linux-kernel - Re: [patch 7/8] fdmap v2

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Date:	Thu, 07 Jun 2007 22:05:12 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	Kyle Moffett <mrmacman_g4@....com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 7/8] fdmap v2 - implement sys_socket2

Davide Libenzi a écrit :
> On Thu, 7 Jun 2007, Eric Dumazet wrote:
> 
>> Davide, are you sure we want FIFO for non sequential allocations ?
>>
>> This tends to use all the fmap slots, and not very cache friendly
>> if an app does a lot of [open(),...,close()] things. We already got a 
>> perf drop because of RCUification of file freeing (FIFO mode instead 
>> of LIFO given by kmalloc()/kfree())
>>
>> If the idea behind this FIFO was security (ie not easy for an app to predict 
>> next glibc file handle), we/glibc might use yet another FD_SECUREMODE flag, 
>> wich ORed with O_NONSEQFD would ask to fdmap_newfd() to take the tail of 
>> fmap->slist, not head.
> 
> Uli, would it be OK to rely only on base randomization and use a LIFO 
> instead? We have base randomization, plus LIFO does not mean strictly 
> sequential like legacy allocator, just more compatc and cache friendly.
> 

I am afraid randomization wont really work if /sbin/init or /bin/bash for 
example uses one (or more) unseq fd :
The 'random base' will be propagated at fork()/exec() time ?





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