mm/memory-failure.c: support use of a dedicated thread to handle SIGBUS(BUS_MCEERR_AO)

Currently memory error handler handles action optional errors in the
deferred manner by default.  And if a recovery aware application wants
to handle it immediately, it can do it by setting PF_MCE_EARLY flag.
However, such signal can be sent only to the main thread, so it's
problematic if the application wants to have a dedicated thread to
handler such signals.

So this patch adds dedicated thread support to memory error handler.  We
have PF_MCE_EARLY flags for each thread separately, so with this patch
AO signal is sent to the thread with PF_MCE_EARLY flag set, not the main
thread.  If you want to implement a dedicated thread, you call prctl()
to set PF_MCE_EARLY on the thread.

Memory error handler collects processes to be killed, so this patch lets
it check PF_MCE_EARLY flag on each thread in the collecting routines.

No behavioral change for all non-early kill cases.

Tony said:

: The old behavior was crazy - someone with a multithreaded process might
: well expect that if they call prctl(PF_MCE_EARLY) in just one thread, then
: that thread would see the SIGBUS with si_code = BUS_MCEERR_A0 - even if
: that thread wasn't the main thread for the process.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Cc: Kamil Iskra <iskra@mcs.anl.gov>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Chen Gong <gong.chen@linux.jf.intel.com>
Cc: <stable@vger.kernel.org>	[3.2+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 43 insertions, 13 deletions

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index ed339c505d55..cd8989c1027e 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -380,15 +380,44 @@ static void kill_procs(struct list_head *to_kill, int forcekill, int trapno,
         }
 }
 
-static int task_early_kill(struct task_struct *tsk, int force_early)
+/*
+ * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO)
+ * on behalf of the thread group. Return task_struct of the (first found)
+ * dedicated thread if found, and return NULL otherwise.
+ *
+ * We already hold read_lock(&tasklist_lock) in the caller, so we don't
+ * have to call rcu_read_lock/unlock() in this function.
+ */
+static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
+{
+        struct task_struct *t;
+
+        for_each_thread(tsk, t)
+                if ((t->flags & PF_MCE_PROCESS) && (t->flags & PF_MCE_EARLY))
+                        return t;
+        return NULL;
+}
+
+/*
+ * Determine whether a given process is "early kill" process which expects
+ * to be signaled when some page under the process is hwpoisoned.
+ * Return task_struct of the dedicated thread (main thread unless explicitly
+ * specified) if the process is "early kill," and otherwise returns NULL.
+ */
+static struct task_struct *task_early_kill(struct task_struct *tsk,
+                                           int force_early)
 {
+        struct task_struct *t;
         if (!tsk->mm)
-                return 0;
+                return NULL;
         if (force_early)
-                return 1;
-        if (tsk->flags & PF_MCE_PROCESS)
-                return !!(tsk->flags & PF_MCE_EARLY);
-        return sysctl_memory_failure_early_kill;
+                return tsk;
+        t = find_early_kill_thread(tsk);
+        if (t)
+                return t;
+        if (sysctl_memory_failure_early_kill)
+                return tsk;
+        return NULL;
 }
 
 /*
@@ -410,16 +439,17 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill,
         read_lock(&tasklist_lock);
         for_each_process (tsk) {
                 struct anon_vma_chain *vmac;
+                struct task_struct *t = task_early_kill(tsk, force_early);
 
-                if (!task_early_kill(tsk, force_early))
+                if (!t)
                         continue;
                 anon_vma_interval_tree_foreach(vmac, &av->rb_root,
                                                pgoff, pgoff) {
                         vma = vmac->vma;
                         if (!page_mapped_in_vma(page, vma))
                                 continue;
-                        if (vma->vm_mm == tsk->mm)
-                                add_to_kill(tsk, page, vma, to_kill, tkc);
+                        if (vma->vm_mm == t->mm)
+                                add_to_kill(t, page, vma, to_kill, tkc);
                 }
         }
         read_unlock(&tasklist_lock);
@@ -440,10 +470,10 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill,
         read_lock(&tasklist_lock);
         for_each_process(tsk) {
                 pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+                struct task_struct *t = task_early_kill(tsk, force_early);
 
-                if (!task_early_kill(tsk, force_early))
+                if (!t)
                         continue;
-
                 vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff,
                                       pgoff) {
                         /*
@@ -453,8 +483,8 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill,
                          * Assume applications who requested early kill want
                          * to be informed of all such data corruptions.
                          */
-                        if (vma->vm_mm == tsk->mm)
-                                add_to_kill(tsk, page, vma, to_kill, tkc);
+                        if (vma->vm_mm == t->mm)
+                                add_to_kill(t, page, vma, to_kill, tkc);
                 }
         }
         read_unlock(&tasklist_lock);