fs: inode and open-file tables that grow, with a hash for iget

The inode table was NINODE (200) entries, searched end to end on every
iget(), and a panic -- "iget: no inodes" -- when all of them were held.
Held means open, a cwd, a running binary or a directory mid-walk, so a
few busy processes reached it. iget() now finds an inode through a hash
on (dev, inum), takes a free one from a list, and makes more from a
slab cache once the NINODE in the image are all in use.

The open-file table had the same shape, NFILE (512) and then ENFILE,
and gets the same treatment: a free list, and the slab past NFILE.
/proc/fsstat's peaks now read against how many exist.

getrlimit(RLIMIT_NOFILE) said unlimited, which musl turns into
sysconf(_SC_OPEN_MAX) == -1, after which zsh believed it had no
descriptors and `exec {fd}<file` failed on the first one. It reports
NOFILE now. And a comment in sys_fs.c held a "*/" that ended it early.

Checked under QEMU: six processes holding 115 files each, 713 open
descriptions and 341 inodes at once, all back to the baseline after.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/file.h

@@ -21,6 +21,7 @@ struct file {
   // invite somebody to keep one.
   int memfd_seg;
   uint64 off;
+  struct file *fnext;   // file.c's free list, while ref is 0
 };
 
 // ---------------------------------------------------------------------------
@@ -105,6 +106,8 @@ struct inode {
   struct sleeplock lock;
   int valid;
   const struct fs_ops *ops;   // chosen by iget from dev; never null once held
+  struct inode *hnext;        // fs.c: hash chain while held, free list after
+  struct inode *anext;        // fs.c: every inode there is, never reordered
 
   short type;
   short major;
@@ -191,6 +194,10 @@ int sysfs_mount(const char *target);
 int iheld(uint dev, uint64 inum);
 // 1 if any inode on dev but `except` is referenced (0: no exception). In fs.c.
 int ibusy(uint dev, uint64 except);
+// How many in-memory inodes exist; the table grows past NINODE. In fs.c.
+uint inode_slots(void);
+// How many open file descriptions exist; the table grows past NFILE. file.c.
+uint file_slots(void);
 // Path lookup that follows a final symlink too; 0 with *err set on failure.
 // `start` 0 means the cwd. In fs.c.
 struct inode *nameifollow(struct inode *start, char *path, int *err);

kernel/dev/procfs.c

@@ -295,10 +295,10 @@ gen_fsstat(struct sbuf *sb)
   // leak, and which line it is on says where to look.
   sb_puts(sb, "\ninode_used       "); sb_putu(sb, fsstat.inode_used);
   sb_puts(sb, "\ninode_peak       "); sb_putu(sb, fsstat.inode_peak);
-  sb_puts(sb, " / "); sb_putu(sb, NINODE);
+  sb_puts(sb, " / "); sb_putu(sb, inode_slots());
   sb_puts(sb, "\nfile_used        "); sb_putu(sb, fsstat.file_used);
   sb_puts(sb, "\nfile_peak        "); sb_putu(sb, fsstat.file_peak);
-  sb_puts(sb, " / "); sb_putu(sb, NFILE);
+  sb_puts(sb, " / "); sb_putu(sb, file_slots());
   sb_puts(sb, "\nbuf_pinned       "); sb_putu(sb, fsstat.buf_pinned);
   {
     uint nb, maxb;

kernel/fs/file.c

@@ -9,18 +9,38 @@
 #include "fsstat.h"
 #include "posix.h"
 #include "errno.h"
+#include "kmem.h"
 
 extern struct devsw devsw[];
 
+// Open file descriptions, shared by every process. NFILE of them in the
+// kernel image and more from a slab cache once those are all open; a closed
+// one goes on the free list. This was NFILE and then ENFILE, which a few
+// processes each holding a hundred descriptors -- an editor, a build, a
+// shell -- were already a third of the way to.
 struct {
   struct spinlock lock;
   struct file file[NFILE];
+  struct file *free;
+  struct kmem_cache *more;
+  uint n;
 } ftable;
 
 void
 fileinit(void)
 {
   initlock(&ftable.lock, "ftable");
+  for(int i = NFILE - 1; i >= 0; i--){
+    ftable.file[i].fnext = ftable.free;
+    ftable.free = &ftable.file[i];
+  }
+  ftable.n = NFILE;
+}
+
+uint
+file_slots(void)
+{
+  return ftable.n;
 }
 
 struct file*
@@ -29,8 +49,19 @@ filealloc(void)
   struct file *f;
 
   acquire(&ftable.lock);
-  for(f = ftable.file; f < ftable.file + NFILE; f++){
+  if((f = ftable.free) != 0){
-    if(f->ref == 0){
+    ftable.free = f->fnext;
+  } else {
+    // The slab allocator takes only its own lock and kalloc's.
+    if(ftable.more == 0)
+      ftable.more = kmem_cache_create("file", sizeof(struct file));
+    if(ftable.more == 0 || (f = kmem_cache_alloc(ftable.more)) == 0){
+      release(&ftable.lock);
+      return 0;
+    }
+    memset(f, 0, sizeof *f);
+    ftable.n++;
+  }
   f->ref = 1;
   FSSTAT_TAKE(fsstat.file_used, fsstat.file_peak);   // 0 -> 1
   // Cleared here rather than left to the caller: every creator sets
@@ -41,10 +72,6 @@ filealloc(void)
   release(&ftable.lock);
   return f;
 }
-  }
-  release(&ftable.lock);
-  return 0;
-}
 
 struct file*
 filedup(struct file *f)
@@ -72,6 +99,8 @@ fileclose(struct file *f)
   ff = *f;
   f->ref = 0;
   f->type = FD_NONE;
+  f->fnext = ftable.free;
+  ftable.free = f;
   FSSTAT_DROP(fsstat.file_used);                          // 1 -> 0
   release(&ftable.lock);
 

kernel/fs/fs.c

@@ -45,6 +45,7 @@
 #include "file.h"
 #include "fat.h"
 #include "errno.h"
+#include "kmem.h"
 
 extern uint uptime(void);
 
@@ -215,17 +216,80 @@ bfree(int dev, uint b)
 // Inode table
 // ===========================================================================
 
+// Every inode somebody holds, found by a hash on (dev, inum). One with no
+// reference goes on the free list and out of the hash, so a lookup never has
+// to skip it.
+//
+// This was NINODE entries searched end to end on every iget(), and "iget: no
+// inodes" -- a panic -- when all 200 were held. Held means open, a cwd, an
+// exec'd binary, or a directory some path walk is in the middle of, so 128
+// processes with a few files each reached it. The table now starts as those
+// NINODE and takes more from a slab cache when they are all held; it never
+// gives them back, which costs 280 bytes for each inode that was ever held at
+// once, and makes the panic mean "out of kernel memory".
+#define INODE_NHASH 4096        // a power of two
+
 struct {
   struct spinlock lock;
   struct inode    inode[NINODE];
+  struct inode   *hash[INODE_NHASH];
+  struct inode   *free;         // through hnext, ref 0
+  struct inode   *all;          // through anext
+  struct kmem_cache *more;
+  uint            n;
 } itable;
 
+static inline uint
+ihash(uint dev, uint64 inum)
+{
+  uint64 h = (inum ^ ((uint64)dev << 40)) * 0x9E3779B97F4A7C15ULL;
+  return (uint)(h >> 52) & (INODE_NHASH - 1);
+}
+
+static void
+inode_new(struct inode *ip)
+{
+  initsleeplock(&ip->lock, "inode");
+  ip->anext = itable.all;
+  itable.all = ip;
+  ip->hnext = itable.free;
+  itable.free = ip;
+  itable.n++;
+}
+
 void
 iinit(void)
 {
   initlock(&itable.lock, "itable");
-  for (int i = 0; i < NINODE; i++)
+  for (int i = NINODE - 1; i >= 0; i--)
-    initsleeplock(&itable.inode[i].lock, "inode");
+    inode_new(&itable.inode[i]);
+}
+
+// How many inode slots exist, for /proc/fsstat.
+uint
+inode_slots(void)
+{
+  return itable.n;
+}
+
+// The held inode for (dev, inum), or 0. Under itable.lock.
+static struct inode *
+ifind(uint dev, uint64 inum)
+{
+  for (struct inode *ip = itable.hash[ihash(dev, inum)]; ip; ip = ip->hnext)
+    if (ip->dev == dev && ip->inum == inum)
+      return ip;
+  return 0;
+}
+
+static void
+iunhash(struct inode *ip)
+{
+  struct inode **pp = &itable.hash[ihash(ip->dev, ip->inum)];
+  while (*pp && *pp != ip)
+    pp = &(*pp)->hnext;
+  if (*pp)
+    *pp = ip->hnext;
 }
 
 struct inode *iget(uint dev, uint64 inum);
@@ -450,20 +514,28 @@ iget(uint dev, uint64 inum)
   struct inode *ip, *empty;
 
   acquire(&itable.lock);
-  empty = 0;
+  if ((ip = ifind(dev, inum)) != 0) {
-  for (ip = &itable.inode[0]; ip < &itable.inode[NINODE]; ip++) {
-    if (ip->ref > 0 && ip->dev == dev && ip->inum == inum) {
     ip->ref++;
     release(&itable.lock);
     return ip;
   }
-    if (empty == 0 && ip->ref == 0)
+
-      empty = ip;
+  if ((empty = itable.free) != 0) {
-  }
+    itable.free = empty->hnext;
-  if (empty == 0)
+  } else {
+    // The slab allocator takes only its own lock and kalloc's.
+    if (itable.more == 0)
+      itable.more = kmem_cache_create("inode", sizeof(struct inode));
+    if (itable.more == 0 || (empty = kmem_cache_alloc(itable.more)) == 0)
       panic("iget: no inodes");
+    memset(empty, 0, sizeof *empty);
+    inode_new(empty);
+    itable.free = empty->hnext;
+  }
 
   ip        = empty;
+  ip->hnext = itable.hash[ihash(dev, inum)];
+  itable.hash[ihash(dev, inum)] = ip;
   ip->dev   = dev;
   ip->inum  = inum;
   ip->ref   = 1;
@@ -481,11 +553,7 @@ iheld(uint dev, uint64 inum)
 {
   int held = 0;
   acquire(&itable.lock);
-  for (struct inode *ip = &itable.inode[0]; ip < &itable.inode[NINODE]; ip++)
+  held = ifind(dev, inum) != 0;
-    if (ip->ref > 0 && ip->dev == dev && ip->inum == inum) {
-      held = 1;
-      break;
-    }
   release(&itable.lock);
   return held;
 }
@@ -497,7 +565,7 @@ ibusy(uint dev, uint64 except)
 {
   int busy = 0;
   acquire(&itable.lock);
-  for (struct inode *ip = &itable.inode[0]; ip < &itable.inode[NINODE]; ip++)
+  for (struct inode *ip = itable.all; ip; ip = ip->anext)
     if (ip->ref > 0 && ip->dev == dev && ip->inum != except) {
       busy = 1;
       break;
@@ -584,8 +652,12 @@ iput(struct inode *ip)
     acquire(&itable.lock);
   }
 
-  if (--ip->ref == 0)
+  if (--ip->ref == 0) {
     FSSTAT_DROP(fsstat.inode_used);
+    iunhash(ip);
+    ip->hnext = itable.free;
+    itable.free = ip;
+  }
   release(&itable.lock);
 }
 

kernel/sys/sys_compat.c

@@ -74,6 +74,20 @@ sys_arch_prctl(void)
     return -EINVAL;
 }
 
+// Limits are reported, not enforced: everything is unlimited except what the
+// kernel actually has a ceiling for. RLIMIT_NOFILE was "unlimited" too, and
+// musl turns that into sysconf(_SC_OPEN_MAX) == -1 -- after which zsh decided
+// it had no descriptors at all, and `exec {fd}<file` failed on the first try.
+#define RLIMIT_NOFILE_ 7
+
+static void
+rlimit_of(int resource, struct rlimit *r)
+{
+    r->rlim_cur = r->rlim_max = (uint64)-1;
+    if (resource == RLIMIT_NOFILE_)
+        r->rlim_cur = r->rlim_max = NOFILE;
+}
+
 addr_t
 sys_getrlimit(void)
 {
@@ -82,19 +96,18 @@ sys_getrlimit(void)
     argint(0, &resource);
     if (argptr(1, (char**)&rlp, sizeof(*rlp)) < 0)
         return -1;
-    rlp->rlim_cur = (uint64)-1;
+    rlimit_of(resource, rlp);
-    rlp->rlim_max = (uint64)-1;
     return 0;
 }
 
 addr_t
 sys_prlimit(void)
 {
+    int resource;
     struct rlimit *oldrlp;
-    if (argptr(3, (char**)&oldrlp, sizeof(*oldrlp)) == 0 && oldrlp) {
+    argint(1, &resource);
-        oldrlp->rlim_cur = (uint64)-1;
+    if (argptr(3, (char**)&oldrlp, sizeof(*oldrlp)) == 0 && oldrlp)
-        oldrlp->rlim_max = (uint64)-1;
+        rlimit_of(resource, oldrlp);
-    }
     return 0;
 }
 

kernel/sys/sys_fs.c

@@ -16,7 +16,7 @@
 #include "errno.h"
 #include "schfs.h"      /* SCHFS_BLKSIZE, SCHFS_NAME_MAX — statfs reports them */
 #include "gfx.h"        /* gfx_cols/gfx_rows — TIOCGWINSZ reports them */
-#include "vt.h"         /* the terminals TIOC*/VT_* act on */
+#include "vt.h"         /* the terminals the TIOC and VT_ ioctls act on */
 #include "pciuser.h"
 #include "keymap.h"    /* KB_*LAYOUT — the keyboard layout catalogue */
 #include "netuser.h"