Re: [PATCH v8 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl

From: Peter Xu <peterx@redhat.com>
To: Axel Rasmussen <axelrasmussen@google.com>
Cc: "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>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	"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>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	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 v8 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl
Date: Thu, 25 Feb 2021 15:51:04 -0500	[thread overview]
Message-ID: <20210225205104.GA261488@xz-x1> (raw)
In-Reply-To: <20210225002658.2021807-5-axelrasmussen@google.com>

On Wed, Feb 24, 2021 at 04:26:56PM -0800, Axel Rasmussen wrote:
> This ioctl is how userspace ought to resolve "minor" userfaults. The
> idea is, userspace is notified that a minor fault has occurred. It might
> change the contents of the page using its second non-UFFD mapping, or
> not. Then, it calls UFFDIO_CONTINUE to tell the kernel "I have ensured
> the page contents are correct, carry on setting up the mapping".
> 
> Note that it doesn't make much sense to use UFFDIO_{COPY,ZEROPAGE} for
> MINOR registered VMAs. ZEROPAGE maps the VMA to the zero page; but in
> the minor fault case, we already have some pre-existing underlying page.
> Likewise, UFFDIO_COPY isn't useful if we have a second non-UFFD mapping.
> We'd just use memcpy() or similar instead.
> 
> It turns out hugetlb_mcopy_atomic_pte() already does very close to what
> we want, if an existing page is provided via `struct page **pagep`. We
> already special-case the behavior a bit for the UFFDIO_ZEROPAGE case, so
> just extend that design: add an enum for the three modes of operation,
> and make the small adjustments needed for the MCOPY_ATOMIC_CONTINUE
> case. (Basically, look up the existing page, and avoid adding the
> existing page to the page cache or calling set_page_huge_active() on
> it.)
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu