From: Linus Torvalds <torvalds@linux-foundation.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>,
Paul Mackerras <paulus@ozlabs.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Christoph Hellwig <hch@infradead.org>,
"Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
Matthew Wilcox <willy@infradead.org>,
cluster-devel <cluster-devel@redhat.com>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
ocfs2-devel@oss.oracle.com, kvm-ppc@vger.kernel.org,
linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks
Date: Thu, 28 Oct 2021 15:32:23 -0700 [thread overview]
Message-ID: <CAHk-=wgzEKEYKRoR_abQRDO=R8xJX_FK+XC3gNhKfu=KLdxt3g@mail.gmail.com> (raw)
In-Reply-To: <YXsUNMWFpmT1eQcX@arm.com>
On Thu, Oct 28, 2021 at 2:21 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> They do look fairly similar but we should have the information in the
> fault handler to distinguish: not a page fault (pte permission or p*d
> translation), in_task(), user address, fixup handler. But I agree the
> logic looks fragile.
So thinking about this a bit more, I think I have a possible
suggestion for how to handle this..
The pointer color fault (or whatever some other architecture may do to
generate sub-page faults) is not only not recoverable in the sense
that we can't fix it up, it also ends up being a forced SIGSEGV (ie it
can't be blocked - it has to either be caught or cause the process to
be killed).
And the thing is, I think we could just make the rule be that kernel
code that has this kind of retry loop with fault_in_pages() would
force an EFAULT on a pending SIGSEGV.
IOW, the pending SIGSEGV could effectively be exactly that "thread flag".
And that means that fault_in_xyz() wouldn't need to worry about this
situation at all: the regular copy_from_user() (or whatever flavor it
is - to/from/iter/whatever) would take the fault. And if it's a
regular page fault,. it would act exactly like it does now, so no
changes.
If it's a sub-page fault, we'd just make the rule be that we send a
SIGSEGV even if the instruction in question has a user exception
fixup.
Then we just need to add the logic somewhere that does "if active
pending SIGSEGV, return -EFAULT".
Of course, that logic might be in fault_in_xyz(), but it migth also be
a separate function entirely.
So this does effectively end up being a thread flag, but it's also
slightly more than that - it's that a sub-page fault from kernel mode
has semantics that a regular page fault does not.
The whole "kernel access doesn't cause SIGSEGV, but returns -EFAULT
instead" has always been an odd and somewhat wrong-headed thing. Of
course it should cause a SIGSEGV, but that's not how Unix traditionall
worked. We would just say "color faults always raise a signal, even if
the color fault was triggered in a system call".
(And I didn't check: I say "SIGSEGV" above, but maybe the pointer
color faults are actually SIGBUS? Doesn't change the end result).
Linus
next prev parent reply other threads:[~2021-10-28 22:33 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-19 13:41 [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 01/17] iov_iter: Fix iov_iter_get_pages{,_alloc} page fault return value Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 02/17] powerpc/kvm: Fix kvm_use_magic_page Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 03/17] gup: Turn fault_in_pages_{readable,writeable} into fault_in_{readable,writeable} Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 04/17] iov_iter: Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 05/17] iov_iter: Introduce fault_in_iov_iter_writeable Andreas Gruenbacher
2021-10-20 16:25 ` Catalin Marinas
2021-10-19 13:41 ` [PATCH v8 06/17] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 07/17] gfs2: Clean up function may_grant Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 08/17] gfs2: Introduce flag for glock holder auto-demotion Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 09/17] gfs2: Move the inode glock locking to gfs2_file_buffered_write Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 10/17] gfs2: Eliminate ip->i_gh Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 11/17] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher
2021-10-19 13:41 ` [PATCH v8 12/17] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-10-19 13:42 ` [PATCH v8 13/17] iomap: Support partial direct I/O on user copy failures Andreas Gruenbacher
2021-10-19 13:42 ` [PATCH v8 14/17] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher
2021-10-19 15:51 ` Darrick J. Wong
2021-10-19 19:30 ` Andreas Gruenbacher
2021-10-20 1:57 ` Darrick J. Wong
2021-10-19 13:42 ` [PATCH v8 15/17] gup: Introduce FOLL_NOFAULT flag to disable page faults Andreas Gruenbacher
2021-10-19 13:42 ` [PATCH v8 16/17] iov_iter: Introduce nofault " Andreas Gruenbacher
2021-10-19 13:42 ` [PATCH v8 17/17] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
2021-10-19 15:40 ` [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks Linus Torvalds
2021-10-19 16:00 ` Bob Peterson
2021-10-20 16:36 ` Catalin Marinas
2021-10-20 20:11 ` Linus Torvalds
2021-10-20 22:44 ` Catalin Marinas
2021-10-21 6:19 ` Linus Torvalds
2021-10-22 18:06 ` Catalin Marinas
2021-10-22 19:22 ` Linus Torvalds
2021-10-25 19:00 ` Andreas Gruenbacher
2021-10-26 18:24 ` Catalin Marinas
2021-10-26 18:50 ` Linus Torvalds
2021-10-26 19:18 ` Linus Torvalds
2021-10-27 19:13 ` Catalin Marinas
2021-10-27 21:14 ` Linus Torvalds
2021-10-28 21:20 ` Catalin Marinas
2021-10-28 21:40 ` Catalin Marinas
2021-10-28 22:15 ` Andreas Grünbacher
2021-10-29 12:50 ` Catalin Marinas
2021-10-28 22:32 ` Linus Torvalds [this message]
2021-10-29 17:50 ` Catalin Marinas
2021-10-29 18:47 ` Linus Torvalds
2021-10-25 18:24 ` Andreas Gruenbacher
2021-10-26 5:12 ` Theodore Ts'o
2021-10-26 9:44 ` Andreas Gruenbacher
2021-10-27 21:21 ` Andreas Gruenbacher
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