linux-kernel - Re: COW optimization on exec

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Date:	Sat, 14 Mar 2009 21:20:42 -0700 (PDT)
From:	sidc7 <siddhartha.chhabra@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: COW optimization on exec


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
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