Re: [PATCH v5 05/10] userfaultfd: add minor fault registration mode

From: Peter Xu <peterx@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Axel Rasmussen" <axelrasmussen@google.com>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Alexey Dobriyan" <adobriyan@gmail.com>,
	"Andrea Arcangeli" <aarcange@redhat.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Anshuman Khandual" <anshuman.khandual@arm.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Chinwen Chang" <chinwen.chang@mediatek.com>,
	"Huang Ying" <ying.huang@intel.com>,
	"Ingo Molnar" <mingo@redhat.com>, "Jann Horn" <jannh@google.com>,
	"Jerome Glisse" <jglisse@redhat.com>,
	"Lokesh Gidra" <lokeshgidra@google.com>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Michal Koutný" <mkoutny@suse.com>,
	"Michel Lespinasse" <walken@google.com>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Mike Rapoport" <rppt@linux.vnet.ibm.com>,
	"Nicholas Piggin" <npiggin@gmail.com>, "Shaohua Li" <shli@fb.com>,
	"Shawn Anastasio" <shawn@anastas.io>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"Steven Price" <steven.price@arm.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, "Linux MM" <linux-mm@kvack.org>,
	"Adam Ruprecht" <ruprecht@google.com>,
	"Cannon Matthews" <cannonmatthews@google.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	"David Rientjes" <rientjes@google.com>,
	"Mina Almasry" <almasrymina@google.com>,
	"Oliver Upton" <oupton@google.com>
Subject: Re: [PATCH v5 05/10] userfaultfd: add minor fault registration mode
Date: Fri, 12 Feb 2021 17:44:05 -0500	[thread overview]
Message-ID: <20210212224405.GF3171@xz-x1> (raw)
In-Reply-To: <20210212222145.GB2858050@casper.infradead.org>

On Fri, Feb 12, 2021 at 10:21:45PM +0000, Matthew Wilcox wrote:
> On Thu, Feb 11, 2021 at 11:28:09AM -0800, Axel Rasmussen wrote:
> > Ah, I had added this just after VM_UFFD_WP, without noticing that this
> > would be sharing a bit with VM_LOCKED. That seems like not such a
> > great idea.
> > 
> > I don't see another unused bit, and I don't see some other obvious
> > candidate to share with. So, the solution that comes to mind is
> 
> it'd be even better if you didn't use the last unused bit for UFFD_WP.
> not sure how feasible that is, but you can see we're really short on
> bits here.

UFFD_WP is used now for anonymouse already.. And the support for hugetlbfs and
shmem is in rfc stage on the list.

Is it possible to use CONFIG_ARCH_USES_HIGH_VMA_FLAGS here?  So far uffd-wp is
only working for 64 bit x86 too due to enlarged pte space.  Maybe we can also
let minor mode to only support 64 bit hosts.

Thanks,

-- 
Peter Xu