75f187aba5
Currently, the NBD device does not accept flush requests from the Linux block layer. If the NBD server opened the target with neither O_SYNC nor O_DSYNC, however, the device will be effectively backed by a writeback cache. Without issuing flushes properly, operation of the NBD device will not be safe against power losses. The NBD protocol has support for both a cache flush command and a FUA command flag; the server will also pass a flag to note its support for these features. This patch adds support for the cache flush command and flag. In the kernel, we receive the flags via the NBD_SET_FLAGS ioctl, and map NBD_FLAG_SEND_FLUSH to the argument of blk_queue_flush. When the flag is active the block layer will send REQ_FLUSH requests, which we translate to NBD_CMD_FLUSH commands. FUA support is not included in this patch because all free software servers implement it with a full fdatasync; thus it has no advantage over supporting flush only. Because I [Paolo] cannot really benchmark it in a realistic scenario, I cannot tell if it is a good idea or not. It is also not clear if it is valid for an NBD server to support FUA but not flush. The Linux block layer gives a warning for this combination, the NBD protocol documentation says nothing about it. The patch also fixes a small problem in the handling of flags: nbd->flags must be cleared at the end of NBD_DO_IT, but the driver was not doing that. The bug manifests itself as follows. Suppose you two different client/server pairs to start the NBD device. Suppose also that the first client supports NBD_SET_FLAGS, and the first server sends NBD_FLAG_SEND_FLUSH; the second pair instead does neither of these two things. Before this patch, the second invocation of NBD_DO_IT will use a stale value of nbd->flags, and the second server will issue an error every time it receives an NBD_CMD_FLUSH command. This bug is pre-existing, but it becomes much more important after this patch; flush failures make the device pretty much unusable, unlike Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Alex Bligh <alex@alex.org.uk> Acked-by: Paul Clements <Paul.Clements@steeleye.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
78 lines
2.4 KiB
C
78 lines
2.4 KiB
C
/*
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* 1999 Copyright (C) Pavel Machek, pavel@ucw.cz. This code is GPL.
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* 1999/11/04 Copyright (C) 1999 VMware, Inc. (Regis "HPReg" Duchesne)
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* Made nbd_end_request() use the io_request_lock
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* 2001 Copyright (C) Steven Whitehouse
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* New nbd_end_request() for compatibility with new linux block
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* layer code.
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* 2003/06/24 Louis D. Langholtz <ldl@aros.net>
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* Removed unneeded blksize_bits field from nbd_device struct.
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* Cleanup PARANOIA usage & code.
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* 2004/02/19 Paul Clements
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* Removed PARANOIA, plus various cleanup and comments
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*/
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#ifndef _UAPILINUX_NBD_H
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#define _UAPILINUX_NBD_H
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#include <linux/types.h>
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#define NBD_SET_SOCK _IO( 0xab, 0 )
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#define NBD_SET_BLKSIZE _IO( 0xab, 1 )
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#define NBD_SET_SIZE _IO( 0xab, 2 )
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#define NBD_DO_IT _IO( 0xab, 3 )
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#define NBD_CLEAR_SOCK _IO( 0xab, 4 )
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#define NBD_CLEAR_QUE _IO( 0xab, 5 )
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#define NBD_PRINT_DEBUG _IO( 0xab, 6 )
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#define NBD_SET_SIZE_BLOCKS _IO( 0xab, 7 )
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#define NBD_DISCONNECT _IO( 0xab, 8 )
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#define NBD_SET_TIMEOUT _IO( 0xab, 9 )
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#define NBD_SET_FLAGS _IO( 0xab, 10)
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enum {
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NBD_CMD_READ = 0,
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NBD_CMD_WRITE = 1,
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NBD_CMD_DISC = 2,
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NBD_CMD_FLUSH = 3,
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NBD_CMD_TRIM = 4
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};
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/* values for flags field */
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#define NBD_FLAG_HAS_FLAGS (1 << 0) /* nbd-server supports flags */
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#define NBD_FLAG_READ_ONLY (1 << 1) /* device is read-only */
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#define NBD_FLAG_SEND_FLUSH (1 << 2) /* can flush writeback cache */
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/* there is a gap here to match userspace */
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#define NBD_FLAG_SEND_TRIM (1 << 5) /* send trim/discard */
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#define nbd_cmd(req) ((req)->cmd[0])
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/* userspace doesn't need the nbd_device structure */
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/* These are sent over the network in the request/reply magic fields */
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#define NBD_REQUEST_MAGIC 0x25609513
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#define NBD_REPLY_MAGIC 0x67446698
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/* Do *not* use magics: 0x12560953 0x96744668. */
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/*
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* This is the packet used for communication between client and
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* server. All data are in network byte order.
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*/
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struct nbd_request {
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__be32 magic;
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__be32 type; /* == READ || == WRITE */
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char handle[8];
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__be64 from;
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__be32 len;
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} __attribute__((packed));
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/*
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* This is the reply packet that nbd-server sends back to the client after
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* it has completed an I/O request (or an error occurs).
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*/
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struct nbd_reply {
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__be32 magic;
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__be32 error; /* 0 = ok, else error */
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char handle[8]; /* handle you got from request */
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};
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#endif /* _UAPILINUX_NBD_H */
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