linux-kernel - Re: Flushing whole page instead of work for ptrace

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Date:	Wed, 01 Dec 2010 18:10:12 +0100
From:	Michal Simek <michal.simek@...alogix.com>
To:	Roland McGrath <roland@...hat.com>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	John Williams <john.williams@...alogix.com>,
	"Edgar E. Iglesias" <edgar.iglesias@...il.com>
Subject: Re: Flushing whole page instead of work for ptrace

Roland McGrath wrote:
> This is a VM question more than a ptrace question.  
> I can't give you any authoritative answers about the VM issues.
> 
> Documentation/cachetlb.txt says:
> 
> 	Any time the kernel writes to a page cache page, _OR_
> 	the kernel is about to read from a page cache page and
> 	user space shared/writable mappings of this page potentially
> 	exist, this routine is called.
> 
> In your case, the kernel is only reading (write=0 passed to
> access_process_vm and get_user_pages).  In normal situations,
> the page in question will have only a private and read-only
> mapping in user space.  So the call should not be required in
> these cases--if the code can tell that's so.
> 
> Perhaps something like the following would be safe.
> But you really need some VM folks to tell you for sure.
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 02e48aa..2864ee7 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1484,7 +1484,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  				pages[i] = page;
>  
>  				flush_anon_page(vma, page, start);
> -				flush_dcache_page(page);
> +				if ((vm_flags & VM_WRITE) || (vma->vm_flags & VM_SHARED)
> +					flush_dcache_page(page);
>  			}
>  			if (vmas)
>  				vmas[i] = vma;
> 
> 
> Thanks,
> Roland

Andrew any comment?

Thanks,
Michal



-- 
Michal Simek, Ing. (M.Eng)
PetaLogix - Linux Solutions for a Reconfigurable World
w: www.petalogix.com p: +61-7-30090663,+42-0-721842854 f: +61-7-30090663
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