tmpfs: fix link accounting when a tmpfile is linked in

Linux Kernel / Linux Kernel - Darrick J. Wong [] - 21 February 2019 17:01 EST

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: Fixes: f4e0c30c191 ("allow the temp files created by open() to be linked to")

1062af920c07 tmpfs: fix link accounting when a tmpfile is linked in
mm/shmem.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)


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