linux-kernel - Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4a65586e-9282-beb0-1880-1ef8da03727c@nvidia.com>
Date:   Tue, 22 Sep 2020 13:01:13 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
CC:     Peter Xu <peterx@...hat.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill Shutemov" <kirill@...temov.name>,
        Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Hugh Dickins <hughd@...gle.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when
 fork()

On 9/22/20 3:33 AM, Jan Kara wrote:
> On Mon 21-09-20 23:41:16, John Hubbard wrote:
>> On 9/21/20 2:20 PM, Peter Xu wrote:
>> ...
>>> +	if (unlikely(READ_ONCE(src_mm->has_pinned) &&
>>> +		     page_maybe_dma_pinned(src_page))) {
>>
>> This condition would make a good static inline function. It's used in 3
>> places, and the condition is quite special and worth documenting, and
>> having a separate function helps with that, because the function name
>> adds to the story. I'd suggest approximately:
>>
>>      page_likely_dma_pinned()
>>
>> for the name.
> 
> Well, but we should also capture that this really only works for anonymous
> pages. For file pages mm->has_pinned does not work because the page may be
> still pinned by completely unrelated process as Jann already properly
> pointed out earlier in the thread. So maybe anon_page_likely_pinned()?
> Possibly also assert PageAnon(page) in it if we want to be paranoid...
> 
> 								Honza

The file-backed case doesn't really change anything, though:
page_maybe_dma_pinned() is already a "fuzzy yes" in the same sense: you
can get a false positive. Just like here, with an mm->has_pinned that
could be a false positive for a process.

And for that reason, I'm also not sure an "assert PageAnon(page)" is
desirable. That assertion would prevent file-backed callers from being
able to call a function that provides a fuzzy answer, but I don't see
why you'd want or need to do that. The goal here is to make the fuzzy
answer a little bit more definite, but it's not "broken" just because
the result is still fuzzy, right?

Apologies if I'm missing a huge point here... :)


thanks,
-- 
John Hubbard
NVIDIA