linux-kernel - Re: [PATCH 0/1] uprobes: Kill __replace_page(), change uprobe_write

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Message-ID: <CA+55aFyHLsC7HvTJpgWy4wmTEJedB=SOUmgYikX7OENHW3h4kQ@mail.gmail.com>
Date:	Tue, 10 Dec 2013 12:19:23 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Borislav Petkov <bp@...en8.de>,
	Hugh Dickins <hughd@...gle.com>,
	Ingo Molnar <mingo@...nel.org>, Jiri Kosina <jkosina@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/1] uprobes: Kill __replace_page(), change
 uprobe_write_opcode() to rely on gup(WRITE)

On Tue, Dec 10, 2013 at 12:16 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 12/10, Oleg Nesterov wrote:
>>
>> On 12/10, Linus Torvalds wrote:
>> >
>> > We'd be *much* better off using get_user_pages_fast() if possible -
>> > and I bet _is_ possible in 99% of all cases.
>>
>> We can't. get_user_pages_fast() takes mmap_sem.
>
> Hmm. I am stupid, there is __get_user_pages_fast(). OK, I'll try
> to think more.

There is indeed the non-locking version, however, since we want to get
the pte-pointer anyway, I really think you were right in complaining
about get_user_pages_fast() in the first place, and we'd be much
better off just doing the page table lookup ourselves. Which we have
to do later *anyway*.

The whole [__]get_user_pages[_fast]() family of functions are for
people who want the *page*. We want more than that. We really want the
page table entry, since we have to potentially change it.

           Linus
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