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Date: Fri, 6 Sep 2019 11:02:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: Hugh Dickins <hughd@...gle.com>, Maya Gokhale <gokhale2@...l.gov>,
Jerome Glisse <jglisse@...hat.com>,
Pavel Emelyanov <xemul@...tuozzo.com>,
Johannes Weiner <hannes@...xchg.org>,
Martin Cracauer <cracauer@...s.org>,
Marty McFadden <mcfadden8@...l.gov>, Shaohua Li <shli@...com>,
Andrea Arcangeli <aarcange@...hat.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Denis Plotnikov <dplotnikov@...tuozzo.com>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Mel Gorman <mgorman@...e.de>,
"Kirill A . Shutemov" <kirill@...temov.name>,
"Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v2 3/7] mm: Introduce FAULT_FLAG_INTERRUPTIBLE
On 05.09.19 12:15, Peter Xu wrote:
> handle_userfaultfd() is currently the only one place in the kernel
> page fault procedures that can respond to non-fatal userspace signals.
> It was trying to detect such an allowance by checking against USER &
> KILLABLE flags, which was "un-official".
>
> In this patch, we introduced a new flag (FAULT_FLAG_INTERRUPTIBLE) to
> show that the fault handler allows the fault procedure to respond even
> to non-fatal signals. Meanwhile, add this new flag to the default
> fault flags so that all the page fault handlers can benefit from the
> new flag. With that, replacing the userfault check to this one.
>
> Since the line is getting even longer, clean up the fault flags a bit
> too to ease TTY users.
>
> Although we've got a new flag and applied it, we shouldn't have any
> functional change with this patch so far.
>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> fs/userfaultfd.c | 4 +---
> include/linux/mm.h | 39 ++++++++++++++++++++++++++++-----------
> 2 files changed, 29 insertions(+), 14 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index ccbdbd62f0d8..4a8ad2dc2b6f 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -462,9 +462,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> uwq.ctx = ctx;
> uwq.waken = false;
>
> - return_to_userland =
> - (vmf->flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
> - (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
> + return_to_userland = vmf->flags & FAULT_FLAG_INTERRUPTIBLE;
> blocking_state = return_to_userland ? TASK_INTERRUPTIBLE :
> TASK_KILLABLE;
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 57fb5c535f8e..53ec7abb8472 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -383,22 +383,38 @@ extern unsigned int kobjsize(const void *objp);
> */
> extern pgprot_t protection_map[16];
>
> -#define FAULT_FLAG_WRITE 0x01 /* Fault was a write access */
> -#define FAULT_FLAG_MKWRITE 0x02 /* Fault was mkwrite of existing pte */
> -#define FAULT_FLAG_ALLOW_RETRY 0x04 /* Retry fault if blocking */
> -#define FAULT_FLAG_RETRY_NOWAIT 0x08 /* Don't drop mmap_sem and wait when retrying */
> -#define FAULT_FLAG_KILLABLE 0x10 /* The fault task is in SIGKILL killable region */
> -#define FAULT_FLAG_TRIED 0x20 /* Second try */
> -#define FAULT_FLAG_USER 0x40 /* The fault originated in userspace */
> -#define FAULT_FLAG_REMOTE 0x80 /* faulting for non current tsk/mm */
> -#define FAULT_FLAG_INSTRUCTION 0x100 /* The fault was during an instruction fetch */
> +/**
> + * Fault flag definitions.
> + *
> + * @FAULT_FLAG_WRITE: Fault was a write fault.
> + * @FAULT_FLAG_MKWRITE: Fault was mkwrite of existing PTE.
> + * @FAULT_FLAG_ALLOW_RETRY: Allow to retry the fault if blocked.
> + * @FAULT_FLAG_RETRY_NOWAIT: Don't drop mmap_sem and wait when retrying.
> + * @FAULT_FLAG_KILLABLE: The fault task is in SIGKILL killable region.
> + * @FAULT_FLAG_TRIED: The fault has been tried once.
> + * @FAULT_FLAG_USER: The fault originated in userspace.
> + * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
> + * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
> + * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
> + */
> +#define FAULT_FLAG_WRITE 0x01
> +#define FAULT_FLAG_MKWRITE 0x02
> +#define FAULT_FLAG_ALLOW_RETRY 0x04
> +#define FAULT_FLAG_RETRY_NOWAIT 0x08
> +#define FAULT_FLAG_KILLABLE 0x10
> +#define FAULT_FLAG_TRIED 0x20
> +#define FAULT_FLAG_USER 0x40
> +#define FAULT_FLAG_REMOTE 0x80
> +#define FAULT_FLAG_INSTRUCTION 0x100
> +#define FAULT_FLAG_INTERRUPTIBLE 0x200
>
I'd probably split off the unrelated doc changes. Just a matter of taste.
> /*
> * The default fault flags that should be used by most of the
> * arch-specific page fault handlers.
> */
> #define FAULT_FLAG_DEFAULT (FAULT_FLAG_ALLOW_RETRY | \
> - FAULT_FLAG_KILLABLE)
> + FAULT_FLAG_KILLABLE | \
> + FAULT_FLAG_INTERRUPTIBLE)
So by default, all faults are marked interruptible, also
!FAULT_FLAG_USER. I assume the trick right now is that
handle_userfault() will indeed only be called on user faults and the
flag is used nowhere else ;)
Would it make sense to name it FAULT_FLAG_USER_INTERRUPTIBLE, to stress
that the flag only applies to user faults? (or am I missing something
and this could also apply to !user faults somewhen in the future?
(I am no expert on the fault paths yet, so sorry for the silly questions)
>
> #define FAULT_FLAG_TRACE \
> { FAULT_FLAG_WRITE, "WRITE" }, \
> @@ -409,7 +425,8 @@ extern pgprot_t protection_map[16];
> { FAULT_FLAG_TRIED, "TRIED" }, \
> { FAULT_FLAG_USER, "USER" }, \
> { FAULT_FLAG_REMOTE, "REMOTE" }, \
> - { FAULT_FLAG_INSTRUCTION, "INSTRUCTION" }
> + { FAULT_FLAG_INSTRUCTION, "INSTRUCTION" }, \
> + { FAULT_FLAG_INTERRUPTIBLE, "INTERRUPTIBLE" }
>
> /*
> * vm_fault is filled by the the pagefault handler and passed to the vma's
>
--
Thanks,
David / dhildenb
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