[Libguestfs] [libnbd PATCH] api: Speed up nbd_pread_structured when reading holes
Richard W.M. Jones
rjones at redhat.com
Sat May 28 17:18:27 UTC 2022
On Fri, May 27, 2022 at 05:25:43PM -0500, Eric Blake wrote:
> Commit c7fbc71d missed an optimization: when we guarantee that a pread
> buffer is pre-zeroed, we don't need to waste time memset()ing it when
> the server sends a hole. But the next commit e0953cb7 lets the user
> skip pre-zeroing, at which point the memset is required to avoid data
> corruption. Thankfully, we are not leaking bogus bytes when a server
> sends a hole, but we CAN speed up the case where we have pre-zeroed
> the buffer to avoid a second memset(). Because the user can change
> the state of pread_initialize on the fly (including while a pread is
> still in flight), we must track the state per-command as it was before
> we send the request to the server.
> ---
> lib/internal.h | 1 +
> generator/states-reply-structured.c | 5 +++--
> lib/rw.c | 18 +++++++++++-------
> 3 files changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/lib/internal.h b/lib/internal.h
> index 6aaced3..885cee1 100644
> --- a/lib/internal.h
> +++ b/lib/internal.h
> @@ -353,6 +353,7 @@ struct command {
> struct command_cb cb;
> enum state state; /* State to resume with on next POLLIN */
> bool data_seen; /* For read, true if at least one data chunk seen */
> + bool initialized; /* For read, true if getting a hole may skip memset */
> uint32_t error; /* Local errno value */
> };
>
> diff --git a/generator/states-reply-structured.c b/generator/states-reply-structured.c
> index 7001047..12c24f5 100644
> --- a/generator/states-reply-structured.c
> +++ b/generator/states-reply-structured.c
> @@ -1,5 +1,5 @@
> /* nbd client library in userspace: state machine
> - * Copyright (C) 2013-2019 Red Hat Inc.
> + * Copyright (C) 2013-2022 Red Hat Inc.
> *
> * This library is free software; you can redistribute it and/or
> * modify it under the terms of the GNU Lesser General Public
> @@ -436,7 +436,8 @@ STATE_MACHINE {
> * 0-length replies are broken. Still, it's easy enough to support
> * them as an extension, and this works even when length == 0.
> */
> - memset (cmd->data + offset, 0, length);
> + if (!cmd->initialized)
> + memset (cmd->data + offset, 0, length);
> if (CALLBACK_IS_NOT_NULL (cmd->cb.fn.chunk)) {
> int error = cmd->error;
>
> diff --git a/lib/rw.c b/lib/rw.c
> index c5d041d..f32481a 100644
> --- a/lib/rw.c
> +++ b/lib/rw.c
> @@ -244,14 +244,18 @@ nbd_internal_command_common (struct nbd_handle *h,
> if (cb)
> cmd->cb = *cb;
>
> - /* For NBD_CMD_READ, cmd->data was pre-zeroed in the prologue
> - * created by the generator. Thus, if a (non-compliant) server with
> - * structured replies fails to send back sufficient data to cover
> - * the whole buffer, we still behave as if it had sent zeroes for
> - * those portions, rather than leaking any uninitialized data, and
> - * without having to complicate our state machine to track which
> - * portions of the read buffer were actually populated.
> + /* For NBD_CMD_READ, cmd->data defaults to being pre-zeroed in the
> + * prologue created by the generator. Thus, if a (non-compliant)
> + * server with structured replies fails to send back sufficient data
> + * to cover the whole buffer, we still behave as if it had sent
> + * zeroes for those portions, rather than leaking any uninitialized
> + * data, and without having to complicate our state machine to track
> + * which portions of the read buffer were actually populated. But
> + * if the user opts in to disabling set_pread_initialize, then we
> + * need to memset zeroes as they are read (and the user gets their
> + * own garbage back in the case of a non-compliant server).
> */
> + cmd->initialized = h->pread_initialize;
>
> /* Add the command to the end of the queue. Kick the state machine
> * if there is no other command being processed, otherwise, it will
Reviewed-by: Richard W.M. Jones <rjones at redhat.com>
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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