On Tue, 2018-10-09 at 15:32 +0000, richard.weiyang@xxxxxxxxx wrote: > > If the PTE is indeed writable, we can also create the shadow PTE with > > write permissions even though we're handling a read fault.????Marking > > the SPTE as writable means we don't take another page fault if/when > > the guest writes the page, e.g. in a RMW scenario we take one page > > fault instead of two. > > > This makes sence to me. > > As you mentioned in the last sentense of previous paragraph, a read-only > PTE with writable VMA means the PTE is indeed writalbe. In this case we > could set the SPTE as write to reduce another page fault in a RMW > scenario. > > After all I still have one confusion: > > My confusion is why the 2nd __get_user_pages_fast() would succeed > while the 1st failed with same parameters passed. Do we touch the PTE > mapping during get_user_pages_unlocked()? Yep, get_user_pages_unlocked() will fault-in the page if necessary.
