Re: [PATCH v2 2/5] mm/page_alloc: place pages to tail in __putback_isolated_page()

[Vlastimil Babka <vbabka@xxxxxxx>]

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@xxxxxxxxxxxxxxx>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>