Re: [PATCH] multifd: Copy pages before compressing them with zlib

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Ilya Leoshkevich (iii@linux.ibm.com) wrote:
> 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().

Thanks for fixing that!

> [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://gitlab.com/qemu-project/qemu/-/issues/1099
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
> 
> v1: https://lists.gnu.org/archive/html/qemu-devel/2022-03/msg06841.html
> v1 -> v2: Rebase, mention Mark Adler's reply in the commit message.
> 
>  migration/multifd-zlib.c | 35 ++++++++++++++++++++++-------------
>  1 file changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/migration/multifd-zlib.c b/migration/multifd-zlib.c
> index 3a7ae44485..b6b22b7d1f 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 */
> +    uint8_t buf[];
>  };
>  
>  /* Multifd zlib compression */
> @@ -43,9 +45,18 @@ struct zlib_data {
>   */
>  static int zlib_send_setup(MultiFDSendParams *p, Error **errp)
>  {
> -    struct zlib_data *z = g_new0(struct zlib_data, 1);
> -    z_stream *zs = &z->zs;
> +    /* This is the maximum size of the compressed buffer */
> +    uint32_t zbuff_len = compressBound(MULTIFD_PACKET_SIZE);
> +    size_t buf_len = qemu_target_page_size();
> +    struct zlib_data *z;
> +    z_stream *zs;
>  
> +    z = g_try_malloc0(sizeof(struct zlib_data) + buf_len + zbuff_len);

So I think this works; but wouldn't life be easier if you just used
separate malloc's for the buffers?  You've got a lot of hairy pointer
maths below that would go away if they were separate.

Dave

> +    if (!z) {
> +        error_setg(errp, "multifd %u: out of memory for zlib_data", p->id);
> +        return -1;
> +    }
> +    zs = &z->zs;
>      zs->zalloc = Z_NULL;
>      zs->zfree = Z_NULL;
>      zs->opaque = Z_NULL;
> @@ -54,15 +65,8 @@ static int zlib_send_setup(MultiFDSendParams *p, Error **errp)
>          error_setg(errp, "multifd %u: deflate init failed", p->id);
>          return -1;
>      }
> -    /* 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;
> -    }
> +    z->zbuff_len = zbuff_len;
> +    z->zbuff = z->buf + buf_len;
>      p->data = z;
>      return 0;
>  }
> @@ -80,7 +84,6 @@ static void zlib_send_cleanup(MultiFDSendParams *p, Error **errp)
>      struct zlib_data *z = p->data;
>  
>      deflateEnd(&z->zs);
> -    g_free(z->zbuff);
>      z->zbuff = NULL;
>      g_free(p->data);
>      p->data = NULL;
> @@ -114,8 +117,14 @@ 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;
> -- 
> 2.35.3
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK