Re: [PATCH 1/1] mm: memory-failure: Re-split hw-poisoned huge page on -EAGAIN

[Naoya Horiguchi <naoya.horiguchi@linux.dev>]

On Fri, Dec 15, 2023 at 04:12:04PM +0800, Qiuxu Zhuo wrote:
> During the process of splitting a hw-poisoned huge page, it is possible
> for the reference count of the huge page to be increased by the threads
> within the affected process, leading to a failure in splitting the
> hw-poisoned huge page with an error code of -EAGAIN.
> 
> This issue can be reproduced when doing memory error injection to a
> multiple-thread process, and the error occurs within a huge page.
> The call path with the returned -EAGAIN during the testing is shown below:
> 
>   memory_failure()
>     try_to_split_thp_page()
>       split_huge_page()
>         split_huge_page_to_list() {
>           ...
>           Step A: can_split_folio() - Checked that the thp can be split.
>           Step B: unmap_folio()
>           Step C: folio_ref_freeze() - Failed and returned -EAGAIN.
>           ...
>         }
> 
> The testing logs indicated that some huge pages were split successfully
> via the call path above (Step C was successful for these huge pages).
> However, some huge pages failed to split due to a failure at Step C, and
> it was observed that the reference count of the huge page increased between
> Step A and Step C.
> 
> Testing has shown that after receiving -EAGAIN, simply re-splitting the
> hw-poisoned huge page within memory_failure() always results in the same
> -EAGAIN. This is possible because memory_failure() is executed in the
> currently affected process. Before this process exits memory_failure() and
> is terminated, its threads could increase the reference count of the
> hw-poisoned page.
> 
> To address this issue, employ the kernel worker to re-split the hw-poisoned
> huge page. By the time this worker begins re-splitting the hw-poisoned huge
> page, the affected process has already been terminated, preventing its
> threads from increasing the reference count. Experimental results have
> consistently shown that this worker successfully re-splits these
> hw-poisoned huge pages on its first attempt.
> 
> The kernel log (before):
>   [ 1116.862895] Memory failure: 0x4097fa7: recovery action for unsplit thp: Ignored
> 
> The kernel log (after):
>   [  793.573536] Memory failure: 0x2100dda: recovery action for unsplit thp: Delayed
>   [  793.574666] Memory failure: 0x2100dda: split unsplit thp successfully.

I'm unclear about the user-visible benefit of ensuring that the error thp is split.
So could you explain about it?

I think that the raw error page is not unmapped (with hwpoisoned entry) after
delayed re-splitting, so recovery action seems not complete even with this patch.
So this patch seems to just convert a hwpoisoned unrecovered thp into a hwpoisoned
unrecovered raw page.

Thanks,
Naoya Horiguchi