linux-kernel - Re: [PATCH v14 01/13] sk_run_filter: add BPF_S_ANC_SECCOMP_LD

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Message-ID: <1331704535.2456.37.camel@edumazet-laptop>
Date:	Tue, 13 Mar 2012 22:55:35 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Indan Zupancic <indan@....nu>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org,
	x86@...nel.org, arnd@...db.de, davem@...emloft.net, hpa@...or.com,
	mingo@...hat.com, oleg@...hat.com, peterz@...radead.org,
	rdunlap@...otime.net, mcgrathr@...omium.org, tglx@...utronix.de,
	luto@....edu, eparis@...hat.com, serge.hallyn@...onical.com,
	djm@...drot.org, scarybeasts@...il.com, pmoore@...hat.com,
	akpm@...ux-foundation.org, corbet@....net, markus@...omium.org,
	coreyb@...ux.vnet.ibm.com, keescook@...omium.org
Subject: Re: [PATCH v14 01/13] sk_run_filter: add BPF_S_ANC_SECCOMP_LD_W

Le mercredi 14 mars 2012 à 06:12 +0100, Indan Zupancic a écrit :

> Problem is that 'old_size' can be up to 32kB in size and it would be nice
> if that memory could be released. If it isn't, then using JIT increases
> memory usage, while also not accounting it to the socket.
> 

It is accounted for, since jit size is in relation with standard filter
size. Check sock_kmalloc()

Fact we can have a litle underestimation was already the case without
jit, since kmalloc() does a roundup to next power of two.

I dont think this discussion has anything to do with SECCOMP anyway.

These accounting dont need to be 100% precise, we only want a limit to
prevent rogue users from using all kernel memory.

> >
> > If it did, this kind of 'optimization' can actually be not good, because
> > sizeof(*fp) is small enough (less than half cache line size) to trigger
> > a possible false sharing issue. (other part of the cache line could be
> > used to hold a often dirtied object)
> 
> It could avoid this by allocating at least a cache size. But this is a
> problem for all small kmalloc's, isn't it?

Its a problem that was already met on several critical paths :

# find net|xargs grep -n L1_CACHE_BYTES
net/core/dev_addr_lists.c:51:	if (alloc_size < L1_CACHE_BYTES)
net/core/dev_addr_lists.c:52:		alloc_size = L1_CACHE_BYTES;
net/core/net-sysfs.c:586:	    RPS_MAP_SIZE(cpumask_weight(mask)), L1_CACHE_BYTES),
net/core/net-sysfs.c:1111:	    XPS_DEV_MAPS_SIZE, L1_CACHE_BYTES), GFP_KERNEL);
net/ipv6/ip6_fib.c:1612:	size = max_t(size_t, size, L1_CACHE_BYTES);
net/ipv4/fib_frontend.c:1049:	size = max_t(size_t, size, L1_CACHE_BYTES);



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