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

[Kevin Wolf]

Am 25.11.2016 um 12:27 hat Alberto Garcia geschrieben:
> 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.

Thanks, applied to the block branch.

Kevin