linux-kernel - Re: [PATCH AUTOSEL 4.20 37/60] tmpfs: fix link accounting when a tmpfile is linked in

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Date:   Wed, 13 Mar 2019 12:58:26 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Sasha Levin <sashal@...nel.org>
cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org
Subject: Re: [PATCH AUTOSEL 4.20 37/60] tmpfs: fix link accounting when a
 tmpfile is linked in

AUTOSEL is wrong to select this commit without also selecting
29b00e609960 ("tmpfs: fix uninitialized return value in shmem_link")
which contains the tag
Fixes: 1062af920c07 ("tmpfs: fix link accounting when a tmpfile is linked in")
Please add 29b00e609960 for those 6 trees, or else omit 1062af920c07 for now.

Thanks,
Hugh

On Wed, 13 Mar 2019, Sasha Levin wrote:

> From: "Darrick J. Wong" <darrick.wong@...cle.com>
> 
> [ Upstream commit 1062af920c07f5b54cf5060fde3339da6df0cf6b ]
> 
> tmpfs has a peculiarity of accounting hard links as if they were
> separate inodes: so that when the number of inodes is limited, as it is
> by default, a user cannot soak up an unlimited amount of unreclaimable
> dcache memory just by repeatedly linking a file.
> 
> But when v3.11 added O_TMPFILE, and the ability to use linkat() on the
> fd, we missed accommodating this new case in tmpfs: "df -i" shows that
> an extra "inode" remains accounted after the file is unlinked and the fd
> closed and the actual inode evicted.  If a user repeatedly links
> tmpfiles into a tmpfs, the limit will be hit (ENOSPC) even after they
> are deleted.
> 
> Just skip the extra reservation from shmem_link() in this case: there's
> a sense in which this first link of a tmpfile is then cheaper than a
> hard link of another file, but the accounting works out, and there's
> still good limiting, so no need to do anything more complicated.
> 
> Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1902182134370.7035@eggly.anvils
> Fixes: f4e0c30c191 ("allow the temp files created by open() to be linked to")
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Reported-by: Matej Kupljen <matej.kupljen@...il.com>
> Acked-by: Al Viro <viro@...iv.linux.org.uk>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  mm/shmem.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 5d07e0b1352f..7872e3b75e57 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2852,10 +2852,14 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
>  	 * No ordinary (disk based) filesystem counts links as inodes;
>  	 * but each new link needs a new dentry, pinning lowmem, and
>  	 * tmpfs dentries cannot be pruned until they are unlinked.
> +	 * But if an O_TMPFILE file is linked into the tmpfs, the
> +	 * first link must skip that, to get the accounting right.
>  	 */
> -	ret = shmem_reserve_inode(inode->i_sb);
> -	if (ret)
> -		goto out;
> +	if (inode->i_nlink) {
> +		ret = shmem_reserve_inode(inode->i_sb);
> +		if (ret)
> +			goto out;
> +	}
>  
>  	dir->i_size += BOGO_DIRENT_SIZE;
>  	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
> -- 
> 2.19.1