Re: [Qemu-devel] [PATCH 0/5] hostmem-file: Add "persistent" option

[Eduardo Habkost]

On Fri, Aug 11, 2017 at 05:44:55PM +0100, Daniel P. Berrange wrote:
> On Fri, Aug 11, 2017 at 01:33:00PM -0300, Eduardo Habkost wrote:
> > CCing Zack Cornelius.
> > 
> > On Wed, Jun 14, 2017 at 05:29:55PM -0300, Eduardo Habkost wrote:
> > > This series adds a new "persistent" option to
> > > memory-backend-file.  The new option it will be useful if
> > > somebody is sharing RAM contents on a file using share=on, but
> > > don't need it to be flushed to disk when QEMU exits.
> > > 
> > > Internally, it will trigger a madvise(MADV_REMOVE) or
> > > fallocate(FALLOC_FL_PUNCH_HOLE) call when the memory backend is
> > > destroyed.
> > > 
> > > To make we actually trigger the new code when QEMU exits, the
> > > first patch in the series ensures we destroy all user-created
> > > objects when exiting QEMU.
> > 
> > So, before sending a new version of this, we need to clarify one
> > thing: why exactly unlink()+close() wouldn't be enough to avoid
> > having data unnecessarily flushed to the backing store and make
> > the new option unnecessary?
> 
> If the backend file is shared between processes, unlinking
> it feels bad - you're assuming no /future/ process wants to
> attach to the file. Also if QEMU aborts for any reason, the
> cleanup code is never going to run

If mem-path is a directory, QEMU will create a file, open() it
and unlink() it immediately.

This solves the problem of not running the cleanup code when QEMU
aborts (which is not solved by the madvise() method).

> 
> > I would expect close() to not write any data unnecessarily if
> > there are no remaining references to the file.  Why/when this is
> > not the case?
> 
> Isn't the unlink() delayed until such time as *all* open handles
> on that file are closed. If so, it seems that if 2 processes
> have the file open, and one closes it, it is still valid for the
> kernel to want to flush data out to the backing store if it needs
> to free up working memory consumed by i/o cache.
> 
> If this wasn't the case, then one process could write 20 GB of data,
> unlink + close the file, and that 20 GB would never be able to be
> purge from I/O cache for as long as another process had that FD
> open. That would be pretty bad denial of sevice for memory management
> system.

I'm assuming QEMU is the only process opening the file.  Are
there use cases where 1) there's a need for another process to
the keep the file open; but 2) there's no need for the data on
the file to be kept?

-- 
Eduardo