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From: Roland Dreier <>
To: Hugh Dickins <>
	Andrea Arcangeli <>,,
Subject: Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters
Date: Fri, 27 Jan 2012 09:28:03 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

> Sigh, what a mess ... it seems what we really want to do is know
> if userspace might trigger a COW because or not, and only do a
> preemptive COW in that case.  (We're not really concerned with
> userspace fork()ing and setting up a COW in the future, since that's
> what we have MADV_DONTFORK for)
> The status quo works for userspace anonymous mappings but
> it doesn't work for my case of mapping a kernel buffer read-only
> into userspace.  And fixing my case breaks the anonymous case.
> Do you see a way out of this dilemma?  Do we need to add yet
> another flag to get_user_pages()?

So thinking about this a bit more... it seems what we want is at least
to first order that we do the equivalent of write==1 exactly when the vma
for a mapping has VM_WRITE set (or is it VMA_MAYWRITE / force==1?
I don't quite understand the distinction between WRITE and MAYWRITE).

Right now, one call to get_user_pages() might involve more than one vma,
but we could simulate the above by doing find_vma() and making sure our
call to get_user_pages() goes one vma at a time.  Of course that would be
inefficient since get_user_pages() will redo the find_vma() internally, so it
would I guess make sense to add another FOLL_ flag to tell
get_user_pages() to do this?

Am I all wet, or am I becoming an MM hacker?


  reply	other threads:[~2012-01-27 17:28 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-26  5:59 [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters Roland Dreier
2012-01-26 20:01 ` Hugh Dickins
2012-01-26 22:45   ` Roland Dreier
2012-01-27 17:28     ` Roland Dreier [this message]
2012-01-28  2:31       ` Hugh Dickins
2012-01-28 19:25         ` Jason Gunthorpe
2012-01-30 19:19           ` Roland Dreier
2012-01-28  2:19     ` Hugh Dickins
2012-01-30 19:16       ` Roland Dreier
2012-01-30 20:20         ` Andrea Arcangeli
2012-02-06 17:46           ` Roland Dreier
2012-01-30 20:34         ` Hugh Dickins
2012-02-06 17:39           ` Roland Dreier
2012-02-07 20:39             ` Hugh Dickins
2012-02-08 23:10               ` Hugh Dickins
2012-02-09 17:50                 ` Roland Dreier
2012-02-09 22:57                   ` Hugh Dickins

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