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SubjectRe: [patch 3/5] x86: finish fault error path with fatal signal
On Fri, Jul 19, 2013 at 12:25:02AM -0400, Johannes Weiner wrote:
> The x86 fault handler bails in the middle of error handling when the
> task has been killed. For the next patch this is a problem, because
> it relies on pagefault_out_of_memory() being called even when the task
> has been killed, to perform proper OOM state unwinding.
>
> This is a rather minor optimization, just remove it.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
> arch/x86/mm/fault.c | 11 -----------
> 1 file changed, 11 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 1cebabe..90248c9 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -846,17 +846,6 @@ static noinline int
> mm_fault_error(struct pt_regs *regs, unsigned long error_code,
> unsigned long address, unsigned int fault)
> {
> - /*
> - * Pagefault was interrupted by SIGKILL. We have no reason to
> - * continue pagefault.
> - */
> - if (fatal_signal_pending(current)) {
> - if (!(fault & VM_FAULT_RETRY))
> - up_read(&current->mm->mmap_sem);
> - if (!(error_code & PF_USER))
> - no_context(regs, error_code, address);
> - return 1;

This is broken but I only hit it now after testing for a while.

The patch has the right idea: in case of an OOM kill, we should
continue the fault and not abort. What I missed is that in case of a
kill during lock_page, i.e. VM_FAULT_RETRY && fatal_signal, we have to
exit the fault and not do up_read() etc. This introduced a locking
imbalance that would get everybody hung on mmap_sem.

I moved the retry handling outside of mm_fault_error() (come on...)
and stole some documentation from arm. It's now a little bit more
explicit and comparable to other architectures.

I'll send an updated series, patch for reference:

---
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: [patch] x86: finish fault error path with fatal signal

The x86 fault handler bails in the middle of error handling when the
task has been killed. For the next patch this is a problem, because
it relies on pagefault_out_of_memory() being called even when the task
has been killed, to perform proper OOM state unwinding.

This is a rather minor optimization that cuts short the fault handling
by a few instructions in rare cases. Just remove it.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
arch/x86/mm/fault.c | 33 +++++++++++++--------------------
1 file changed, 13 insertions(+), 20 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 6d77c38..0c18beb 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -842,31 +842,17 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
force_sig_info_fault(SIGBUS, code, address, tsk, fault);
}

-static noinline int
+static noinline void
mm_fault_error(struct pt_regs *regs, unsigned long error_code,
unsigned long address, unsigned int fault)
{
- /*
- * Pagefault was interrupted by SIGKILL. We have no reason to
- * continue pagefault.
- */
- if (fatal_signal_pending(current)) {
- if (!(fault & VM_FAULT_RETRY))
- up_read(&current->mm->mmap_sem);
- if (!(error_code & PF_USER))
- no_context(regs, error_code, address, 0, 0);
- return 1;
- }
- if (!(fault & VM_FAULT_ERROR))
- return 0;
-
if (fault & VM_FAULT_OOM) {
/* Kernel mode? Handle exceptions or die: */
if (!(error_code & PF_USER)) {
up_read(&current->mm->mmap_sem);
no_context(regs, error_code, address,
SIGSEGV, SEGV_MAPERR);
- return 1;
+ return;
}

up_read(&current->mm->mmap_sem);
@@ -884,7 +870,6 @@ mm_fault_error(struct pt_regs *regs, unsigned long error_code,
else
BUG();
}
- return 1;
}

static int spurious_fault_check(unsigned long error_code, pte_t *pte)
@@ -1189,9 +1174,17 @@ good_area:
*/
fault = handle_mm_fault(mm, vma, address, flags);

- if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
- if (mm_fault_error(regs, error_code, address, fault))
- return;
+ /*
+ * If we need to retry but a fatal signal is pending, handle the
+ * signal first. We do not need to release the mmap_sem because it
+ * would already be released in __lock_page_or_retry in mm/filemap.c.
+ */
+ if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+ return;
+
+ if (unlikely(fault & VM_FAULT_ERROR)) {
+ mm_fault_error(regs, error_code, address, fault);
+ return;
}

/*
--
1.8.3.2


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