x86: finish user fault error path with fatal signal

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

Shortcutting the fault like this is a rather minor optimization that
saves a few instructions in rare cases.  Just remove it for
user-triggered faults.

Use the opportunity to split the fault retry handling from actual fault
errors and add locking documentation that reads suprisingly similar to
ARM's.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: azurIt <azurit@pobox.sk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 6d77c38..3aaeffc 100644 (file)
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -842,23 +842,15 @@ 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 (fatal_signal_pending(current) && !(error_code & PF_USER)) {
+               up_read(&current->mm->mmap_sem);
+               no_context(regs, error_code, address, 0, 0);
+               return;
        }
-       if (!(fault & VM_FAULT_ERROR))
-               return 0;
 
        if (fault & VM_FAULT_OOM) {
                /* Kernel mode? Handle exceptions or die: */
@@ -866,7 +858,7 @@ mm_fault_error(struct pt_regs *regs, unsigned long error_code,
                        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 +876,6 @@ 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 +1180,17 @@ 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 (unlikely((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)))
+               return;
+
+       if (unlikely(fault & VM_FAULT_ERROR)) {
+               mm_fault_error(regs, error_code, address, fault);
+               return;
        }
 
        /*