On Thu, 23 Sep 2021, Zi Yan wrote: > On 23 Sep 2021, at 17:54, Yang Shi wrote: > > On Thu, Sep 23, 2021 at 2:10 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > >> > >> NR_FILE_MAPPED being used for /proc/meminfo's "Mapped:" and a couple > >> of other such stats files, and for a reclaim heuristic in mm/vmscan.c. > >> > >> Allow ourselves more slack in NR_FILE_MAPPED accounting (either count > >> each pte as if it mapped the whole THP, or don't count a THP's ptes > >> at all - you opted for the latter in the "Mlocked:" accounting), > >> and I suspect subpage _mapcount could be abandoned. > > > > AFAIK, partial THP unmap may need the _mapcount information of every > > subpage otherwise the deferred split can't know what subpages could be > > freed. I believe Yang Shi is right insofar as the decision on whether it's worth queuing for deferred split is being done based on those subpage _mapcounts. That is a use I had not considered, and I've given no thought to how important or not it is. > > Could we just scan page tables of a THP during deferred split process > instead? Deferred split is a slow path already, so maybe it can afford > the extra work. But unless I misunderstand, actually carrying out the deferred split already unmaps, uses migration entries, and remaps the remaining ptes: needing no help from subpage _mapcounts to do those, and free the rest. Hugh
