Messages in this thread | | | Subject | Re: [PATCH v2 2/5] mm/page_alloc: place pages to tail in __putback_isolated_page() | From | Vlastimil Babka <> | Date | Tue, 20 Oct 2020 19:18:00 +0200 |
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On 10/5/20 2:15 PM, David Hildenbrand wrote: > __putback_isolated_page() already documents that pages will be placed to > the tail of the freelist - this is, however, not the case for > "order >= MAX_ORDER - 2" (see buddy_merge_likely()) - which should be > the case for all existing users. > > This change affects two users: > - free page reporting > - page isolation, when undoing the isolation (including memory onlining). > > This behavior is desireable for pages that haven't really been touched > lately, so exactly the two users that don't actually read/write page > content, but rather move untouched pages. > > The new behavior is especially desirable for memory onlining, where we > allow allocation of newly onlined pages via undo_isolate_page_range() > in online_pages(). Right now, we always place them to the head of the > freelist, resulting in undesireable behavior: Assume we add > individual memory chunks via add_memory() and online them right away to > the NORMAL zone. We create a dependency chain of unmovable allocations > e.g., via the memmap. The memmap of the next chunk will be placed onto > previous chunks - if the last block cannot get offlined+removed, all > dependent ones cannot get offlined+removed. While this can already be > observed with individual DIMMs, it's more of an issue for virtio-mem > (and I suspect also ppc DLPAR). > > Document that this should only be used for optimizations, and no code > should rely on this behavior for correction (if the order of the > freelists ever changes). > > We won't care about page shuffling: memory onlining already properly > shuffles after onlining. free page reporting doesn't care about > physically contiguous ranges, and there are already cases where page > isolation will simply move (physically close) free pages to (currently) > the head of the freelists via move_freepages_block() instead of > shuffling. If this becomes ever relevant, we should shuffle the whole > zone when undoing isolation of larger ranges, and after > free_contig_range(). > > Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> > Reviewed-by: Oscar Salvador <osalvador@suse.de> > Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com> > Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com> > Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
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