qemu-kvm/kvm-multifd-Copy-pages-before-compressing-them-with-zlib.patch

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5.0 KiB
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From 1d280070748b604c60a7be4d4c3c3a28e3964f37 Mon Sep 17 00:00:00 2001
From: Thomas Huth <thuth@redhat.com>
Date: Tue, 2 Aug 2022 10:11:21 +0200
Subject: [PATCH 31/32] multifd: Copy pages before compressing them with zlib
RH-Author: Thomas Huth <thuth@redhat.com>
RH-MergeRequest: 112: Fix postcopy migration on s390x
RH-Commit: [1/2] fd5a0221e22b4563bd1cb7f8a8b95f0bfe8f5fc9 (thuth/qemu-kvm-cs9)
RH-Bugzilla: 2099934
RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>
RH-Acked-by: Peter Xu <peterx@redhat.com>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2099934
zlib_send_prepare() compresses pages of a running VM. zlib does not
make any thread-safety guarantees with respect to changing deflate()
input concurrently with deflate() [1].
One can observe problems due to this with the IBM zEnterprise Data
Compression accelerator capable zlib [2]. When the hardware
acceleration is enabled, migration/multifd/tcp/plain/zlib test fails
intermittently [3] due to sliding window corruption. The accelerator's
architecture explicitly discourages concurrent accesses [4]:
Page 26-57, "Other Conditions":
As observed by this CPU, other CPUs, and channel
programs, references to the parameter block, first,
second, and third operands may be multiple-access
references, accesses to these storage locations are
not necessarily block-concurrent, and the sequence
of these accesses or references is undefined.
Mark Adler pointed out that vanilla zlib performs double fetches under
certain circumstances as well [5], therefore we need to copy data
before passing it to deflate().
[1] https://zlib.net/manual.html
[2] https://github.com/madler/zlib/pull/410
[3] https://lists.nongnu.org/archive/html/qemu-devel/2022-03/msg03988.html
[4] http://publibfp.dhe.ibm.com/epubs/pdf/a227832c.pdf
[5] https://lists.gnu.org/archive/html/qemu-devel/2022-07/msg00889.html
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20220705203559.2960949-1-iii@linux.ibm.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
(cherry picked from commit 007e179ef0e97eafda4c9ff2a9d665a1947c7c6d)
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
migration/multifd-zlib.c | 38 ++++++++++++++++++++++++++++++--------
1 file changed, 30 insertions(+), 8 deletions(-)
diff --git a/migration/multifd-zlib.c b/migration/multifd-zlib.c
index 3a7ae44485..18213a9513 100644
--- a/migration/multifd-zlib.c
+++ b/migration/multifd-zlib.c
@@ -27,6 +27,8 @@ struct zlib_data {
uint8_t *zbuff;
/* size of compressed buffer */
uint32_t zbuff_len;
+ /* uncompressed buffer of size qemu_target_page_size() */
+ uint8_t *buf;
};
/* Multifd zlib compression */
@@ -45,26 +47,38 @@ static int zlib_send_setup(MultiFDSendParams *p, Error **errp)
{
struct zlib_data *z = g_new0(struct zlib_data, 1);
z_stream *zs = &z->zs;
+ const char *err_msg;
zs->zalloc = Z_NULL;
zs->zfree = Z_NULL;
zs->opaque = Z_NULL;
if (deflateInit(zs, migrate_multifd_zlib_level()) != Z_OK) {
- g_free(z);
- error_setg(errp, "multifd %u: deflate init failed", p->id);
- return -1;
+ err_msg = "deflate init failed";
+ goto err_free_z;
}
/* This is the maxium size of the compressed buffer */
z->zbuff_len = compressBound(MULTIFD_PACKET_SIZE);
z->zbuff = g_try_malloc(z->zbuff_len);
if (!z->zbuff) {
- deflateEnd(&z->zs);
- g_free(z);
- error_setg(errp, "multifd %u: out of memory for zbuff", p->id);
- return -1;
+ err_msg = "out of memory for zbuff";
+ goto err_deflate_end;
+ }
+ z->buf = g_try_malloc(qemu_target_page_size());
+ if (!z->buf) {
+ err_msg = "out of memory for buf";
+ goto err_free_zbuff;
}
p->data = z;
return 0;
+
+err_free_zbuff:
+ g_free(z->zbuff);
+err_deflate_end:
+ deflateEnd(&z->zs);
+err_free_z:
+ g_free(z);
+ error_setg(errp, "multifd %u: %s", p->id, err_msg);
+ return -1;
}
/**
@@ -82,6 +96,8 @@ static void zlib_send_cleanup(MultiFDSendParams *p, Error **errp)
deflateEnd(&z->zs);
g_free(z->zbuff);
z->zbuff = NULL;
+ g_free(z->buf);
+ z->buf = NULL;
g_free(p->data);
p->data = NULL;
}
@@ -114,8 +130,14 @@ static int zlib_send_prepare(MultiFDSendParams *p, Error **errp)
flush = Z_SYNC_FLUSH;
}
+ /*
+ * Since the VM might be running, the page may be changing concurrently
+ * with compression. zlib does not guarantee that this is safe,
+ * therefore copy the page before calling deflate().
+ */
+ memcpy(z->buf, p->pages->block->host + p->normal[i], page_size);
zs->avail_in = page_size;
- zs->next_in = p->pages->block->host + p->normal[i];
+ zs->next_in = z->buf;
zs->avail_out = available;
zs->next_out = z->zbuff + out_size;
--
2.31.1