Date:   Tue, 14 Dec 2021 17:20:25 +0100
In-Reply-To: <20211214162050.660953-1-glider@google.com>
Message-Id: <20211214162050.660953-19-glider@google.com>
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References: <20211214162050.660953-1-glider@google.com>
Subject: [PATCH 18/43] kmsan: unpoison @tlb in arch_tlb_gather_mmu()
From:   Alexander Potapenko <glider@google.com>
To:     glider@google.com
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Christoph Hellwig <hch@lst.de>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ilya Leoshkevich <iii@linux.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Kees Cook <keescook@chromium.org>,
        Marco Elver <elver@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Pekka Enberg <penberg@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Vegard Nossum <vegard.nossum@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

This is a hack to reduce stackdepot pressure.

struct mmu_gather contains 7 1-bit fields packed into a 32-bit unsigned
int value. The remaining 25 bits remain uninitialized and are never used,
but KMSAN updates the origin for them in zap_pXX_range() in mm/memory.c,
thus creating very long origin chains. This is technically correct, but
consumes too much memory.

Unpoisoning the whole structure will prevent creating such chains.

Signed-off-by: Alexander Potapenko <glider@google.com>
---
Link: https://linux-review.googlesource.com/id/I76abee411b8323acfdbc29bc3a60dca8cff2de77
---
 mm/mmu_gather.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index 1b9837419bf9c..72e4c4ca01d27 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -1,6 +1,7 @@
 #include <linux/gfp.h>
 #include <linux/highmem.h>
 #include <linux/kernel.h>
+#include <linux/kmsan-checks.h>
 #include <linux/mmdebug.h>
 #include <linux/mm_types.h>
 #include <linux/pagemap.h>
@@ -252,6 +253,15 @@ void tlb_flush_mmu(struct mmu_gather *tlb)
 static void __tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
 			     bool fullmm)
 {
+	/*
+	 * struct mmu_gather contains 7 1-bit fields packed into a 32-bit
+	 * unsigned int value. The remaining 25 bits remain uninitialized
+	 * and are never used, but KMSAN updates the origin for them in
+	 * zap_pXX_range() in mm/memory.c, thus creating very long origin
+	 * chains. This is technically correct, but consumes too much memory.
+	 * Unpoisoning the whole structure will prevent creating such chains.
+	 */
+	kmsan_unpoison_memory(tlb, sizeof(*tlb));
 	tlb->mm = mm;
 	tlb->fullmm = fullmm;
 
-- 
2.34.1.173.g76aa8bc2d0-goog