Date: Fri, 17 Mar 2006 18:20:12 +0000 (GMT)
From: Hugh Dickins <hugh@veritas.com>
To: Linus Torvalds <torvalds@osdl.org>
cc: "Bryan O'Sullivan" <bos@pathscale.com>, Roland Dreier <rdreier@cisco.com>,
       Andrew Morton <akpm@osdl.org>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
       hch@infradead.org, linux-kernel@vger.kernel.org
Subject: Re: Remapping pages mapped to userspace (was: [PATCH 10 of 20] ipath
 - support for userspace apps using core driver)
In-Reply-To: <Pine.LNX.4.64.0603170921470.3618@g5.osdl.org>
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On Fri, 17 Mar 2006, Linus Torvalds wrote:
> 
> Anyway, zap_page_range() would do what you want, but it's not exported, 
> and I'm not convinced it's even something we want to export. You can only 
> zap a page range from within the context of the zappee, not from an 
> external module/driver.
> 
> [ Maybe it works if somebody else calls it, maybe it doesn't. I wouldn't 
>   bet on it, and more importantly, I can pretty much _guarantee_ that a 
>   driver will get the "struct mm_struct" reference counting wrong. ]

But vmtruncate or unmap_mapping_range is quite used to operating on
whatever mm's have mapped the file, so should be a safe route.  Except
here it's a device mapped VM_PFNMAP, so there might prove to be some
gotchas.  I'd be happy to make a nopage fault on VM_PFNMAP give SIGBUS,
for sensible behaviour after the "truncate" - but recent history warns
we're liable then to discover some app expecting otherwise ;)

Hugh
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