Re: [PATCH v5 1/2] tmpfs: Add per-superblock i_ino support

Amir Goldstein
2020-01-06T06:41:18Z

On Mon, Jan 6, 2020 at 4:04 AM zhengbin (A) <zhengbin13@huawei.com> wrote:
>
>
> On 2020/1/5 20:06, Chris Down 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@chrisdown.name>
> > Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> > Cc: Hugh Dickins <hughd@google.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Jeff Layton <jlayton@kernel.org>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: linux-mm@kvack.org
> > Cc: linux-fsdevel@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: kernel-team@fb.com
> > ---
> > include/linux/shmem_fs.h | 1 +
> > mm/shmem.c | 30 +++++++++++++++++++++++++++++-
> > 2 files changed, 30 insertions(+), 1 deletion(-)
> >
> > v5: Nothing in code, just resending with correct linux-mm domain.
> >
> > 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..9e97ba972225 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -2236,6 +2236,12 @@ 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 this tmpfs is from kern_mount we use get_next_ino, which is global, since
> > + * inum churn there is low and this avoids taking locks.
> > + */
> > 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 +2254,28 @@ 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 stat_lock.
> > + */
> > + inode->i_ino = get_next_ino();
> > + } else {
> > + spin_lock(&sbinfo->stat_lock);
>
> Use spin_lock will affect performance, how about define
>
> unsigned long __percpu *last_ino_number; /* Last inode number */
> atomic64_t shared_last_ino_number; /* Shared last inode number */
> in shmem_sb_info, whose performance will be better?
>

Please take a look at shmem_reserve_inode().
spin lock is already being taken in shmem_get_inode()
so there is nothing to be gained from complicating next_ino counter.

This fact would have been a lot clearer if next_ino was incremented
inside shmem_reserve_inode() and its value returned to be used by
shmem_get_inode(), but I am also fine with code as it is with the
comment above.

Thanks,
Amir.