linux-kernel - [PATCH stable 4.14 1/1] mm: kvmalloc does not fallback to vmalloc for incompatible gfp flags

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Message-ID: <20230131043815.14989-2-risbhat@amazon.com>
Date:   Tue, 31 Jan 2023 04:38:15 +0000
From:   Rishabh Bhatnagar <risbhat@...zon.com>
To:     <stable@...r.kernel.org>, <gregkh@...uxfoundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <mhocko@...e.co>, Michal Hocko <mhocko@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Herbert <tom@...ntonium.net>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        Rishabh Bhatnagar <risbhat@...zon.com>
Subject: [PATCH stable 4.14 1/1] mm: kvmalloc does not fallback to vmalloc for incompatible gfp flags

From: Michal Hocko <mhocko@...e.com>

Commit ce91f6ee5b3bbbad8caff61b1c46d845c8db19bf upstream.

kvmalloc warned about incompatible gfp_mask to catch abusers (mostly
GFP_NOFS) with an intention that this will motivate authors of the code
to fix those.  Linus argues that this just motivates people to do even
more hacks like

	if (gfp == GFP_KERNEL)
		kvmalloc
	else
		kmalloc

I haven't seen this happening much (Linus pointed to bucket_lock special
cases an atomic allocation but my git foo hasn't found much more) but it
is true that we can grow those in future.  Therefore Linus suggested to
simply not fallback to vmalloc for incompatible gfp flags and rather
stick with the kmalloc path.

Link: http://lkml.kernel.org/r/20180601115329.27807-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@...e.com>
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Tom Herbert <tom@...ntonium.net>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Rishabh Bhatnagar <risbhat@...zon.com>
---
Changelog:
- Removed lib/bucket_locks.c diff as not present in 4.14

 mm/util.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/util.c b/mm/util.c
index 84e775f5216f..7d80c8119dce 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -400,7 +400,8 @@ EXPORT_SYMBOL(vm_mmap);
  * __GFP_RETRY_MAYFAIL is supported, and it should be used only if kmalloc is
  * preferable to the vmalloc fallback, due to visible performance drawbacks.
  *
- * Any use of gfp flags outside of GFP_KERNEL should be consulted with mm people.
+ * Please note that any use of gfp flags outside of GFP_KERNEL is careful to not
+ * fall back to vmalloc.
  */
 void *kvmalloc_node(size_t size, gfp_t flags, int node)
 {
@@ -411,7 +412,8 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
 	 * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
 	 * so the given set of flags has to be compatible.
 	 */
-	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
+	if ((flags & GFP_KERNEL) != GFP_KERNEL)
+		return kmalloc_node(size, flags, node);
 
 	/*
 	 * We want to attempt a large physically contiguous block first because
-- 
2.38.1