linux-kernel - Re: Re: [RFC] PAGE_RW Should be added to PAGE

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Date:	Fri, 15 Sep 2006 04:32:28 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Yingchao Zhou <yc_zhou@...c.ac.cn>
cc:	linux-kernel <linux-kernel@...r.kernel.org>, akpm <akpm@...l.org>,
	alan <alan@...hat.com>, zxc <zxc@...c.ac.cn>
Subject: Re: Re: [RFC] PAGE_RW Should be added to PAGE_COPY ?

On Fri, 15 Sep 2006, Yingchao Zhou wrote:
> >
> >You want to mmap MAP_SHARED, which will use PAGE_SHARED instead,
> >including the write bit, both before and after the mprotects.
> >There should be no problem then: do you actually see a problem
> >when MAP_SHARED is used?
> It's ok to mmap MAP_SHARED. But is it not a normal way to malloc() a space and
> then registered to NIC ?

Not that I know of.  How would one register malloc()ed space to a NIC?
Sorry, I may well be misunderstanding you.

> >>      Adding PAGE_RW to PAGE_COPY will resolve this problem.  
> >
> >No!  That would give every user write access to shared files they
> >should have no write access to.
> I guess you refer to mmap a file MAP_READ|MAP_WRITE in MAP_PRIVATE way.
> I think it is probably more logical to read the file data into an anoymous page and filled the pte with _PAGE_RW in the first time page-fault. It will probably reduce numbers of page fault interrupt.

do_no_page() does just that when its fault demands write access; but
doesn't waste memory and time on copying when it's only a read access.

Hugh
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