get_next_ino has a number of problems:
- It uses and returns a uint, which is susceptible to become overflowed
if a lot of volatile inodes that use get_next_ino are created.
- It's global, with no specificity per-sb or even per-filesystem. This
means it's not that difficult to cause inode number wraparounds on a
single device, which can result in having multiple distinct inodes
with the same inode number.
This patch adds a per-superblock counter that mitigates the second case.
This design also allows us to later have a specific i_ino size
per-device, for example, allowing users to choose whether to use 32- or
64-bit inodes for each tmpfs mount. This is implemented in the next
commit.
For internal shmem mounts which may be less tolerant to spinlock delays,
we implement a percpu batching scheme which only takes the stat_lock at
each batch boundary.
Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
Cc: Amir Goldstein <amir73il@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: kernel-team@xxxxxx
---
include/linux/fs.h | 15 +++++++++
include/linux/shmem_fs.h | 2 ++
mm/shmem.c | 66 +++++++++++++++++++++++++++++++++++++---
3 files changed, 78 insertions(+), 5 deletions(-)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f15848899945..b70b334f8e16 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2961,6 +2961,21 @@ extern void discard_new_inode(struct inode *);
extern unsigned int get_next_ino(void);
extern void evict_inodes(struct super_block *sb);
+/*
+ * Userspace may rely on the the inode number being non-zero. For example, glibc
+ * simply ignores files with zero i_ino in unlink() and other places.
+ *
+ * As an additional complication, if userspace was compiled with
+ * _FILE_OFFSET_BITS=32 on a 64-bit kernel we'll only end up reading out the
+ * lower 32 bits, so we need to check that those aren't zero explicitly. With
+ * _FILE_OFFSET_BITS=64, this may cause some harmless false-negatives, but
+ * better safe than sorry.
+ */
+static inline bool is_zero_ino(ino_t ino)
+{
+ return (u32)ino == 0;
+}
+
extern void __iget(struct inode * inode);
extern void iget_failed(struct inode *);
extern void clear_inode(struct inode *);
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 7a35a6901221..eb628696ec66 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -36,6 +36,8 @@ struct shmem_sb_info {
unsigned char huge; /* Whether to try for hugepages */
kuid_t uid; /* Mount uid for root directory */
kgid_t gid; /* Mount gid for root directory */
+ ino_t next_ino; /* The next per-sb inode number to use */
+ ino_t __percpu *ino_batch; /* The next per-cpu inode number to use */
struct mempolicy *mpol; /* default memory policy for mappings */
spinlock_t shrinklist_lock; /* Protects shrinklist */
struct list_head shrinklist; /* List of shinkable inodes */
diff --git a/mm/shmem.c b/mm/shmem.c
index a0dbe62f8042..f70ab1623081 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -260,18 +260,67 @@ bool vma_is_shmem(struct vm_area_struct *vma)
static LIST_HEAD(shmem_swaplist);
static DEFINE_MUTEX(shmem_swaplist_mutex);
-static int shmem_reserve_inode(struct super_block *sb)
+/*
+ * shmem_reserve_inode() performs bookkeeping to reserve a shmem inode, and
+ * produces a novel ino for the newly allocated inode.
+ *
+ * It may also be called when making a hard link to permit the space needed by
+ * each dentry. However, in that case, no new inode number is needed since that
+ * internally draws from another pool of inode numbers (currently global
+ * get_next_ino()). This case is indicated by passing NULL as inop.
+ */
+#define SHMEM_INO_BATCH 1024U
+static int shmem_reserve_inode(struct super_block *sb, ino_t *inop)
{
struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
- if (sbinfo->max_inodes) {
+ ino_t ino;
+
+ if (!(sb->s_flags & SB_KERNMOUNT)) {
spin_lock(&sbinfo->stat_lock);
if (!sbinfo->free_inodes) {
spin_unlock(&sbinfo->stat_lock);
return -ENOSPC;
}
sbinfo->free_inodes--;
+ if (inop) {
+ ino = sbinfo->next_ino++;
+ if (unlikely(is_zero_ino(ino)))
+ ino = sbinfo->next_ino++;
+ if (unlikely(ino > UINT_MAX)) {
+ /*
+ * Emulate get_next_ino uint wraparound for
+ * compatibility
+ */
+ ino = 1;
+ }
+ *inop = ino;
+ }
spin_unlock(&sbinfo->stat_lock);
+ } else if (inop) {
+ /*
+ * __shmem_file_setup, one of our callers, is lock-free: it
+ * doesn't hold stat_lock in shmem_reserve_inode since
+ * max_inodes is always 0, and is called from potentially
+ * unknown contexts. As such, use a per-cpu batched allocator
+ * which doesn't require the per-sb stat_lock unless we are at
+ * the batch boundary.
+ */
+ ino_t *next_ino;
+ next_ino = per_cpu_ptr(sbinfo->ino_batch, get_cpu());
+ ino = *next_ino;
+ if (unlikely((ino & ~SHMEM_INO_BATCH) == 0)) {
+ spin_lock(&sbinfo->stat_lock);
+ ino = sbinfo->next_ino;
+ sbinfo->next_ino += SHMEM_INO_BATCH;
+ spin_unlock(&sbinfo->stat_lock);
+ if (unlikely(is_zero_ino(ino)))
+ ino++;
+ }
+ *inop = ino;
+ *next_ino = ++ino;
+ put_cpu();
}
+
return 0;
}
@@ -2222,13 +2271,14 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
struct inode *inode;
struct shmem_inode_info *info;
struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
+ ino_t ino;
- if (shmem_reserve_inode(sb))
+ if (shmem_reserve_inode(sb, &ino))
return NULL;
inode = new_inode(sb);
if (inode) {
- inode->i_ino = get_next_ino();
+ inode->i_ino = ino;
inode_init_owner(inode, dir, mode);
inode->i_blocks = 0;
inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
@@ -2932,7 +2982,7 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
* first link must skip that, to get the accounting right.
*/
if (inode->i_nlink) {
- ret = shmem_reserve_inode(inode->i_sb);
+ ret = shmem_reserve_inode(inode->i_sb, NULL);
if (ret)
goto out;
}
@@ -3584,6 +3634,7 @@ static void shmem_put_super(struct super_block *sb)
{
struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
+ free_percpu(sbinfo->ino_batch);
percpu_counter_destroy(&sbinfo->used_blocks);
mpol_put(sbinfo->mpol);
kfree(sbinfo);
@@ -3626,6 +3677,11 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
#endif
sbinfo->max_blocks = ctx->blocks;
sbinfo->free_inodes = sbinfo->max_inodes = ctx->inodes;
+ if (sb->s_flags & SB_KERNMOUNT) {
+ sbinfo->ino_batch = alloc_percpu(ino_t);
+ if (!sbinfo->ino_batch)
+ goto failed;
+ }
sbinfo->uid = ctx->uid;
sbinfo->gid = ctx->gid;
sbinfo->mode = ctx->mode;
--
2.27.0
