Re: [PATCH RFC v0 0/6] mm: proof-of-concept memory compaction without isolation

[Vlastimil Babka <vbabka@suse.cz>]

On 06/15/2015 09:50 AM, Konstantin Khlebnikov wrote:
> This is incomplete implementation of non-isolating memory migration and
> compaction. It's alive!
>
> The main reason -- it can preserve lru order during compaction.

That's nice, and there's also another benefit - no lru_lock taken during 
migration scanner.

So I think it's worth pursuing. But after brief checking, I'm not sure 
it can work as it is (but maybe I just overlooked something). What 
prevents somebody else to isolate the old page from the lru while you 
are migrating it? Especially in Patch 6, it appears that a PageLRU() is 
tested without any lock and only then insert_lru_page() takes the 
lru_lock to do something. This seems racy to me.

>
> Also it makes implementation of migration for various types of pages: zram,
> balloon, ptes, kernel stacks [ Why not? I've already migrated them accidentally
> and kernel have crashed in very funny places ] much easier: owner just have to
> set page->mappingw with valid method a_ops->migratepage.
>
> ---
>
> Konstantin Khlebnikov (6):
>        pagevec: segmented page vectors
>        mm/migrate: move putback of old page out of unmap_and_move
>        mm/cma: repalce reclaim_clean_pages_from_list with try_to_reclaim_page
>        mm/migrate: page migration without page isolation
>        mm/compaction: use migration without isolation
>        mm/migrate: preserve lru order if possible
>
>
>   include/linux/migrate.h           |    4 +
>   include/linux/mm.h                |    1
>   include/linux/pagevec.h           |   48 ++++++++-
>   include/trace/events/compaction.h |   12 +-
>   mm/compaction.c                   |  205 +++++++++++++++++++++----------------
>   mm/filemap.c                      |   20 ++++
>   mm/internal.h                     |   12 +-
>   mm/migrate.c                      |  141 +++++++++++++++++++++----
>   mm/page_alloc.c                   |   35 ++++--
>   mm/swap.c                         |   69 ++++++++++++
>   mm/vmscan.c                       |   42 +-------
>   11 files changed, 410 insertions(+), 179 deletions(-)
>
> --
> Konstantin
>
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