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

[Eduardo Habkost]

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?

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?

-- 
Eduardo