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

From: Michal Hocko <mhocko@kernel.org>
To: William Kucharski <william.kucharski@oracle.com>
Cc: LKML <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>,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v5 1/2] mm: Allow the page cache to allocate large pages
Date: Wed, 4 Sep 2019 10:28:05 +0200	[thread overview]
Message-ID: <20190904082805.GJ3838@dhcp22.suse.cz> (raw)
In-Reply-To: <68E123A9-22A8-40ED-B2ED-897FC02D7D75@oracle.com>

On Tue 03-09-19 21:30:30, William Kucharski wrote:
> 
> 
> > On Sep 3, 2019, at 5:57 AM, Michal Hocko <mhocko@kernel.org> 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.
> 
> All the EXISTING code in patch [1/2] is changed to call it with an order
> of 0, as you would expect.
> 
> However, new code in part [2/2] of the patch calls it with an order of
> HPAGE_PMD_ORDER, as it seems cleaner to have those routines operate on
> a page, regardless of the order of the page desired.
> 
> I certainly can change this as you request, but once again the question
> is whether "page" should MEAN "page" regardless of the order desired,
> or whether the assumption will always be "page" means base PAGESIZE.
> 
> Either approach works, but what is the semantic we want going forward?

I do not have anything against handling page as compound, if that is the
question. All I was interested in whether adding a new helper to
_allocate_ the comound page wouldn't be easier than touching all
existing __page_cache_alloc users.
-- 
Michal Hocko
SUSE Labs