From: Yuri Tikhonov <yur@emcraft.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: wd@denx.de, dzu@denx.de, linux-kernel@vger.kernel.org,
miltonm@bga.com, linuxppc-dev@ozlabs.org,
viro@zeniv.linux.org.uk, Geert.Uytterhoeven@sonycom.com,
yanok@emcraft.com
Subject: Re[2]: [PATCH][v2] fork_init: fix division by zero
Date: Fri, 12 Dec 2008 01:22:32 +0300 [thread overview]
Message-ID: <483237973.20081212012232@emcraft.com> (raw)
In-Reply-To: <20081211121635.ff58193f.akpm@linux-foundation.org>
=0D=0AHello Andrew,
On Thursday, December 11, 2008 you wrote:
[snip]
> The expression you've chosen here can be quite inacccurate, because
> ((PAGE_SIZE / (8 * THREAD_SIZE)) is a small number.=20
But why is it bad? We do multiplication to 'mempages', not division.=20
All the numbers in the multiplier are the power of 2, so both=20
expressions:
mempages * (PAGE_SIZE / (8 * THREAD_SIZE))
and
max_threads =3D (mempages * PAGE_SIZE) / (8 * THREAD_SIZE)
are finally equal.=20
> The way to preserve accuracy is
> max_threads =3D (mempages * PAGE_SIZE) / (8 * THREAD_SIZE);
> so how about avoiding the nasty ifdefs and doing
I'm OK with the approach below, but, leading resulting to the same,=20
this involves some overhead to the code where there was no this=20
overhead before this patch: e.g. your implementation is finally boils=20
down to ~5 times more processor instructions than there were before,
plus operations with stack for the 'm' variable.
On the other hand, my approach with nasty (I agree) ifdefs doesn't=20
lead to overheads to the code which does not need this: i.e. the most=20
common situation of small PAGE_SIZEs. Big PAGE_SIZE is the exception,=20
so I believe that the more common cases should not suffer because of=20
this.
> --- a/kernel/fork.c~fork_init-fix-division-by-zero
> +++ a/kernel/fork.c
> @@ -69,6 +69,7 @@
> #include <asm/mmu_context.h>
> #include <asm/cacheflush.h>
> #include <asm/tlbflush.h>
> +#include <asm/div64.h>
> =20
> /*
> * Protected counters by write_lock_irq(&tasklist_lock)
> @@ -185,10 +186,15 @@ void __init fork_init(unsigned long memp
> =20
> /*
> * The default maximum number of threads is set to a safe
> - * value: the thread structures can take up at most half
> - * of memory.
> + * value: the thread structures can take up at most
> + * (1/8) part of memory.
> */
> - max_threads =3D mempages / (8 * THREAD_SIZE / PAGE_SIZE);
> + {
> + /* max_threads =3D (mempages * PAGE_SIZE) / THREAD_SIZE /=
8; */
> + u64 m =3D mempages * PAGE_SIZE;
> + do_div(m, THREAD_SIZE * 8);
> + max_threads =3D m;
> + }
> =20
> /*
> * we need to allow at least 20 threads to boot a system
> _
> ?
> The code is also inaccurate because it assumes that <whatever allocator
is used for threads>> will pack the thread_structs into pages with best
> possible density, which isn't necessarily the case. Let's not worry
> about that.
> OT:
> max_threads is widly wrong anyway.
> - the caller passes in num_physpages, which includes highmem. And we
> can't allocate thread structs from highmem.
> - num_physpages includes kernel pages and other stuff which can never
> be allocated via the page allocator.
> A suitable fix would be to switch the caller to the strangely-named
> nr_free_buffer_pages().
> If you grep the tree for `num_physpages', you will find a splendid
> number of similar bugs. num_physpages should be unexported, burnt,
> deleted, etc. It's just an invitation to write buggy code.
Regards, Yuri
--
Yuri Tikhonov, Senior Software Engineer
Emcraft Systems, www.emcraft.com
next prev parent reply other threads:[~2008-12-11 22:22 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-12-10 16:50 [PATCH][v2] fork_init: fix division by zero Yuri Tikhonov
2008-12-11 20:16 ` Andrew Morton
2008-12-11 20:28 ` Al Viro
2008-12-11 20:43 ` Andrew Morton
2008-12-12 2:31 ` Nick Piggin
2008-12-12 2:47 ` Andrew Morton
2008-12-12 3:36 ` Nick Piggin
2008-12-11 22:22 ` Yuri Tikhonov [this message]
2008-12-11 22:26 ` Re[2]: " Andrew Morton
2008-12-12 0:48 ` Paul Mackerras
2008-12-12 1:07 ` Andrew Morton
2008-12-18 7:47 ` Yuri Tikhonov
2008-12-18 22:45 ` Andrew Morton
2008-12-19 5:49 ` Re[2]: " Yuri Tikhonov
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