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Date:   Tue, 19 Feb 2019 05:48:07 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Matej Kupljen <matej.kupljen@gmail.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] tmpfs: fix link accounting when a tmpfile is linked in
Message-ID: <20190219054807.GX2217@ZenIV.linux.org.uk>
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On Mon, Feb 18, 2019 at 09:37:52PM -0800, Hugh Dickins wrote:
> From: "Darrick J. Wong" <darrick.wong@oracle.com>
> 
> 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.
> 
> Fixes: f4e0c30c191 ("allow the temp files created by open() to be linked to")
> Reported-by: Matej Kupljen <matej.kupljen@gmail.com>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> Signed-off-by: Hugh Dickins <hughd@google.com>

FWIW, Acked-by: Al Viro <viro@zeniv.linux.org.uk>

Or I can drop it into vfs.git - up to you.