linux-kernel - Re: [PATCH v3 1/2] tmpfs: Add per-superblock i

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Date:   Sat, 4 Jan 2020 21:09:20 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Chris Down <chris@...isdown.name>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, kernel-team@...com
Subject: Re: [PATCH v3 1/2] tmpfs: Add per-superblock i_ino support

On Fri, Jan 3, 2020 at 7:30 PM Chris Down <chris@...isdown.name> 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@...isdown.name>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Amir Goldstein <amir73il@...il.com>
> Cc: Jeff Layton <jlayton@...nel.org>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: linux-fsdevel@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: kernel-team@...com
> ---

Some nits. When fixed you may add:
Reviewed-by: Amir Goldstein <amir73il@...il.com>


>  include/linux/shmem_fs.h |  1 +
>  mm/shmem.c               | 33 ++++++++++++++++++++++++++++++++-
>  2 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index de8e4b71e3ba..7fac91f490dc 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 next_ino;             /* The next per-sb 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 8793e8cc1a48..638b1e30625f 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2236,6 +2236,15 @@ 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 SB_KERNMOUNT, we use the per-sb inode allocator to avoid wraparound.
> + * Otherwise, we use get_next_ino, which is global.

Its the other way around.

> + *
> + * If max_inodes is greater than 0 (ie. non-SB_KERNMOUNT), we may have to grab
> + * the per-sb stat_lock.

It's not a "may" it's for sure, but I don't see what this comment adds
in this context.
The comment about stat_lock below seems enough to me.

> + */
>  static struct inode *shmem_get_inode(struct super_block *sb, const struct inode *dir,
>                                      umode_t mode, dev_t dev, unsigned long flags)
>  {
> @@ -2248,7 +2257,28 @@ 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 (sb->s_flags & SB_KERNMOUNT) {
> +                       /*
> +                        * __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
.
> +                        */
> +                       inode->i_ino = get_next_ino();
> +               } else {
> +                       spin_lock(&sbinfo->stat_lock);
> +                       if (unlikely(sbinfo->next_ino > UINT_MAX)) {
> +                               /*
> +                                * Emulate get_next_ino uint wraparound for
> +                                * compatibility
> +                                */
> +                               sbinfo->next_ino = 1;
> +                       }
> +                       inode->i_ino = sbinfo->next_ino++;
> +                       spin_unlock(&sbinfo->stat_lock);
> +               }
> +
>                 inode_init_owner(inode, dir, mode);
>                 inode->i_blocks = 0;
>                 inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
> @@ -3662,6 +3692,7 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
>  #else
>         sb->s_flags |= SB_NOUSER;
>  #endif
> +       sbinfo->next_ino = 1;
>         sbinfo->max_blocks = ctx->blocks;
>         sbinfo->free_inodes = sbinfo->max_inodes = ctx->inodes;
>         sbinfo->uid = ctx->uid;
> --
> 2.24.1
>