linux-kernel - Re: [PATCH v2] hugetlbfs: add O

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Date:   Tue, 12 Nov 2019 10:22:52 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Piotr Sarna <p.sarna@...n.pl>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, mhocko@...nel.org,
        syzbot+136d2439a4e6561ea00c@...kaller.appspotmail.com,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] hugetlbfs: add O_TMPFILE support

Cc: Andrew

On 11/8/19 11:50 PM, Piotr Sarna wrote:
> With hugetlbfs, a common pattern for mapping anonymous huge pages
> is to create a temporary file first. Currently libraries like
> libhugetlbfs and seastar create these with a standard mkstemp+unlink
> trick, but it would be more robust to be able to simply pass
> the O_TMPFILE flag to open(). O_TMPFILE is already supported by several
> file systems like ext4 and xfs. The implementation simply uses the existing
> d_tmpfile utility function to instantiate the dcache entry for the file.
> 
> Tested manually by successfully creating a temporary file by opening
> it with (O_TMPFILE|O_RDWR) on mounted hugetlbfs and successfully
> mapping 2M huge pages with it. Without the patch, trying to open
> a file with O_TMPFILE results in -ENOSUP.
> 
> v2 changes:
>  * syzkaller thankfully discovered a bug during unmount - tmpfile
> erroneously called dget() on a dentry when creating a tmpfile,
> and it was never countered by a dput(), because tmpfile is never
> explicitly unlinked. In v2, dget() is simply not called for tmpfile.
> Verified manually, and also with the reproducer provided by syzkaller.
> Reported-by: syzbot+136d2439a4e6561ea00c@...kaller.appspotmail.com
> 
> Signed-off-by: Piotr Sarna <p.sarna@...n.pl>

My apologies for not catching the extra dget().

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
-- 
Mike Kravetz

> ---
>  fs/hugetlbfs/inode.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index a478df035651..a39d7a0a158e 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -815,8 +815,11 @@ static struct inode *hugetlbfs_get_inode(struct super_block *sb,
>  /*
>   * File creation. Allocate an inode, and we're done..
>   */
> -static int hugetlbfs_mknod(struct inode *dir,
> -			struct dentry *dentry, umode_t mode, dev_t dev)
> +static int do_hugetlbfs_mknod(struct inode *dir,
> +			struct dentry *dentry,
> +			umode_t mode,
> +			dev_t dev,
> +			bool tmpfile)
>  {
>  	struct inode *inode;
>  	int error = -ENOSPC;
> @@ -824,13 +827,23 @@ static int hugetlbfs_mknod(struct inode *dir,
>  	inode = hugetlbfs_get_inode(dir->i_sb, dir, mode, dev);
>  	if (inode) {
>  		dir->i_ctime = dir->i_mtime = current_time(dir);
> -		d_instantiate(dentry, inode);
> -		dget(dentry);	/* Extra count - pin the dentry in core */
> +		if (tmpfile) {
> +			d_tmpfile(dentry, inode);
> +		} else {
> +			d_instantiate(dentry, inode);
> +			dget(dentry);/* Extra count - pin the dentry in core */
> +		}
>  		error = 0;
>  	}
>  	return error;
>  }
>  
> +static int hugetlbfs_mknod(struct inode *dir,
> +			struct dentry *dentry, umode_t mode, dev_t dev)
> +{
> +	return do_hugetlbfs_mknod(dir, dentry, mode, dev, false);
> +}
> +
>  static int hugetlbfs_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
>  {
>  	int retval = hugetlbfs_mknod(dir, dentry, mode | S_IFDIR, 0);
> @@ -844,6 +857,12 @@ static int hugetlbfs_create(struct inode *dir, struct dentry *dentry, umode_t mo
>  	return hugetlbfs_mknod(dir, dentry, mode | S_IFREG, 0);
>  }
>  
> +static int hugetlbfs_tmpfile(struct inode *dir,
> +			struct dentry *dentry, umode_t mode)
> +{
> +	return do_hugetlbfs_mknod(dir, dentry, mode | S_IFREG, 0, true);
> +}
> +
>  static int hugetlbfs_symlink(struct inode *dir,
>  			struct dentry *dentry, const char *symname)
>  {
> @@ -1102,6 +1121,7 @@ static const struct inode_operations hugetlbfs_dir_inode_operations = {
>  	.mknod		= hugetlbfs_mknod,
>  	.rename		= simple_rename,
>  	.setattr	= hugetlbfs_setattr,
> +	.tmpfile	= hugetlbfs_tmpfile,
>  };
>  
>  static const struct inode_operations hugetlbfs_inode_operations = {
>