Re: [PATCH 2/2] msync: start async writeout when MS_ASYNC

On Thu, 31 May 2012 22:43:55 +0200
Paolo Bonzini <pbonzini@redhat.com> wrote:

> msync.c says that applications had better use fsync() or fadvise(FADV_DONTNEED)
> instead of MS_ASYNC.  Both advices are really bad:
> 
> * fsync() can be a replacement for MS_SYNC, not for MS_ASYNC;
> 
> * fadvise(FADV_DONTNEED) invalidates the pages completely, which will make
>   later accesses expensive.
> 
> Having the possibility to schedule a writeback immediately is an advantage
> for the applications.  They can do the same thing that fadvise does,
> but without the invalidation part.  The implementation is also similar
> to fadvise, but with tag-and-write enabled.
> 
> One example is if you are implementing a persistent dirty bitmap.
> Whenever you set bits to 1 you need to synchronize it with MS_SYNC, so
> that dirtiness is reported properly after a host crash.  If you have set
> any bits to 0, getting them to disk is not needed for correctness, but
> it is still desirable to save some work after a host crash.  You could
> simply use MS_SYNC in a separate thread, but MS_ASYNC provides exactly
> the desired semantics and is easily done in the kernel.
> 
> If the application does not want to start I/O, it can simply call msync
> with flags equal to MS_INVALIDATE.  This one remains a no-op, as it should
> be on a reasonable implementation.

Means that people will find that their msync(MS_ASYNC) call will newly
start IO.  This may well be undesirable for some.

Also, it hardwires into the kernel behaviour which userspace itself
could have initiated, with sync_file_range().  ie: reduced flexibility.

Perhaps we can update the msync.c code comments to direct people to
sync_file_range()?


One wonders how msync() works with nonlinear mappings.  I guess
"badly".  I think this was all discussed when we merged
remap_file_pages() (what a mistake that was) and we decided "too hard".