On 10/24/2022 9:56 AM, Huang, Ying wrote:
Yang Shi <shy828301@xxxxxxxxx> writes:
On Fri, Oct 21, 2022 at 11:41 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 21 Oct 2022 18:16:23 +0800 Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
When THP migration, if THPs are split and all subpages are migrated successfully
, the migrate_pages() will still return the number of THP that were not migrated.
That will confuse the callers of migrate_pages(), for example, which will make
the longterm pinning failed though all pages are migrated successfully.
Thus we should return 0 to indicate all pages are migrated in this case.
This had me puzzled for a while. I think this wording is clearer?
: During THP migration, if THPs are not migrated but they are split and all
: subpages are migrated successfully, migrate_pages() will still return the
: number of THP pages that were not migrated. This will confuse the callers
: of migrate_pages(). For example, the longterm pinning will failed though
: all pages are migrated successfully.
:
: Thus we should return 0 to indicate that all pages are migrated in this
: case.
This is a fairly longstanding problem? No Fixes: we can identify?
It doesn't seem like a long standing issue. It seems like commit
b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
fixed one problem, but introduced this new one IIUC.
Before this commit, the code did:
nr_failed += retry + thp_retry;
rc = nr_failed;
But retry and thp_retry were actually reset for each retry until the
last one. So as long as there is no permanent migration failure and
THP split failure, nr_failed should be 0 IIUC. TBH the code is a
little bit hard to follow, please correct me if I'm wrong.
I think that you are correct. We can added
Fixes: b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
I think so too. Thanks Yang and Ying for pointing it out.
Did you consider the desirability of a -stable backport?
I think this can be backport to -stable.
Agree.
Andrew, could you help to add the Fixes tag and cc -stable? Thanks.
