linux-kernel - Re: [PATCH v2 1/2] mm: migrate: Fix return value if all subpages of THPs are migrated successfully

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Message-Id: <20221021114149.eadf6fafdd0b2c2409a46389@linux-foundation.org>
Date:   Fri, 21 Oct 2022 11:41:49 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     david@...hat.com, ying.huang@...el.com, ziy@...dia.com,
        shy828301@...il.com, apopple@...dia.com,
        jingshan@...ux.alibaba.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: migrate: Fix return value if all subpages of
 THPs are migrated successfully

On Fri, 21 Oct 2022 18:16:23 +0800 Baolin Wang <baolin.wang@...ux.alibaba.com> 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?

Did you consider the desirability of a -stable backport?