Date:   Fri, 15 Dec 2017 15:04:29 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Oded Gabbay <oded.gabbay@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>,
        Joerg Roedel <joro@...tes.org>,
        Doug Ledford <dledford@...hat.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Sean Hefty <sean.hefty@...el.com>,
        Dimitri Sivanich <sivanich@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v2 1/2] mm, mmu_notifier: annotate mmu notifiers with
 blockable invalidate callbacks

On Thu, 14 Dec 2017 13:30:56 -0800 (PST) David Rientjes <rientjes@...gle.com> wrote:

> Commit 4d4bbd8526a8 ("mm, oom_reaper: skip mm structs with mmu notifiers")
> prevented the oom reaper from unmapping private anonymous memory with the
> oom reaper when the oom victim mm had mmu notifiers registered.
> 
> The rationale is that doing mmu_notifier_invalidate_range_{start,end}()
> around the unmap_page_range(), which is needed, can block and the oom
> killer will stall forever waiting for the victim to exit, which may not
> be possible without reaping.
> 
> That concern is real, but only true for mmu notifiers that have blockable
> invalidate_range_{start,end}() callbacks.  This patch adds a "flags" field
> to mmu notifier ops that can set a bit to indicate that these callbacks do
> not block.
> 
> The implementation is steered toward an expensive slowpath, such as after
> the oom reaper has grabbed mm->mmap_sem of a still alive oom victim.

some tweakage, please review.

From: Andrew Morton <akpm@...ux-foundation.org>
Subject: mm-mmu_notifier-annotate-mmu-notifiers-with-blockable-invalidate-callbacks-fix

make mm_has_blockable_invalidate_notifiers() return bool, use rwsem_is_locked()

Cc: Alex Deucher <alexander.deucher@....com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Christian König <christian.koenig@....com>
Cc: David Airlie <airlied@...ux.ie>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Dimitri Sivanich <sivanich@....com>
Cc: Doug Ledford <dledford@...hat.com>
Cc: Jani Nikula <jani.nikula@...ux.intel.com>
Cc: Jérôme Glisse <jglisse@...hat.com>
Cc: Joerg Roedel <joro@...tes.org>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Mike Marciniszyn <mike.marciniszyn@...el.com>
Cc: Oded Gabbay <oded.gabbay@...il.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: Sean Hefty <sean.hefty@...el.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 include/linux/mmu_notifier.h |    7 ++++---
 mm/mmu_notifier.c            |    8 ++++----
 2 files changed, 8 insertions(+), 7 deletions(-)

diff -puN include/linux/mmu_notifier.h~mm-mmu_notifier-annotate-mmu-notifiers-with-blockable-invalidate-callbacks-fix include/linux/mmu_notifier.h
--- a/include/linux/mmu_notifier.h~mm-mmu_notifier-annotate-mmu-notifiers-with-blockable-invalidate-callbacks-fix
+++ a/include/linux/mmu_notifier.h
@@ -2,6 +2,7 @@
 #ifndef _LINUX_MMU_NOTIFIER_H
 #define _LINUX_MMU_NOTIFIER_H
 
+#include <linux/types.h>
 #include <linux/list.h>
 #include <linux/spinlock.h>
 #include <linux/mm_types.h>
@@ -233,7 +234,7 @@ extern void __mmu_notifier_invalidate_ra
 				  bool only_end);
 extern void __mmu_notifier_invalidate_range(struct mm_struct *mm,
 				  unsigned long start, unsigned long end);
-extern int mm_has_blockable_invalidate_notifiers(struct mm_struct *mm);
+extern bool mm_has_blockable_invalidate_notifiers(struct mm_struct *mm);
 
 static inline void mmu_notifier_release(struct mm_struct *mm)
 {
@@ -473,9 +474,9 @@ static inline void mmu_notifier_invalida
 {
 }
 
-static inline int mm_has_blockable_invalidate_notifiers(struct mm_struct *mm)
+static inline bool mm_has_blockable_invalidate_notifiers(struct mm_struct *mm)
 {
-	return 0;
+	return false;
 }
 
 static inline void mmu_notifier_mm_init(struct mm_struct *mm)
diff -puN mm/mmu_notifier.c~mm-mmu_notifier-annotate-mmu-notifiers-with-blockable-invalidate-callbacks-fix mm/mmu_notifier.c
--- a/mm/mmu_notifier.c~mm-mmu_notifier-annotate-mmu-notifiers-with-blockable-invalidate-callbacks-fix
+++ a/mm/mmu_notifier.c
@@ -240,13 +240,13 @@ EXPORT_SYMBOL_GPL(__mmu_notifier_invalid
  * Must be called while holding mm->mmap_sem for either read or write.
  * The result is guaranteed to be valid until mm->mmap_sem is dropped.
  */
-int mm_has_blockable_invalidate_notifiers(struct mm_struct *mm)
+bool mm_has_blockable_invalidate_notifiers(struct mm_struct *mm)
 {
 	struct mmu_notifier *mn;
 	int id;
-	int ret = 0;
+	bool ret = false;
 
-	WARN_ON_ONCE(down_write_trylock(&mm->mmap_sem));
+	WARN_ON_ONCE(!rwsem_is_locked(&mm->mmap_sem));
 
 	if (!mm_has_notifiers(mm))
 		return ret;
@@ -259,7 +259,7 @@ int mm_has_blockable_invalidate_notifier
 				continue;
 
 		if (!(mn->ops->flags & MMU_INVALIDATE_DOES_NOT_BLOCK)) {
-			ret = 1;
+			ret = true;
 			break;
 		}
 	}
_

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