linux-kernel - Re: [patch 1/2] mm: fincore()

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Date:	Fri, 15 Feb 2013 13:27:38 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Stewart Smith <stewart@...mingspork.com>, linux-mm@...ck.org,
	linux-arch@...r.kernel.org
Subject: Re: [patch 1/2] mm: fincore()

On Fri, 15 Feb 2013 01:34:50 -0500
Johannes Weiner <hannes@...xchg.org> wrote:

> + * The status is returned in a vector of bytes.  The least significant
> + * bit of each byte is 1 if the referenced page is in memory, otherwise
> + * it is zero.

Also, this is going to be dreadfully inefficient for some obvious cases.

We could address that by returning the info in some more efficient
representation.  That will be run-length encoded in some fashion.

The obvious way would be to populate an array of

struct page_status {
	u32 present:1;
	u32 count:31;
};

or whatever.

Another way would be to define the syscall so it returns "number of
pages present/absent starting at offset `start'".  In other words, one
call to fincore() will return a single `struct page_status'.  Userspace
can then walk through the file and generate the full picture, if needed.

This also gets inefficient in obvious cases, but it's not as obviously
bad?

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