Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

From: John Hubbard <jhubbard@nvidia.com>
To: Linus Torvalds <torvalds@linux-foundation.org>, Jan Kara <jack@suse.cz>
Cc: David Hildenbrand <david@redhat.com>,
	Jason Gunthorpe <jgg@nvidia.com>, Nadav Amit <namit@vmware.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	David Rientjes <rientjes@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Yang Shi <shy828301@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Matthew Wilcox <willy@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
	Michal Hocko <mhocko@kernel.org>, Rik van Riel <riel@surriel.com>,
	Roman Gushchin <guro@fb.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Peter Xu <peterx@redhat.com>, Donald Dutile <ddutile@redhat.com>,
	Christoph Hellwig <hch@lst.de>, Oleg Nesterov <oleg@redhat.com>,
	Linux-MM <linux-mm@kvack.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)
Date: Tue, 21 Dec 2021 13:16:23 -0800	[thread overview]
Message-ID: <3eb81a1d-a5fd-eb76-d23e-0f6998c647e9@nvidia.com> (raw)
In-Reply-To: <CAHk-=wiyxQ==vnHFHW99S_OPwA=u1Qrfg2OGr_6zPcBAuhQY2w@mail.gmail.com>

On 12/21/21 10:30, Linus Torvalds wrote:
> On Tue, Dec 21, 2021 at 10:07 AM Jan Kara <jack@suse.cz> wrote:
>>
>> For record we always intended (and still intend) to make O_DIRECT use
>> FOLL_PIN. Just it is tricky because some users mix pages pinned with GUP
>> and pages acquired through get_page() in a single bio (such as zero page)
>> and thus it is non-trivial to do the right thing on IO completion (unpin or
>> just put_page).
> 
> Side note: the new "exclusive VM" bit wouldn't _solve_ this issue, but
> it might make it much easier to debug and catch.
> 
> If we only set the exclusive VM bit on pages that get mapped into user
> space, and we guarantee that GUP only looks up such pages, then we can
> also add a debug test to the "unpin" case that the bit is still set.
> 
> And that would catch anybody who ends up using other pages for
> unpin(), and you could have a WARN_ON() for it (obviously also trigger
> on the page count being too small to unpin).
> 
> That way, at least from a kernel debugging and development standpoint
> it would make it easy to see "ok, this unpinning got a page that
> wasn't pinned", and it would help find these cases where some
> situation had used just a get_page() rather than a pin to get a page
> pointer.
> 
> No?
> 
>                    Linus

Yes, this is especially welcome, because it means that after enough time
sitting in the -mm tree, we can reasonably expect to catch the most important
cases, if any were missed. That makes it a whole other level of useful, as
compared to local testing hacks.

thanks,
-- 
John Hubbard
NVIDIA