Re: [PATCH] move_pages.2: not return ENOENT if the page are already on the target nodes

On 12/6/19 1:45 AM, Michal Hocko wrote:

On Fri 06-12-19 00:25:53, John Hubbard wrote:

quoted

On 12/5/19 5:34 PM, Yang Shi wrote:

quoted

Since commit e78bbfa82624 ("mm: stop returning -ENOENT
from sys_move_pages() if nothing got migrated"), move_pages doesn't
return -ENOENT anymore if the pages are already on the target nodes, but
this change is never reflected in manpage.

Cc: Michael Kerrisk <redacted>
Cc: Christoph Lameter <redacted>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Qian Cai <redacted>
Signed-off-by: Yang Shi <redacted>
---
   man2/move_pages.2 | 5 ++---
   1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/man2/move_pages.2 b/man2/move_pages.2
index 2d96468..2a2f3cd 100644
--- a/man2/move_pages.2
+++ b/man2/move_pages.2

@@ -192,9 +192,8 @@ was specified or an attempt was made to migrate pages of a kernel thread.
   One of the target nodes is not online.
   .TP
   .B ENOENT
-No pages were found that require moving.
-All pages are either already
-on the target node, not present, had an invalid address or could not be
+No pages were found.
+All pages are either not present, had an invalid address or could not be
   moved because they were mapped by multiple processes.
   .TP
   .B EPERM

whoa, hold on. If I'm reading through the various error paths correctly, then this
code is *never* going to return ENOENT for the whole function. It can fill in that
value per-page, in the status array, but that's all. Did I get that right?

You are right. Both store_status and do_move_pages_to_node do overwrite
the error code. So you are right that ENOENT return value is not
possible. I haven't checked since when this is the case. This whole
syscall is a disaster from the API and documentation POV.

It looks since commit e78bbfa82624 ("mm: stop returning -ENOENT from sys_move_pages() if nothing got migrated") too, which reset err to 0 unconditionally. It seems it is on purpose by that commit the syscall caller should check status for the details according to the commit log.

Btw. Page states error codes could see some refinements as well.