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

[Joao Martins <joao.m.martins@oracle.com>]

On 2/1/22 15:46, 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.
> 
Considering that on device-dax we would unmap the whole 2M page regardless
of the poisoned subpage isn't that actually representative still?

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