Re: [PATCH v2 1/2] tmpfs: Add per-superblock i_ino support

[Amir Goldstein]

On Thu, Jan 2, 2020 at 8:49 PM Chris Down <chris@xxxxxxxxxxxxxx> wrote:
>
> 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.
>
> Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> Cc: Jeff Layton <jlayton@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kernel-team@xxxxxx
> ---
>  include/linux/shmem_fs.h |  1 +
>  mm/shmem.c               | 55 ++++++++++++++++++++++++++++++++++------
>  2 files changed, 48 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index de8e4b71e3ba..dec4353cf3b7 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -35,6 +35,7 @@ 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 last_ino;             /* The last used per-sb inode number */
>         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 165fa6332993..8af9fb922a96 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2235,8 +2235,18 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
>         return 0;
>  }
>
> +/*
> + * shmem_get_inode - reserve, allocate, and initialise a new inode
> + *
> + * If usb_sb_ino is true, we use the per-sb inode allocator to avoid wraparound.
> + * Otherwise, we use get_next_ino, which is global.
> + *
> + * If use_sb_ino is true or max_inodes is greater than 0, we may have to grab
> + * the per-sb stat_lock.

Wouldn't it be easier to check max_inodes instead of passing this
use_sb_ino arg?
Is there any case where they *need* to differ?

> + */
>  static struct inode *shmem_get_inode(struct super_block *sb, const struct inode *dir,
> -                                    umode_t mode, dev_t dev, unsigned long flags)
> +                                    umode_t mode, dev_t dev,
> +                                    unsigned long flags, bool use_sb_ino)
>  {
>         struct inode *inode;
>         struct shmem_inode_info *info;
> @@ -2247,7 +2257,30 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
>
>         inode = new_inode(sb);
>         if (inode) {
> -               inode->i_ino = get_next_ino();
> +               if (use_sb_ino) {
> +                       spin_lock(&sbinfo->stat_lock);
> +                       inode->i_ino = sbinfo->last_ino++;
> +                       if (unlikely(inode->i_ino >= UINT_MAX)) {
> +                               /*
> +                                * Emulate get_next_ino uint wraparound for
> +                                * compatibility
> +                                */
> +                               pr_warn("%s: inode number overflow on device %d, consider using inode64 mount option\n",
> +                                       __func__, MINOR(sb->s_dev));
> +                               inode->i_ino = sbinfo->last_ino = 1;
> +                       }
> +                       spin_unlock(&sbinfo->stat_lock);
> +               } else {
> +                       /*
> +                        * __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 the global
> +                        * allocator which doesn't require the per-sb stat_lock.
> +                        */
> +                       inode->i_ino = get_next_ino();
> +               }
> +
>                 inode_init_owner(inode, dir, mode);
>                 inode->i_blocks = 0;
>                 inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
> @@ -2881,7 +2914,7 @@ shmem_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
>         struct inode *inode;
>         int error = -ENOSPC;
>
> -       inode = shmem_get_inode(dir->i_sb, dir, mode, dev, VM_NORESERVE);
> +       inode = shmem_get_inode(dir->i_sb, dir, mode, dev, VM_NORESERVE, true);
>         if (inode) {
>                 error = simple_acl_create(dir, inode);
>                 if (error)
> @@ -2910,7 +2943,7 @@ shmem_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
>         struct inode *inode;
>         int error = -ENOSPC;
>
> -       inode = shmem_get_inode(dir->i_sb, dir, mode, 0, VM_NORESERVE);
> +       inode = shmem_get_inode(dir->i_sb, dir, mode, 0, VM_NORESERVE, true);
>         if (inode) {
>                 error = security_inode_init_security(inode, dir,
>                                                      NULL,
> @@ -3106,7 +3139,7 @@ static int shmem_symlink(struct inode *dir, struct dentry *dentry, const char *s
>                 return -ENAMETOOLONG;
>
>         inode = shmem_get_inode(dir->i_sb, dir, S_IFLNK | 0777, 0,
> -                               VM_NORESERVE);
> +                               VM_NORESERVE, true);
>         if (!inode)
>                 return -ENOSPC;
>
> @@ -3378,6 +3411,8 @@ enum shmem_param {
>         Opt_nr_inodes,
>         Opt_size,
>         Opt_uid,
> +       Opt_inode32,
> +       Opt_inode64,

Does not belong to this patch..

>  };
>
>  static const struct fs_parameter_spec shmem_param_specs[] = {
> @@ -3389,6 +3424,8 @@ static const struct fs_parameter_spec shmem_param_specs[] = {
>         fsparam_string("nr_inodes",     Opt_nr_inodes),
>         fsparam_string("size",          Opt_size),
>         fsparam_u32   ("uid",           Opt_uid),
> +       fsparam_flag  ("inode32",       Opt_inode32),
> +       fsparam_flag  ("inode64",       Opt_inode64),

Ditto

>         {}
>  };
>
> @@ -3690,7 +3727,8 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
>  #endif
>         uuid_gen(&sb->s_uuid);
>
> -       inode = shmem_get_inode(sb, NULL, S_IFDIR | sbinfo->mode, 0, VM_NORESERVE);
> +       inode = shmem_get_inode(sb, NULL, S_IFDIR | sbinfo->mode, 0,
> +                               VM_NORESERVE, true);

Should usb_sb_ino be true for the kern_mount??
In any case, it wouldn't matter if it was false, hence no need to pass
an argument
and can either check for sbinfo->max_inodes or the SB_KERNMOUNT flag in
shmem_get_inode().

Thanks,
Amir.