Re: [PATCH v2 13/15] mm: support GUP-triggered unsharing of anonymous pages

From: Jason Gunthorpe <jgg@nvidia.com>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	John Hubbard <jhubbard@nvidia.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>, Nadav Amit <namit@vmware.com>,
	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>,
	Jan Kara <jack@suse.cz>, Liang Zhang <zhangliang5@huawei.com>,
	Pedro Gomes <pedrodemargomes@gmail.com>,
	Oded Gabbay <oded.gabbay@gmail.com>,
	linux-mm@kvack.org
Subject: Re: [PATCH v2 13/15] mm: support GUP-triggered unsharing of anonymous pages
Date: Mon, 21 Mar 2022 13:18:23 -0300	[thread overview]
Message-ID: <20220321161823.GZ11336@nvidia.com> (raw)
In-Reply-To: <de543fbc-0051-120b-c91a-74cf4dd6156b@redhat.com>

On Mon, Mar 21, 2022 at 05:15:06PM +0100, David Hildenbrand wrote:
> On 19.03.22 00:30, Jason Gunthorpe wrote:
> > On Tue, Mar 15, 2022 at 11:47:39AM +0100, David Hildenbrand wrote:
> >> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
> >> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
> >> on the page table entry will end up replacing the mapped anonymous page
> >> due to COW, resulting in the GUP pin no longer being consistent with the
> >> page actually mapped into the page table.
> >>
> >> The possible ways to deal with this situation are:
> >>  (1) Ignore and pin -- what we do right now.
> >>  (2) Fail to pin -- which would be rather surprising to callers and
> >>      could break user space.
> >>  (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
> >>      pins.
> >>
> >> We want to implement 3) because it provides the clearest semantics and
> >> allows for checking in unpin_user_pages() and friends for possible BUGs:
> >> when trying to unpin a page that's no longer exclusive, clearly
> >> something went very wrong and might result in memory corruptions that
> >> might be hard to debug. So we better have a nice way to spot such
> >> issues.
> >>
> >> To implement 3), we need a way for GUP to trigger unsharing:
> >> FAULT_FLAG_UNSHARE. FAULT_FLAG_UNSHARE is only applicable to R/O mapped
> >> anonymous pages and resembles COW logic during a write fault. However, in
> >> contrast to a write fault, GUP-triggered unsharing will, for example, still
> >> maintain the write protection.
> > 
> > Given the way this series has developed you might want to call this
> > FAULT_FLAG_MAKE_ANON_EXCLUSIVE
> > 
> > Which strikes me as more directly connected to what it is trying to
> > do.
> 
> I thought about something similar along those lines, and I think it
> would apply even when extending that mechanism to anything !anon inside
> a MAP_PRIVATE mapping.
> 
> The whole
> 
> const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;

I think the extra words are worthwhile, share makes me think about
MAP_SHARED as we don't really use shared anywhere else FWICT..

Jason