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From: Hugh Dickins <hughd@google.com>
To: Axel Rasmussen <axelrasmussen@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	Jerome Glisse <jglisse@redhat.com>, Joe Perches <joe@perches.com>,
	Lokesh Gidra <lokeshgidra@google.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Peter Xu <peterx@redhat.com>, Shaohua Li <shli@fb.com>,
	Shuah Khan <shuah@kernel.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Wang Qing <wangqing@vivo.com>,
	linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-mm@kvack.org, Brian Geffon <bgeffon@google.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Mina Almasry <almasrymina@google.com>,
	Oliver Upton <oupton@google.com>
Subject: Re: [PATCH v4 03/10] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem
Date: Mon, 26 Apr 2021 19:19:58 -0700 (PDT)
Message-ID: <alpine.LSU.2.11.2104261906390.2998@eggly.anvils> (raw)
In-Reply-To: <20210420220804.486803-4-axelrasmussen@google.com>

On Tue, 20 Apr 2021, Axel Rasmussen wrote:

> With this change, userspace can resolve a minor fault within a
> shmem-backed area with a UFFDIO_CONTINUE ioctl. The semantics for this
> match those for hugetlbfs - we look up the existing page in the page
> cache, and install a PTE for it.
> 
> This commit introduces a new helper: mcopy_atomic_install_pte.
> 
> Why handle UFFDIO_CONTINUE for shmem in mm/userfaultfd.c, instead of in
> shmem.c? The existing userfault implementation only relies on shmem.c
> for VM_SHARED VMAs. However, minor fault handling / CONTINUE work just
> fine for !VM_SHARED VMAs as well. We'd prefer to handle CONTINUE for
> shmem in one place, regardless of shared/private (to reduce code
> duplication).
> 
> Why add a new mcopy_atomic_install_pte helper? A problem we have with
> continue is that shmem_mcopy_atomic_pte() and mcopy_atomic_pte() are
> *close* to what we want, but not exactly. We do want to setup the PTEs
> in a CONTINUE operation, but we don't want to e.g. allocate a new page,
> charge it (e.g. to the shmem inode), manipulate various flags, etc. Also
> we have the problem stated above: shmem_mcopy_atomic_pte() and
> mcopy_atomic_pte() both handle one-half of the problem (shared /
> private) continue cares about. So, introduce mcontinue_atomic_pte(), to
> handle all of the shmem continue cases. Introduce the helper so it
> doesn't duplicate code with mcopy_atomic_pte().
> 
> In a future commit, shmem_mcopy_atomic_pte() will also be modified to
> use this new helper. However, since this is a bigger refactor, it seems
> most clear to do it as a separate change.
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

If this "03/10" had been numbered 04/10, I would have said
Acked-by: Hugh Dickins <hughd@google.com>

But I find this new ordering incomprehensible - I'm surprised that it
even builds this way around (if it does): this patch is so much about
what has been enabled in "04/10" (references to UFFDIO_CONTINUE shmem
VMAs etc).

Does Peter still think this way round is better? If he does, then we
shall have to compromise by asking you just to squash the two together.

> ---
>  mm/userfaultfd.c | 172 ++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 127 insertions(+), 45 deletions(-)

  parent reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-20 22:07 [PATCH v4 00/10] userfaultfd: add minor fault handling " Axel Rasmussen
2021-04-20 22:07 ` [PATCH v4 01/10] userfaultfd/hugetlbfs: avoid including userfaultfd_k.h in hugetlb.h Axel Rasmussen
2021-04-27  2:05   ` Hugh Dickins
2021-04-20 22:07 ` [PATCH v4 02/10] userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte Axel Rasmussen
2021-04-20 22:07 ` [PATCH v4 03/10] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem Axel Rasmussen
2021-04-22 20:22   ` Axel Rasmussen
2021-04-22 21:18     ` Peter Xu
2021-04-22 22:05       ` Axel Rasmussen
2021-04-27  2:19   ` Hugh Dickins [this message]
2021-04-27 15:54     ` Peter Xu
2021-04-27 16:57       ` Axel Rasmussen
2021-04-27 18:03         ` Peter Xu
2021-04-27 20:29           ` Axel Rasmussen
2021-04-27 20:42             ` Peter Xu
2021-04-27 20:59               ` Hugh Dickins
2021-04-20 22:07 ` [PATCH v4 04/10] userfaultfd/shmem: support minor fault registration " Axel Rasmussen
2021-04-27  2:23   ` Hugh Dickins
2021-04-27 15:57     ` Peter Xu
2021-04-27 17:03       ` Axel Rasmussen
2021-04-20 22:07 ` [PATCH v4 05/10] userfaultfd/selftests: use memfd_create for shmem test type Axel Rasmussen
2021-04-20 22:08 ` [PATCH v4 06/10] userfaultfd/selftests: create alias mappings in the shmem test Axel Rasmussen
2021-04-20 22:08 ` [PATCH v4 07/10] userfaultfd/selftests: reinitialize test context in each test Axel Rasmussen
2021-04-20 22:08 ` [PATCH v4 08/10] userfaultfd/selftests: exercise minor fault handling shmem support Axel Rasmussen
2021-04-20 22:08 ` [PATCH v4 09/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte() Axel Rasmussen
2021-04-21  1:02   ` Peter Xu
2021-04-27  2:59     ` Hugh Dickins
2021-04-20 22:08 ` [PATCH v4 10/10] userfaultfd: update documentation to mention shmem minor faults Axel Rasmussen
2021-04-27  2:27   ` Hugh Dickins

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