Re: [PATCH v2] jffs2: safely remove obsolete dirent from the f->dents list

[yuyufen <yuyufen@huawei.com>]

ping?


On 2019/1/23 15:22, Yufen Yu wrote:
> We may delete a bunch of directory entries, operating such as:
> getdents(), unlink(), getdents()..., until the end of the directory.
> jffs2 handles f_pos on the directory merely as the position on the
> f->dents list. So, the next getdents() may skip some entries
> before f_pos, if we remove some entries from the list between two
> getdents(), resulting in some entries of the directory cannot be deleted.
>
> Commit 15953580e79b is introduced to resolve this bug by not
> removing delete entries from the list immediately.
>
> However, when CONFIG_JFFS2_SUMMARY is not set, it can cause the
> following issues:
>
> * 'deletion' dirents is always in the f->dents list, wasting memory
>      resource. For example:
>          There is a file named 'file1'. Then we rename it:
>          mv file1 file2;
>          mv file2 file3;
>          ...
>          mv file99999 file1000000
>
>      All of file1~file1000000 dirents always in the f->dents list.
>
> * Though, the list we're traversing is already ordered by CRC,
> it could waste much CPU time when the list is very long.
>
> To fix, we define two variables in struct jffs2_inode_info: nr_dir_opening,
> obsolete_count, and two new functions: jffs2_dir_open(), jffs2_dir_release().
>
> When open a directory, jffs2_dir_open() will increase nr_dir_opening,
> which will be decreased by jffs2_dir_release(). if the value is 0,
> it means nobody open the dir and nobody in getdents()/seek() semantics.
> Thus, we can remove all obsolete dirent from the list.
>
> When delete a file, jffs2_do_unlink() can remove the dirent directly if
> nobody open the directory(i.e. nr_dir_opending == 0). Otherwise, it just
> increase obsolete_count, denoting obsolete dirent count of the list.
>
> Fixes: 15953580e79b ("[JFFS2] Improve getdents vs. f_pos handling on NOR flash.")
> Signed-off-by: Yufen Yu <yuyufen@huawei.com>
> ---
>   fs/jffs2/dir.c             | 49 ++++++++++++++++++++++++++++++++++++++++++++++
>   fs/jffs2/jffs2_fs_i.h      |  7 +++++++
>   fs/jffs2/super.c           |  4 ++++
>   fs/jffs2/write.c           | 30 +++++++++++++++++++---------
>   include/uapi/linux/jffs2.h |  4 ++++
>   5 files changed, 85 insertions(+), 9 deletions(-)
>
> diff --git a/fs/jffs2/dir.c b/fs/jffs2/dir.c
> index f20cff1194bb..aed872dcd0ca 100644
> --- a/fs/jffs2/dir.c
> +++ b/fs/jffs2/dir.c
> @@ -37,6 +37,8 @@ static int jffs2_mknod (struct inode *,struct dentry *,umode_t,dev_t);
>   static int jffs2_rename (struct inode *, struct dentry *,
>   			 struct inode *, struct dentry *,
>   			 unsigned int);
> +static int jffs2_dir_release (struct inode *, struct file *);
> +static int jffs2_dir_open (struct inode *, struct file *);
>   
>   const struct file_operations jffs2_dir_operations =
>   {
> @@ -45,6 +47,8 @@ const struct file_operations jffs2_dir_operations =
>   	.unlocked_ioctl=jffs2_ioctl,
>   	.fsync =	jffs2_fsync,
>   	.llseek =	generic_file_llseek,
> +	.open =		jffs2_dir_open,
> +	.release =	jffs2_dir_release,
>   };
>   
>   
> @@ -865,3 +869,48 @@ static int jffs2_rename (struct inode *old_dir_i, struct dentry *old_dentry,
>   	return 0;
>   }
>   
> +static int jffs2_dir_open(struct inode *dir_i, struct file *filp)
> +{
> +#ifndef CONFIG_JFFS2_SUMMARY
> +	struct jffs2_inode_info *dir_f = JFFS2_INODE_INFO(dir_i);
> +	atomic_inc(&dir_f->nr_dir_opening);
> +#endif
> +
> +	return 0;
> +}
> +
> +static int jffs2_dir_release(struct inode *dir_i, struct file *filp)
> +{
> +#ifndef CONFIG_JFFS2_SUMMARY
> +	struct jffs2_inode_info *dir_f = JFFS2_INODE_INFO(dir_i);
> +
> +	BUG_ON(atomic_read(&dir_f->nr_dir_opening) <= 0);
> +
> +	mutex_lock(&dir_f->sem);
> +	/* jffs2_dir_open may increase nr_dir_opening after
> +	 * atomic_dec_and_test() returning true.
> +	 * However, it cannot traverse the list until hold
> +	 * mutex dir_f->sem lock, so that we can go on
> +	 * removing.*/
> +	if (atomic_dec_and_test(&dir_f->nr_dir_opening) &&
> +			dir_f->obsolete_count > JFFS2_OBS_DIRENT_LIMIT) {
> +		struct jffs2_full_dirent **prev = &dir_f->dents;
> +
> +		/* remove all obsolete dirent from the list, which
> +		 * can save memory space and reduce CPU time for
> +		 * traverse the list */
> +		while(*prev) {
> +			if ((*prev)->raw == NULL && (*prev)->ino == 0) {
> +				struct jffs2_full_dirent *this = *prev;
> +				*prev = this->next;
> +				jffs2_free_full_dirent(this);
> +			} else
> +				prev = &((*prev)->next);
> +		}
> +		dir_f->obsolete_count = 0;
> +	}
> +	mutex_unlock(&dir_f->sem);
> +#endif
> +
> +	return 0;
> +}
> diff --git a/fs/jffs2/jffs2_fs_i.h b/fs/jffs2/jffs2_fs_i.h
> index 2e4a86763c07..a4da25d16cb4 100644
> --- a/fs/jffs2/jffs2_fs_i.h
> +++ b/fs/jffs2/jffs2_fs_i.h
> @@ -50,6 +50,13 @@ struct jffs2_inode_info {
>   
>   	uint16_t flags;
>   	uint8_t usercompr;
> +
> +	/* obsolete dirent count in the list of 'dents' */
> +	uint8_t  obsolete_count;
> +
> +	/* Directory open refcount */
> +	atomic_t nr_dir_opening;
> +
>   	struct inode vfs_inode;
>   };
>   
> diff --git a/fs/jffs2/super.c b/fs/jffs2/super.c
> index 87bdf0f4cba1..bf181b0ade9c 100644
> --- a/fs/jffs2/super.c
> +++ b/fs/jffs2/super.c
> @@ -41,6 +41,10 @@ static struct inode *jffs2_alloc_inode(struct super_block *sb)
>   	f = kmem_cache_alloc(jffs2_inode_cachep, GFP_KERNEL);
>   	if (!f)
>   		return NULL;
> +
> +	atomic_set(&f->nr_dir_opening, 0);
> +	f->obsolete_count = 0;
> +
>   	return &f->vfs_inode;
>   }
>   
> diff --git a/fs/jffs2/write.c b/fs/jffs2/write.c
> index cda9a361368e..780b4fd9af51 100644
> --- a/fs/jffs2/write.c
> +++ b/fs/jffs2/write.c
> @@ -600,29 +600,41 @@ int jffs2_do_unlink(struct jffs2_sb_info *c, struct jffs2_inode_info *dir_f,
>   	} else {
>   		uint32_t nhash = full_name_hash(NULL, name, namelen);
>   
> -		fd = dir_f->dents;
> +		struct jffs2_full_dirent **prev = &dir_f->dents;
> +
>   		/* We don't actually want to reserve any space, but we do
>   		   want to be holding the alloc_sem when we write to flash */
>   		mutex_lock(&c->alloc_sem);
>   		mutex_lock(&dir_f->sem);
>   
> -		for (fd = dir_f->dents; fd; fd = fd->next) {
> -			if (fd->nhash == nhash &&
> -			    !memcmp(fd->name, name, namelen) &&
> -			    !fd->name[namelen]) {
> +		while((*prev) && (*prev)->nhash <= nhash) {
> +			if ((*prev)->nhash == nhash &&
> +			    !memcmp((*prev)->name, name, namelen) &&
> +			    !(*prev)->name[namelen]) {
>   
> +				fd = *prev;
>   				jffs2_dbg(1, "Marking old dirent node (ino #%u) @%08x obsolete\n",
>   					  fd->ino, ref_offset(fd->raw));
>   				jffs2_mark_node_obsolete(c, fd->raw);
> -				/* We don't want to remove it from the list immediately,
> -				   because that screws up getdents()/seek() semantics even
> -				   more than they're screwed already. Turn it into a
> -				   node-less deletion dirent instead -- a placeholder */
>   				fd->raw = NULL;
>   				fd->ino = 0;
> +
> +				/* if nr_dir_openning is 0, we think nobody open the dir, and
> +				 * nobody being in getdents()/seek() semantics. Thus, we can
> +				 * safely remove this obsolete dirent from the list. Otherwise,
> +				 * we just increase obsolete_count, and finally delete it in
> +				 * jffs2_dir_release() */
> +				if (atomic_read(&dir_f->nr_dir_opening) == 0) {
> +					*prev = fd->next;
> +					jffs2_free_full_dirent(fd);
> +				} else
> +					dir_f->obsolete_count++;
> +
>   				break;
>   			}
> +			prev = &((*prev)->next);
>   		}
> +
>   		mutex_unlock(&dir_f->sem);
>   	}
>   
> diff --git a/include/uapi/linux/jffs2.h b/include/uapi/linux/jffs2.h
> index a18b719f49d4..dff3ac2d6b0c 100644
> --- a/include/uapi/linux/jffs2.h
> +++ b/include/uapi/linux/jffs2.h
> @@ -35,6 +35,10 @@
>   */
>   #define JFFS2_MAX_NAME_LEN 254
>   
> +/* The obsolete dirent of dents list limit. When the number over
> + * this limit, we can remove the obsoleted dents. */
> +#define JFFS2_OBS_DIRENT_LIMIT 64
> +
>   /* How small can we sensibly write nodes? */
>   #define JFFS2_MIN_DATA_LEN 128
>   



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