linux-kernel - Re: [rfc] no ZERO

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Date:	Wed, 4 Apr 2007 17:27:31 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Valdis.Kletnieks@...edu
cc:	Nick Piggin <npiggin@...e.de>, Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	tee@....com, holt@....com, Andrea Arcangeli <andrea@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] no ZERO_PAGE?



On Wed, 4 Apr 2007, Valdis.Kletnieks@...edu wrote:
> 
> I'd not be surprised if there's sparse-matrix code out there that wants to
> malloc a *huge* array (like a 1025x1025 array of numbers) that then only
> actually *writes* to several hundred locations, and relies on the fact that
> all the untouched pages read back all-zeros.

Good point. In fact, it doesn't need to be a malloc() - I remember people 
doing this with Fortran programs and just having an absolutely incredibly 
big BSS (with traditional Fortran, dymic memory allocations are just not 
done).

> Of course, said code is probably buggy because it doesn't zero the whole 
> thing because you don't usually know if some other function already 
> scribbled on that heap page.

Sure you do. If glibc used mmap() or brk(), it *knows* the new data is 
zero. So if you use calloc(), for example, it's entirely possible that 
a good libc wouldn't waste time zeroing it.

The same is true of BSS. You never clear the BSS with a memset, you just 
know it starts out zeroed.

		Linus
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