Re: [PATCH v7 1/2] memory: Update inline documentation

[Peter Xu]

On Thu, Jan 09, 2025 at 01:30:35PM +0100, BALATON Zoltan wrote:
> On Thu, 9 Jan 2025, Akihiko Odaki wrote:
> > Do not refer to "memory region's reference count"
> > -------------------------------------------------
> > 
> > Now MemoryRegions do have their own reference counts, but they will not
> > be used when their owners are not themselves. However, the documentation
> > of memory_region_ref() says it adds "1 to a memory region's reference
> > count", which is confusing. Avoid referring to "memory region's
> > reference count" and just say: "Add a reference to a memory region".
> > Make a similar change to memory_region_unref() too.
> > 
> > Refer to docs/devel/memory.rst for "owner"
> > ------------------------------------------
> > 
> > memory_region_ref() and memory_region_unref() used to have their own
> > descriptions of "owner", but they are somewhat out-of-date and
> > misleading.
> > 
> > In particular, they say "whenever memory regions are accessed outside
> > the BQL, they need to be preserved against hot-unplug", but protecting
> > against hot-unplug is not mandatory if it is known that they will never
> > be hot-unplugged. They also say "MemoryRegions actually do not have
> > their own reference count", but they actually do. They just will not be
> > used unless their owners are not themselves.
> > 
> > Refer to docs/devel/memory.rst as the single source of truth instead of
> > maintaining duplicate descriptions of "owner".
> > 
> > Clarify that owner may be missing
> > 
> > ---------------------------------
> > A memory region may not have an owner, and memory_region_ref() and
> > memory_region_unref() do nothing for such.
> > 
> > memory: Clarify owner must not call memory_region_ref()
> > --------------------------------------------------------
> > 
> > The owner must not call this function as it results in a circular
> > reference.
> > 
> > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > Reviewed-by: Peter Xu <peterx@redhat.com>
> > ---
> > include/exec/memory.h | 59 
> > ++++++++++++++++++++++++---------------------------
> > 1 file changed, 28 insertions(+), 31 deletions(-)
> > 
> > diff --git a/include/exec/memory.h b/include/exec/memory.h
> > index 9458e2801d50..ca247343f433 100644
> > --- a/include/exec/memory.h
> > +++ b/include/exec/memory.h
> > @@ -1210,7 +1210,7 @@ void 
> > memory_region_section_free_copy(MemoryRegionSection *s);
> >  * memory_region_add_subregion() to add subregions.
> >  *
> >  * @mr: the #MemoryRegion to be initialized
> > - * @owner: the object that tracks the region's reference count
> > + * @owner: the object that keeps the region alive
> >  * @name: used for debugging; not visible to the user or ABI
> >  * @size: size of the region; any subregions beyond this size will be 
> > clipped
> >  */
> > @@ -1220,29 +1220,26 @@ void memory_region_init(MemoryRegion *mr,
> >                         uint64_t size);
> > 
> > /**
> > - * memory_region_ref: Add 1 to a memory region's reference count
> > + * memory_region_ref: Add a reference to the owner of a memory region
> >  *
> > - * Whenever memory regions are accessed outside the BQL, they need to be
> > - * preserved against hot-unplug.  MemoryRegions actually do not have their
> > - * own reference count; they piggyback on a QOM object, their "owner".
> > - * This function adds a reference to the owner.
> > - *
> > - * All MemoryRegions must have an owner if they can disappear, even if the
> > - * device they belong to operates exclusively under the BQL.  This is 
> > because
> > - * the region could be returned at any time by memory_region_find, and this
> > - * is usually under guest control.
> > + * This function adds a reference to the owner of a memory region to keep 
> > the
> > + * memory region alive. It does nothing if the owner is not present as a 
> > memory
> > + * region without owner will never die.
> > + * For references internal to the owner, use object_ref() instead to avoid 
> > a
> > + * circular reference.
> 
> Reading this again I'm still confused by this last sentence. Do you mean
> references internal to the memory region should use object_ref on the memory
> region or that other references to the owner should use object_ref on the
> owner? This sentence is still not clear about that.

Having two refcounts are definitely confusing.. especially IIRC all MRs'
obj->free==NULL, so the MR's refcount isn't working.  Dynamic MR's needs
its g_free() on its own.

I acked both patches, but maybe it could indeed be slightly better we drop
this sentence, meanwhile in patch 2 we can drop the object_ref() too: it
means for parent/child MRs that share the same owner, QEMU does nothing on
the child MRs when add subregion, because it assumes the child MR will
never go away when the parent is there who shares the owner.

So maybe we try not to touch MR's refcount manually, but fix what can be
problematic for owner->ref only.

-- 
Peter Xu