On 25.09.20 15:19, Oscar Salvador wrote:

On Wed, Sep 16, 2020 at 08:34:09PM +0200, David Hildenbrand wrote:

quoted

__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.

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
free list, 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).

Note: If we observe a degradation due to the changed page isolation
behavior (which I doubt), we can always make this configurable by the
instance triggering undo of isolation (e.g., alloc_contig_range(),
memory onlining, memory offlining).

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Duyck <redacted>
Cc: Mel Gorman <redacted>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Dave Hansen <redacted>
Cc: Vlastimil Babka <redacted>
Cc: Wei Yang <redacted>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Scott Cheloha <redacted>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: David Hildenbrand <redacted>

LGTM, the only thing is the shuffe_zone topic that Wei and Vlastimil rose.
Feels a bit odd that takes precedence over something we explicitily demanded.

Thanks, yeah I'll be changing that.

With the comment Vlastimil suggested:

Reviewed-by: Oscar Salvador <osalvador@suse.de>

-- 
Thanks,

David / dhildenb