linux-kernel - Re: [PATCH v3 1/3] mm: introduce fincore()

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Date:	Tue, 08 Jul 2014 12:42:58 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Johannes Weiner <hannes@...xchg.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	David Miller <davem@...emloft.net>,
	Andres Freund <andres@...quadrant.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Naoya Horiguchi <nao.horiguchi@...il.com>,
	Kees Cook <kees@...flux.net>
Subject: Re: [PATCH v3 1/3] mm: introduce fincore()

On 07/08/2014 12:03 PM, Naoya Horiguchi wrote:
>> > The biggest question for me, though, is whether we want to start
>> > designing these per-page interfaces to consider different page sizes, or
>> > whether we're going to just continue to pretend that the entire world is
>> > 4k pages.  Using FINCORE_BMAP on 1GB hugetlbfs files would be a bit
>> > silly, for instance.
> I didn't answer this question, sorry.
> 
> In my option, hugetlbfs pages should be handled as one hugepage (not as
> many 4kB pages) to avoid lots of meaningless data transfer, as you pointed
> out. And the current patch already works like that.

Just reading the code, I don't see any way that pc_shift gets passed
down in to the do_fincore() loop.  I don't see it getting reflected in
to 'nr' or 'nr_pages' in there, and I can't see how:

	jump = iter.index - fc->pgstart - nr;

can possibly be right since iter.index is being kept against the offset
in the userspace buffer (4k pages) and 'nr' and fc->pgstart are
essentially done in the huge page size.

If you had a 2-page 1GB-hpage_size() hugetlbfs file, you would only have
two pages in the radix tree, and only two iterations of
radix_tree_for_each_slot().  It would only set the first two bytes of a
256k BMAP buffer since only two pages were encountered in the radix tree.

Or am I reading your code wrong again?
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