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devmnt: avoid memory copies of I/O rpc buffer by using bwrite()

given that devmnt will almost always write into a pipe
or a network connection, which supports te bwrite routine,
we can avoid the memory copy that would have been done by
devbwrite(). this also means the i/o buffer for writes
will get freed sooner without having to wait for the 9p
rpc to get a response, saving memory.

theres one case where we have to keep the rpc arround and
that is when we write to a cached file, as we want to update
the cache with the data that was written, but the user buffer
cannot be trusted to stay the same during the rpc.

--- a/sys/src/9/port/devmnt.c

+++ b/sys/src/9/port/devmnt.c

@@ -26,8 +26,7 @@

 	Fcall 	reply;		/* Incoming reply */

 	Mnt*	m;		/* Mount device during rpc */

 	Rendez*	z;		/* Place to hang out */

-	uchar*	rpc;		/* I/O Data buffer */

-	uint	rpclen;		/* len of buffer */

+	Block*	w;		/* copy of write rpc for cache */

 	Block*	b;		/* reply blocks */

 	Mntrpc*	flushed;	/* message this one flushes */

 	char	done;		/* Rpc completed */

@@ -68,6 +67,8 @@

 static void	mountrpc(Mnt*, Mntrpc*);

 static int	rpcattn(void*);

+#define cachedchan(c) (((c)->flag & CCACHE) != 0 && (c)->mcp != nil)

+

 char	Esbadstat[] = "invalid directory entry received from server";

 char	Enoversion[] = "version not established for mount channel";

@@ -659,7 +660,7 @@

 	Chan *c;

 	c = r->c;

-	if((c->flag & CCACHE) == 0 || c->mcp == nil)

+	if(!cachedchan(c))

 		return;

 	off = r->request.offset;

 	switch(r->reply.type){

@@ -676,9 +677,13 @@

 		}

 		break;

 	case Rwrite:

-		if(convM2S(r->rpc, r->rpclen, &r->request) == 0)

+		b = r->w;

+		if(convM2S(b->rp, BLEN(b), &r->request) == 0)

 			panic("convM2S");

-		cwrite(c, (uchar*)r->request.data, r->request.count, off);

+		m = r->reply.count;

+		if(m > r->request.count)

+			m = r->request.count;

+		cwrite(c, (uchar*)r->request.data, m, off);

 		break;

 	}

 }

@@ -700,7 +705,7 @@

 		if(nreq > c->iounit)

 			nreq = c->iounit;

-		if(type == Tread && (c->flag&CCACHE) != 0) {

+		if(type == Tread && cachedchan(c)) {

 			nr = cread(c, (uchar*)uba, nreq, off);

 			if(nr > 0) {

 				nreq = nr;

@@ -992,6 +997,7 @@

 static void

 mountio(Mnt *m, Mntrpc *r)

 {

+	Block *b;

 	int n;

 	while(waserror()) {

@@ -1021,23 +1027,22 @@

 	/* Transmit a file system rpc */

 	n = sizeS2M(&r->request);

-	if(n > r->rpclen) {

-		free(r->rpc);

-		r->rpc = mallocz(((uint)n+127) & ~127, 0);

-		if(r->rpc == nil) {

-			r->rpclen = 0;

-			exhausted("mount rpc buffer");

-		}

-		r->rpclen = msize(r->rpc);

+	b = allocb(n);

+	if(waserror()){

+		freeb(b);

+		nexterror();

 	}

-	n = convS2M(&r->request, r->rpc, r->rpclen);

+	n = convS2M(&r->request, b->wp, n);

 	if(n <= 0 || n > m->msize) {

 		print("mountio: proc %s %lud: convS2M returned %d for tag %d fid %d T%d\n",

 			up->text, up->pid, n, r->request.tag, r->request.fid, r->request.type);

 		error(Emountrpc);

 	}

-	if(devtab[m->c->type]->write(m->c, r->rpc, n, 0) != n)

-		error(Emountrpc);

+	b->wp += n;

+	if(r->request.type == Twrite && cachedchan(r->c))

+		r->w = copyblock(b, n);

+	poperror();

+	devtab[m->c->type]->bwrite(m->c, b, 0);

 	/* Gate readers onto the mount point one at a time */

 	for(;;) {

@@ -1288,8 +1293,6 @@

 		new = malloc(sizeof(Mntrpc));

 		if(new == nil)

 			exhausted("mount rpc header");

-		new->rpc = nil;

-		new->rpclen = 0;

 		lock(&mntalloc);

 		new->request.tag = alloctag();

 	} else {

@@ -1308,6 +1311,7 @@

 	new->done = 0;

 	new->flushed = nil;

 	new->b = nil;

+	new->w = nil;

 	return new;

 }

@@ -1314,6 +1318,7 @@

 static void

 mntfree(Mntrpc *r)

 {

+	freeb(r->w);

 	freeblist(r->b);

 	lock(&mntalloc);

 	mntalloc.nrpcused--;

@@ -1326,7 +1331,6 @@

 	}

 	freetag(r->request.tag);

 	unlock(&mntalloc);

-	free(r->rpc);

 	free(r);

 }