>,
	Bob Kasten <robert.a.kasten@intel.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Chad Mynhier <chad.mynhier@oracle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Johannes Weiner <jweiner@fb.com>
Subject: Re: [PATCH v5 1/2] mm: Allow the page cache to allocate large pages
Date: Tue, 3 Sep 2019 14:19:52 +0200	[thread overview]
Message-ID: <20190903121952.GU14028@dhcp22.suse.cz> (raw)
In-Reply-To: <20190903121155.GD29434@bombadil.infradead.org>

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