linux-kernel - Re: [PATCH] proc: pagemap: Hold mmap

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Message-Id: <20100401144329.BE42.A69D9226@jp.fujitsu.com>
Date:	Thu,  1 Apr 2010 14:54:43 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Matt Mackall <mpm@...enic.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	San Mehat <san@...gle.com>, linux-kernel@...r.kernel.org,
	Brian Swetland <swetland@...gle.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] proc: pagemap: Hold mmap_sem during page walk

> > So Matt, please actually address the _bug_ I pointed out rather than talk 
> > about other things. And yes, getting rid of the vma accesses sounds like 
> > it would fix it best. If that means that it doesn't work for hugepages, so 
> > be it.
> 
> That'd actually take us back to where it was when it hit mainline, which
> would make a lot of people unhappy. I wouldn't be one of them as there
> thankfully aren't any huge pages in my world. But I'm convinced
> put_user() must go. In which case, get_user_pages() stays, and I've got
> to switch things to direct physical page access into that array. 

no. direct physical page access for /proc is completely wrong idea, i think.
please imazine the caller process is multi threaded and it use fork case, 

example scenario)
1. the parent process has thread-A and thread-B.
2. thread-A call read_pagemap
3. read_pagemap grab the page-C
3. at the same time, thread-B call fork(), now page-C pointed from two process.
4. thread-B touch page-C, cow occur, then the parent process has cowed page (page-C')
   and the child process has original page-C.
5. thread-A write page-C by physical page access, then the child page is
   modified, instead parent one.

I just recommend simply do double buffering.

thanks.

> Even if I fix that, I believe San's original bug can still be triggered
> though, as all the new callers to find_vma are run outside of the
> target's mm_sem. Fixing that should be reasonably straight-forward.



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