Message-ID: <22519869.post@talk.nabble.com>
Date: Sat, 14 Mar 2009 21:20:42 -0700 (PDT)
From: sidc7 <siddhartha.chhabra@gmail.com>
To: linux-kernel@vger.kernel.org
Subject: Re: COW optimization on exec
In-Reply-To: <20090315034134.GA9829@cmpxchg.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
References: <22519639.post@talk.nabble.com> <20090315034134.GA9829@cmpxchg.org>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2298
Lines: 61


Thanks for clearing this


Johannes Weiner-5 wrote:
> 
> On Sat, Mar 14, 2009 at 07:57:54PM -0700, sidc7 wrote:
>> 
>> The Linux kernel uses the COW optimization for fork, so the processes
>> share
>> the same pages, till on of the processes writes to the page. I was
>> wondering, if I do a fork and do an exec immediately following the fork,
>> will the COW optimization still be applied as it is most likely that the
>> new
>> process is going to write to the shared pages? So doing a COW will not
>> give
>> much benefit here, if it is done at all. Can anyone clarify if COW will
>> be
>> applied in such a case, for e.g. a command shell.
> 
> COWing the pages is not much extrawork, it's handled with this code:
> 
>         /*
>          * If it's a COW mapping, write protect it both
>          * in the parent and the child
>          */
>         if (is_cow_mapping(vm_flags)) {
>                 ptep_set_wrprotect(src_mm, addr, src_pte);
>                 pte = pte_wrprotect(pte);
>         }
> 
> you can find it in mm/memory.c::copy_one_pte().  The fault handler
> will then take care of it (it will notice that the pte is
> write-protected while the mapping itself allows writes) and then
> replace the page with a copy in the faulting process.
> 
> The real overhead is copying the page tables in the first place.  But
> the kernel can not know whether exec() is soon to be called, so fork()
> always must provide the copy-whole-address-space semantics.
> 
> If the forking process knows in advance the child will exec
> immediately, it can use vfork() which doesn't copy the address space.
> 
> 	Hannes
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 
> 

-- 
View this message in context: http://www.nabble.com/COW-optimization-on-exec-tp22519639p22519869.html
Sent from the linux-kernel mailing list archive at Nabble.com.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/