From owner-svn-src-all@FreeBSD.ORG Thu Dec 20 20:18:28 2012 Return-Path: Delivered-To: svn-src-all@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 533D6C13; Thu, 20 Dec 2012 20:18:28 +0000 (UTC) (envelope-from des@FreeBSD.org) Received: from svn.freebsd.org (svn.freebsd.org [IPv6:2001:1900:2254:2068::e6a:0]) by mx1.freebsd.org (Postfix) with ESMTP id 373208FC0C; Thu, 20 Dec 2012 20:18:28 +0000 (UTC) Received: from svn.freebsd.org (localhost [127.0.0.1]) by svn.freebsd.org (8.14.5/8.14.5) with ESMTP id qBKKISEA015346; Thu, 20 Dec 2012 20:18:28 GMT (envelope-from des@svn.freebsd.org) Received: (from des@localhost) by svn.freebsd.org (8.14.5/8.14.5/Submit) id qBKKISw6015344; Thu, 20 Dec 2012 20:18:28 GMT (envelope-from des@svn.freebsd.org) Message-Id: <201212202018.qBKKISw6015344@svn.freebsd.org> From: Dag-Erling Smørgrav Date: Thu, 20 Dec 2012 20:18:28 +0000 (UTC) To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r244510 - head/sys/kern X-SVN-Group: head MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-BeenThere: svn-src-all@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: "SVN commit messages for the entire src tree \(except for " user" and " projects" \)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 20 Dec 2012 20:18:28 -0000 Author: des Date: Thu Dec 20 20:18:27 2012 New Revision: 244510 URL: http://svnweb.freebsd.org/changeset/base/244510 Log: Rewrite fdgrowtable() so common mortals can actually understand what it does and how, and add comments describing the data structures and explaining how they are managed. Modified: head/sys/kern/kern_descrip.c Modified: head/sys/kern/kern_descrip.c ============================================================================== --- head/sys/kern/kern_descrip.c Thu Dec 20 19:59:13 2012 (r244509) +++ head/sys/kern/kern_descrip.c Thu Dec 20 20:18:27 2012 (r244510) @@ -133,12 +133,26 @@ static int fill_socket_info(struct socke static int fill_vnode_info(struct vnode *vp, struct kinfo_file *kif); /* - * A process is initially started out with NDFILE descriptors stored within - * this structure, selected to be enough for typical applications based on - * the historical limit of 20 open files (and the usage of descriptors by - * shells). If these descriptors are exhausted, a larger descriptor table - * may be allocated, up to a process' resource limit; the internal arrays - * are then unused. + * Each process has: + * + * - An array of open file descriptors (fd_ofiles) + * - An array of file flags (fd_ofileflags) + * - A bitmap recording which descriptors are in use (fd_map) + * + * A process starts out with NDFILE descriptors. The value of NDFILE has + * been selected based the historical limit of 20 open files, and an + * assumption that the majority of processes, especially short-lived + * processes like shells, will never need more. + * + * If this initial allocation is exhausted, a larger descriptor table and + * map are allocated dynamically, and the pointers in the process's struct + * filedesc are updated to point to those. This is repeated every time + * the process runs out of file descriptors (provided it hasn't hit its + * resource limit). + * + * Since threads may hold references to individual descriptor table + * entries, the tables are never freed. Instead, they are placed on a + * linked list and freed only when the struct filedesc is released. */ #define NDFILE 20 #define NDSLOTSIZE sizeof(NDSLOTTYPE) @@ -148,34 +162,23 @@ static int fill_vnode_info(struct vnode #define NDSLOTS(x) (((x) + NDENTRIES - 1) / NDENTRIES) /* - * Storage required per open file descriptor. - */ -#define OFILESIZE (sizeof(struct file *) + sizeof(char)) - -/* - * Storage to hold unused ofiles that need to be reclaimed. + * SLIST entry used to keep track of ofiles which must be reclaimed when + * the process exits. */ struct freetable { - struct file **ft_table; + struct file **ft_table; SLIST_ENTRY(freetable) ft_next; }; /* - * Basic allocation of descriptors: - * one of the above, plus arrays for NDFILE descriptors. + * Initial allocation: a filedesc structure + the head of SLIST used to + * keep track of old ofiles + enough space for NDFILE descriptors. */ struct filedesc0 { - struct filedesc fd_fd; - /* - * ofiles which need to be reclaimed on free. - */ - SLIST_HEAD(,freetable) fd_free; - /* - * These arrays are used when the number of open files is - * <= NDFILE, and are then pointed to by the pointers above. - */ - struct file *fd_dfiles[NDFILE]; - char fd_dfileflags[NDFILE]; + struct filedesc fd_fd; + SLIST_HEAD(, freetable) fd_free; + struct file *fd_dfiles[NDFILE]; + char fd_dfileflags[NDFILE]; NDSLOTTYPE fd_dmap[NDSLOTS(NDFILE)]; }; @@ -1414,58 +1417,74 @@ static void fdgrowtable(struct filedesc *fdp, int nfd) { struct filedesc0 *fdp0; - struct freetable *fo; + struct freetable *ft; struct file **ntable; struct file **otable; - char *nfileflags; + char *nfileflags, *ofileflags; int nnfiles, onfiles; - NDSLOTTYPE *nmap; + NDSLOTTYPE *nmap, *omap; FILEDESC_XLOCK_ASSERT(fdp); KASSERT(fdp->fd_nfiles > 0, ("zero-length file table")); - /* compute the size of the new table */ + /* save old values */ onfiles = fdp->fd_nfiles; + otable = fdp->fd_ofiles; + ofileflags = fdp->fd_ofileflags; + omap = fdp->fd_map; + + /* compute the size of the new table */ nnfiles = NDSLOTS(nfd) * NDENTRIES; /* round up */ if (nnfiles <= onfiles) /* the table is already large enough */ return; - /* allocate a new table and (if required) new bitmaps */ - ntable = malloc((nnfiles * OFILESIZE) + sizeof(struct freetable), + /* + * Allocate a new table and map. We need enough space for a) the + * file entries themselves, b) the file flags, and c) the struct + * freetable we will use when we decommission the table and place + * it on the freelist. We place the struct freetable in the + * middle so we don't have to worry about padding. + */ + ntable = malloc(nnfiles * sizeof(*ntable) + + sizeof(struct freetable) + + nnfiles * sizeof(*nfileflags), + M_FILEDESC, M_ZERO | M_WAITOK); + nfileflags = (char *)&ntable[nnfiles] + sizeof(struct freetable); + nmap = malloc(NDSLOTS(nnfiles) * NDSLOTSIZE, M_FILEDESC, M_ZERO | M_WAITOK); - nfileflags = (char *)&ntable[nnfiles]; - if (NDSLOTS(nnfiles) > NDSLOTS(onfiles)) - nmap = malloc(NDSLOTS(nnfiles) * NDSLOTSIZE, - M_FILEDESC, M_ZERO | M_WAITOK); - else - nmap = NULL; - bcopy(fdp->fd_ofiles, ntable, onfiles * sizeof(*ntable)); - bcopy(fdp->fd_ofileflags, nfileflags, onfiles); - otable = fdp->fd_ofiles; - fdp->fd_ofileflags = nfileflags; + /* copy the old data over and point at the new tables */ + memcpy(ntable, otable, onfiles * sizeof(*otable)); + memcpy(nfileflags, ofileflags, onfiles * sizeof(*ofileflags)); + memcpy(nmap, omap, NDSLOTS(onfiles) * sizeof(*omap)); + + /* update the pointers and counters */ + fdp->fd_nfiles = nnfiles; fdp->fd_ofiles = ntable; + fdp->fd_ofileflags = nfileflags; + fdp->fd_map = nmap; + /* - * We must preserve ofiles until the process exits because we can't - * be certain that no threads have references to the old table via - * _fget(). + * Do not free the old file table, as some threads may still + * reference entries within it. Instead, place it on a freelist + * which will be processed when the struct filedesc is released. + * + * Do, however, free the old map. + * + * Note that if onfiles == NDFILE, we're dealing with the original + * static allocation contained within (struct filedesc0 *)fdp, + * which must not be freed. */ if (onfiles > NDFILE) { - fo = (struct freetable *)&otable[onfiles]; + ft = (struct freetable *)&otable[onfiles]; fdp0 = (struct filedesc0 *)fdp; - fo->ft_table = otable; - SLIST_INSERT_HEAD(&fdp0->fd_free, fo, ft_next); - } - if (NDSLOTS(nnfiles) > NDSLOTS(onfiles)) { - bcopy(fdp->fd_map, nmap, NDSLOTS(onfiles) * sizeof(*nmap)); - if (NDSLOTS(onfiles) > NDSLOTS(NDFILE)) - free(fdp->fd_map, M_FILEDESC); - fdp->fd_map = nmap; + ft->ft_table = otable; + SLIST_INSERT_HEAD(&fdp0->fd_free, ft, ft_next); + free(omap, M_FILEDESC); } - fdp->fd_nfiles = nnfiles; } /*