Re: [RFC] Missing compound_head() in memory-failure

[Jane Chu <jane.chu@oracle.com>]

On 2/1/2022 7:46 AM, Matthew Wilcox wrote:
> On Mon, Jan 31, 2022 at 08:54:39PM +0000, Joao Martins wrote:
>> On 1/31/22 20:29, Matthew Wilcox wrote:
>>> Unless I am mistaken, you have to pass the compound head of the page
>>> which has the error to collect_procs().  Am I mistaken?
>>>
>> -rc2 already has a fix for it:
>>
>> https://lore.kernel.org/linux-mm/20220129021420.PgBIZm-q9%25akpm@linux-foundation.org/
>>
>> Earlier in that function there's a:
>>
>> 	page = compound_head(page);
>>
>> So the @page passed to collect_procs() already is a head page.
> 
> It's wrong though ;-(  You set the HWPoison bit on the page after
> calling compound_head(), so you set the bit on the head page instead
> of the precise page that had the poison.

Indeed. The rest of the kernel including  pmem driver still deal with
base page on clearing poison, bookkeeping etc. So the HWpoison bit needs 
to be set precisely on the poisoned base page such that we pass the 
correct 'pfn' to set_mce_nospec() to discourage speculative access.

> 
> I'm fixing this up as part of the folio patches, but you may wish to
> fix it earlier than that.

Thanks for the fix!

-jane