On 05.11.21 12:49, Naoya Horiguchi wrote: > On Fri, Nov 05, 2021 at 11:58:15AM +0100, David Hildenbrand wrote: >> On 05.11.21 06:50, Naoya Horiguchi wrote: >>> Hi, >>> >>> I updated the unpoison patchset based ou discussions over v2. >>> Please see individual patches for details of updates. >>> >>> ----- (cover letter copied from v2) ----- >>> Main purpose of this series is to sync unpoison code to recent changes >>> around how hwpoison code takes page refcount. Unpoison should work or >>> simply fail (without crash) if impossible. >>> >>> The recent works of keeping hwpoison pages in shmem pagecache introduce >>> a new state of hwpoisoned pages, but unpoison for such pages is not >>> supported yet with this series. >>> >>> It seems that soft-offline and unpoison can be used as general purpose >>> page offline/online mechanism (not in the context of memory error). >> >> I'm not sure what the target use case would be TBH ... for proper memory >> offlining/memory hotunplug we have to offline whole memory blocks. For >> memory ballooning based mechanisms we simply allocate random free pages >> and eventually trigger reclaim to make more random free pages available. >> For memory hotunplug via virtio-mem we're using alloc_contig_range() to >> allocate ranges of interest we logically unplug. > > I heard about it from two people independently and I think that that's maybe > a rough idea, so if no one shows the clear use case or someone logically > shows that we don't need it, I do not head for it. I'd love to learn about use cases! > >> >> The only benefit compared to alloc_contig_range() might be that we can >> offline smaller chunks -- alloc_contig_range() isn't optimized for >> sub-MAX_ORDER granularity yet. But then, alloc_contig_range() should >> much rather be extended. > > If alloc_contig_range() supports memory offline in arbitrary size of > granurality (including a single page), maybe soft offline can be (partially > I guess) unified to it. Conceptually, memory offlining, alloc_contig_range(), soft-offlining all perform roughly the same thing just with different flavors: evacuate a given PFN area and decide what shall happen with it. After memory offlining, memory cannot get reused (e.g., allocated via the buddy) before re-onlining that memory. It's some kind of "fake allocation" + advanced magic to actually remove the memory from the system. alloc_contig_range() is just a "real" allocation, and can be used (e.g., by virtio-mem) for fake offlining by some additional magic on top. soft-offlining is just another special type of "special allocation", however, focused on individual page. The biggest difference between soft-offlining and alloc_contig_range()+memory offlining is right now the granularity. While alloc_contig_range() can be used to allocate individual pages on ZONE_MOVABLE and MIGRATE_CMA, it cannot deal with other zones with such small granularity yet -- too many false negatives, meaning an allocation might fail although the single page actually could get allocated. -- Thanks, David / dhildenb
