[Qemu-devel] [PATCH for-2.8 0/4] Allow 'cache-clean-interval' in Linux only

[Alberto Garcia]

Hi all,

The cache-clean-interval setting of qcow2 frees the memory of the L2
cache tables that haven't been used after a certain interval of time.

QEMU uses madvise() with MADV_DONTNEED for this. After that call, the
data in the specified cache tables is discarded by the kernel. The
problem with this behavior is that it is Linux-specific. madvise()
itself is not a standard system call and while other implementations
(e.g. FreeBSD) also have MADV_DONTNEED, they don't share the same
semantics.

POSIX defines posix_madvise(), which has POSIX_MADV_DONTNEED, and
that's what QEMU uses in systems that don't implement madvise().
However POSIX_MADV_DONTNEED also has different semantics and cannot be
used for our purposes. As a matter of fact, in glibc it is a no-op:

https://github.molgen.mpg.de/git-mirror/glibc/blob/glibc-2.23/sysdeps/unix/sysv/linux/posix_madvise.c

So while this all is mentioned in the QEMU documentation, there's
nothing preventing users of other systems from trying to use this
feature. In non-Linux systems it is worse than a no-op: it invalidates
perfectly valid cache tables for no reason without freeing their
memory.

This series makes Linux a hard requirement for cache-clean-interval
and prints an error message in other systems.

Regards,

Berto

Alberto Garcia (4):
  qcow2: Make qcow2_cache_table_release() work only in Linux
  qcow2: Allow 'cache-clean-interval' in Linux only
  qcow2: Remove stale comment
  docs: Specify that cache-clean-interval is only supported in Linux

 block/qcow2-cache.c  | 6 +++---
 block/qcow2.c        | 8 ++++++++
 docs/qcow2-cache.txt | 5 +++--
 3 files changed, 14 insertions(+), 5 deletions(-)

-- 
2.10.2