On Thu, Jun 17, 2021 at 06:00:21PM +0800, Ding Hui wrote:

On 2021/6/14 10:12, Naoya Horiguchi wrote:

quoted

From: Naoya Horiguchi <redacted>

After recent soft-offline rework, error pages can be taken off from
buddy allocator, but the existing unpoison_memory() does not properly
undo the operation.  Moreover, due to the recent change on
__get_hwpoison_page(), get_page_unless_zero() is hardly called for
hwpoisoned pages.  So __get_hwpoison_page() mostly returns zero (meaning
to fail to grab page refcount) and unpoison just clears PG_hwpoison
without releasing a refcount.  That does not lead to a critical issue
like kernel panic, but unpoisoned pages never get back to buddy (leaked
permanently), which is not good.

As I mention in [1], I'm not sure about the exactly meaning of "broken" in
unpoison_memory().

Maybe the misunderstanding is:

I think __get_hwpoison_page() mostly returns one for hwpoisoned page.
In 06be6ff3d2ec ("mm,hwpoison: rework soft offline for free pages"),
page_handle_poison() is introduced, it will add refcount for all
soft-offlineed hwpoison page.
In memory_failure() for hard-offline，page_ref_inc() called on free page
too, and for used page, we do not call put_page() after get_hwpoison_page()
!= 0.
So all hwpoisoned page refcount must be great than zero when
unpoison_memory() if regardless of racy.

Hi, Ding,

Thanks for the comment.  I feel that I failed to define the exact issue in
unpoison.  Maybe I saw and misinterpreted some random error as unpoison's
issue during developing other hwpoison patches, so please don't take serious
my previous wrong word "broken", sorry about that.

Anyway I reconsider how to handle this 6/6, maybe it will be a clear
description of the problem, and will be simplified.

Recently I tested loop soft-offline random pages and unpoison them for days,
it works fine to me. (with bac9c6fa1f92 patched)

Thank you for testing,

Thanks,
Naoya Horiguchi