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 v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()
Date: Tue, 27 Apr 2021 17:58:16 -0700 (PDT) [thread overview]
Message-ID: <alpine.LSU.2.11.2104271704110.7111@eggly.anvils> (raw)
In-Reply-To: <20210427225244.4326-7-axelrasmussen@google.com>
On Tue, 27 Apr 2021, Axel Rasmussen wrote:
> In a previous commit, we added the mcopy_atomic_install_pte() helper.
> This helper does the job of setting up PTEs for an existing page, to map
> it into a given VMA. It deals with both the anon and shmem cases, as
> well as the shared and private cases.
>
> In other words, shmem_mcopy_atomic_pte() duplicates a case it already
> handles. So, expose it, and let shmem_mcopy_atomic_pte() use it
> directly, to reduce code duplication.
>
> This requires that we refactor shmem_mcopy_atomic_pte() a bit:
>
> Instead of doing accounting (shmem_recalc_inode() et al) part-way
> through the PTE setup, do it afterward. This frees up
> mcopy_atomic_install_pte() from having to care about this accounting,
> and means we don't need to e.g. shmem_uncharge() in the error path.
>
> A side effect is this switches shmem_mcopy_atomic_pte() to use
> lru_cache_add_inactive_or_unevictable() instead of just lru_cache_add().
> This wrapper does some extra accounting in an exceptional case, if
> appropriate, so it's actually the more correct thing to use.
>
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Not quite. Two things.
One, in this version, delete_from_page_cache(page) has vanished
from the particular error path which needs it.
Two, and I think this predates your changes (so needs a separate
fix patch first, for backport to stable? a user with bad intentions
might be able to trigger the BUG), in pondering the new error paths
and that /* don't free the page */ one in particular, isn't it the
case that the shmem_inode_acct_block() on entry might succeed the
first time, but atomic copy fail so -ENOENT, then something else
fill up the tmpfs before the retry comes in, so that retry then
fail with -ENOMEM, and hit the BUG_ON(page) in __mcopy_atomic()?
(As I understand it, the shmem_inode_unacct_blocks() has to be
done before returning, because the caller may be unable to retry.)
What the right fix is rather depends on other uses of __mcopy_atomic():
if they obviously cannot hit that BUG_ON(page), you may prefer to leave
it in, and fix it here where shmem_inode_acct_block() fails. Or you may
prefer instead to delete that "else BUG_ON(page);" - looks as if that
would end up doing the right thing. Peter may have a preference.
(Or, we could consider doing the shmem_inode_acct_block() only after
the page has been copied in: its current placing reflects how shmem.c
does it elsewhere, and there's reason for that, but it doesn't always
work out right. Don't be surprised if I change the ordering in future,
but it's probably best not to mess with that ordering now.)
Sorry, if this is a pre-existing issue, then we are taking advantage
of you, in asking you to fix it: but I hope that while you're in there,
it will make sense to do so.
Thanks,
Hugh
> ---
> include/linux/userfaultfd_k.h | 5 ++++
> mm/shmem.c | 48 +++++------------------------------
> mm/userfaultfd.c | 17 +++++--------
> 3 files changed, 18 insertions(+), 52 deletions(-)
next prev parent reply other threads:[~2021-04-28 0:58 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 22:52 [PATCH v5 00/10] userfaultfd: add minor fault handling for shmem Axel Rasmussen
2021-04-27 22:52 ` [PATCH v5 01/10] userfaultfd/hugetlbfs: avoid including userfaultfd_k.h in hugetlb.h Axel Rasmussen
2021-04-27 22:52 ` [PATCH v5 02/10] userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte Axel Rasmussen
2021-04-27 22:52 ` [PATCH v5 03/10] userfaultfd/shmem: support minor fault registration for shmem Axel Rasmussen
2021-04-28 0:02 ` Hugh Dickins
2021-04-27 22:52 ` [PATCH v5 04/10] userfaultfd/shmem: support UFFDIO_CONTINUE " Axel Rasmussen
2021-04-28 0:03 ` Hugh Dickins
2021-04-28 15:10 ` Peter Xu
2021-04-27 22:52 ` [PATCH v5 05/10] userfaultfd/shmem: advertise shmem minor fault support Axel Rasmussen
2021-04-28 0:04 ` Hugh Dickins
2021-04-28 15:11 ` Peter Xu
2021-04-27 22:52 ` [PATCH v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte() Axel Rasmussen
2021-04-28 0:58 ` Hugh Dickins [this message]
2021-04-28 15:56 ` Peter Xu
2021-04-28 15:59 ` Axel Rasmussen
2021-04-28 16:23 ` Peter Xu
2021-04-27 22:52 ` [PATCH v5 07/10] userfaultfd/selftests: use memfd_create for shmem test type Axel Rasmussen
2021-04-27 22:52 ` [PATCH v5 08/10] userfaultfd/selftests: create alias mappings in the shmem test Axel Rasmussen
2021-04-27 22:52 ` [PATCH v5 09/10] userfaultfd/selftests: reinitialize test context in each test Axel Rasmussen
2021-04-28 17:23 ` Peter Xu
2021-05-18 20:57 ` Peter Xu
2021-05-18 22:28 ` Axel Rasmussen
2021-04-27 22:52 ` [PATCH v5 10/10] userfaultfd/selftests: exercise minor fault handling shmem support Axel Rasmussen
2021-04-28 17:26 ` Peter Xu
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