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SubjectRe: [v3 PATCH] mm: move_pages: return valid node id in status if the page is already on the target node
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On 12/5/19 4:19 PM, Qian Cai wrote:
>
>> On Dec 5, 2019, at 7:04 PM, John Hubbard <jhubbard@nvidia.com> wrote:
>>
>> Felix's code is not random test code. It's code he wrote and he expected it to work.
> Sure, but could he show a bit detail if the kernel will start to behavior as expected like what was written in the manpage to return ENOENT in this case, why is it not acceptable? i.e., why is it important to depend on the broken behavior?


I think I found the root cause. It did return -ENOENT when the syscall
was introduced (my first impression was wrong), but the behavior was
changed since 2.6.28 by commit e78bbfa82624 ("mm: stop returning -ENOENT
from sys_move_pages() if nothing got migrated"). The full commit log is
as below:

Author: Brice Goglin <Brice.Goglin@inria.fr>
Date:   Sat Oct 18 20:27:15 2008 -0700

    mm: stop returning -ENOENT from sys_move_pages() if nothing got
migrated

    A patchset reworking sys_move_pages().  It removes the possibly large
    vmalloc by using multiple chunks when migrating large buffers. It also
    dramatically increases the throughput for large buffers since the
lookup
    in new_page_node() is now limited to a single chunk, causing the
quadratic
    complexity to have a much slower impact.  There is no need to use any
    radix-tree-like structure to improve this lookup.

    sys_move_pages() duration on a 4-quadcore-opteron 2347HE (1.9Gz),
    migrating between nodes #2 and #3:

            length          move_pages (us)         move_pages+patch (us)
            4kB             126                     98
            40kB            198                     168
            400kB           963                     937
            4MB             12503                   11930
            40MB            246867                  11848

    Patches #1 and #4 are the important ones:
    1) stop returning -ENOENT from sys_move_pages() if nothing got migrated
    2) don't vmalloc a huge page_to_node array for do_pages_stat()
    3) extract do_pages_move() out of sys_move_pages()
    4) rework do_pages_move() to work on page_sized chunks
    5) move_pages: no need to set pp->page to ZERO_PAGE(0) by default

    This patch:

    There is no point in returning -ENOENT from sys_move_pages() if all
pages
    were already on the right node, while we return 0 if only 1 page
was not.
    Most application don't know where their pages are allocated, so
it's not
    an error to try to migrate them anyway.

    Just return 0 and let the status array in user-space be checked if the
    application needs details.

    It will make the upcoming chunked-move_pages() support much easier.

    Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
    Acked-by: Christoph Lameter <cl@linux-foundation.org>
    Cc: Nick Piggin <nickpiggin@yahoo.com.au>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>


So the behavior was changed in kernel intentionally but never reflected
in the manpage. I will propose a patch to fix the document.

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