Restore fd optimization with a few minor tweaks, to quote tegge:

"fdinit() fails to initialize newfdp->fd_fd.fd_lastfile to -1.  This breaks
fdcopy() which will incorrectly set newfdp->fd_freefile to 1 if no files are
open and the last file descriptor marked as unused for fdp was 0.  This later
causes descriptor 0 to be unavailable in newfdp when the optimization is
enabled.

When the last file descriptor previously marked as used is nonzero and marked
as unused, fdunused() incorrectly sets fdp->fd_lastfile to fd - 1 due to
fd_last_used() returning (size - 1).  This hides the problem that breaks the
optimization."

This allows us to keep the optimization, while un-breaking it.

This is a RELENG_6 candidate.

PR:		kern/87208
MFC after:	1 week
Submitted by:	tegge

1 files changed, 5 insertions, 2 deletions

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 56ab05a..8482307 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -193,7 +193,7 @@ fd_last_used(struct filedesc *fdp, int low, int size)
 	for (minoff = NDSLOT(low); off >= minoff; --off)
 		if (map[off] != 0)
 			return (off * NDENTRIES + flsl(map[off]) - 1);
-	return (size - 1);
+	return (low - 1);
 }
 
 static int
@@ -1257,6 +1257,9 @@ fdalloc(struct thread *td, int minfd, int *result)
 
 	FILEDESC_LOCK_ASSERT(fdp, MA_OWNED);
 
+	if (fdp->fd_freefile > minfd)
+		minfd = fdp->fd_freefile;	   
+
 	PROC_LOCK(p);
 	maxfd = min((int)lim_cur(p, RLIMIT_NOFILE), maxfilesperproc);
 	PROC_UNLOCK(p);
@@ -1286,7 +1289,6 @@ fdalloc(struct thread *td, int minfd, int *result)
 	    ("free descriptor isn't"));
 	fdp->fd_ofileflags[fd] = 0; /* XXX needed? */
 	fdused(fdp, fd);
-	fdp->fd_freefile = fd_first_free(fdp, fd, fdp->fd_nfiles);
 	*result = fd;
 	return (0);
 }
@@ -1420,6 +1422,7 @@ fdinit(struct filedesc *fdp)
 	newfdp->fd_fd.fd_ofileflags = newfdp->fd_dfileflags;
 	newfdp->fd_fd.fd_nfiles = NDFILE;
 	newfdp->fd_fd.fd_map = newfdp->fd_dmap;
+	newfdp->fd_fd.fd_lastfile = -1;
 	return (&newfdp->fd_fd);
 }