Re: + compaction-add-def_blk_aops-migrate-function-for-memory-compaction.patch added to -mm tree

[Vlastimil Babka <vbabka@suse.cz>]

On 03/09/2017 12:55 AM, akpm@linux-foundation.org wrote:
> 
> The patch titled
>      Subject: compaction: add def_blk_aops migrate function for memory compaction
> has been added to the -mm tree.  Its filename is
>      compaction-add-def_blk_aops-migrate-function-for-memory-compaction.patch
> 
> This patch should soon appear at
>     http://ozlabs.org/~akpm/mmots/broken-out/compaction-add-def_blk_aops-migrate-function-for-memory-compaction.patch
> and later at
>     http://ozlabs.org/~akpm/mmotm/broken-out/compaction-add-def_blk_aops-migrate-function-for-memory-compaction.patch
> 
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>    c) Ideally: find the original patch on the mailing list and do a
>       reply-to-all to that, adding suitable additional cc's
> 
> *** Remember to use Documentation/SubmitChecklist when testing your code ***
> 
> The -mm tree is included into linux-next and is updated
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> 
> ------------------------------------------------------
> From: zhouxianrong <zhouxianrong@huawei.com>
> Subject: compaction: add def_blk_aops migrate function for memory compaction

That's not really a mm/compaction patch, but a block layer/migration patch. I
don't know internals of those so well, so I added some CC's.

> The reason for doing this is based on two factors.
> 
> 1. larg file read/write operations with order 0 can fragmentize
>    memory rapidly.
> 
> 2. when a special filesystem does not supply migratepage callback,
>    kernel would fallback to default function fallback_migrate_page.
>    but fallback_migrate_page could not migrate diry page nicely;
>    specially kcompactd with MIGRATE_SYNC_LIGHT could not migrate
>    diry pages due to this until clear_page_dirty_for_io in some
>    procedure. i think it is not suitable here in this scenario.
>    for dirty pages we should migrate it rather than skip or writeout
>    it in kcomapctd with MIGRATE_SYNC_LIGHT. i think this problem is
>    for all filesystem without migratepage not only for block device fs.
> 
> So for compaction under large file writing supply migratepage for
> def_blk_aops.

Is this really safe to do? buffer_migrate_page() has some assumptions listed in
its comment (and maybe more that are not listed). Do we know it's safe to use it
for all def_blk_aops users?

> Link: http://lkml.kernel.org/r/1488937915-78955-1-git-send-email-zhouxianrong@huawei.com
> Signed-off-by: zhouxianrong <zhouxianrong@huawei.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: <Mi.Sophia.Wang@huawei.com>
> Cc: <zhouxiyu@huawei.com>
> Cc: <weidu.du@huawei.com>
> Cc: <zhangshiming5@huawei.com>
> Cc: <won.ho.park@huawei.com>
> Cc: <zhouxiaoyan1@huawei.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  fs/block_dev.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff -puN fs/block_dev.c~compaction-add-def_blk_aops-migrate-function-for-memory-compaction fs/block_dev.c
> --- a/fs/block_dev.c~compaction-add-def_blk_aops-migrate-function-for-memory-compaction
> +++ a/fs/block_dev.c
> @@ -2064,6 +2064,9 @@ static const struct address_space_operat
>  	.releasepage	= blkdev_releasepage,
>  	.direct_IO	= blkdev_direct_IO,
>  	.is_dirty_writeback = buffer_check_dirty_writeback,
> +#ifdef CONFIG_MIGRATION
> +	.migratepage = buffer_migrate_page,
> +#endif
>  };
>  
>  #define	BLKDEV_FALLOC_FL_SUPPORTED					\
> _
> 
> Patches currently in -mm which might be from zhouxianrong@huawei.com are
> 
> compaction-add-def_blk_aops-migrate-function-for-memory-compaction.patch
>