On 14.09.2021 15:34, Roger Pau Monne wrote: > If the new gfn matches the previous one (ie: gfn == old_gpfn)
It took me a while to realize that you probably mean "gpfn" here. > xenmem_add_to_physmap_one will issue a duplicated call to > guest_physmap_remove_page with the same gfn, because the Considering the local variable of this name, may I suggest to upper-case GFN in this case? > get_gpfn_from_mfn call has been moved by commit f8582da041 to be > performed before the original page is removed. This leads to the > second guest_physmap_remove_page failing, which was not the case > before commit f8582da041. > > Fix this by adding a check that prevents a second call to > guest_physmap_remove_page if the previous one has already removed the > backing page from that gfn. Same here (if this remains; see below). > --- a/xen/arch/x86/mm/p2m.c > +++ b/xen/arch/x86/mm/p2m.c > @@ -2813,7 +2813,7 @@ int xenmem_add_to_physmap_one( > } > > /* Unmap from old location, if any. */ > - if ( !rc && old_gpfn != INVALID_M2P_ENTRY ) > + if ( !rc && old_gpfn != INVALID_M2P_ENTRY && !gfn_eq(_gfn(old_gpfn), > gpfn) ) > rc = guest_physmap_remove_page(d, _gfn(old_gpfn), mfn, > PAGE_ORDER_4K); > > /* Map at new location. */ > It feels unbalanced to suppress the remove, but keep the add in this case. Wouldn't we be better off skipping both, or short- circuiting the effective no-op even earlier? I recall considering to install a shortcut, but it didn't seem worth it. But now that you've found actual breakage, perhaps this need reconsidering. Jan
