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

From: John Hubbard <jhubbard@nvidia.com>
To: Jan Kara <jack@suse.cz>
Cc: Peter Xu <peterx@redhat.com>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>,
	"Kirill Shutemov" <kirill@shutemov.name>,
	Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Hugh Dickins <hughd@google.com>,
	Leon Romanovsky <leonro@nvidia.com>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()
Date: Tue, 22 Sep 2020 13:01:13 -0700	[thread overview]
Message-ID: <4a65586e-9282-beb0-1880-1ef8da03727c@nvidia.com> (raw)
In-Reply-To: <20200922103315.GD15112@quack2.suse.cz>

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