linux-kernel - Re: [PATCH v1 2/6] fs/proc/task_mmu: don't indicate PM_MMAP_EXCLUSIVE without PM

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Date: Tue, 11 Jun 2024 12:45:25 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v1 2/6] fs/proc/task_mmu: don't indicate PM_MMAP_EXCLUSIVE
 without PM_PRESENT

On 10.06.24 06:38, Oscar Salvador wrote:
> On Fri, Jun 07, 2024 at 02:23:53PM +0200, David Hildenbrand wrote:
>> Relying on the mapcount for non-present PTEs that reference pages
>> doesn't make any sense: they are not accounted in the mapcount, so
>> page_mapcount() == 1 won't return the result we actually want to know.
>>
>> While we don't check the mapcount for migration entries already, we
>> could end up checking it for swap, hwpoison, device exclusive, ...
>> entries, which we really shouldn't.
>>
>> There is one exception: device private entries, which we consider
>> fake-present (e.g., incremented the mapcount). But we won't care about
>> that for now for PM_MMAP_EXCLUSIVE, because indicating PM_SWAP for them
>> although they are fake-present already sounds suspiciously wrong.
>>
>> Let's never indicate PM_MMAP_EXCLUSIVE without PM_PRESENT.
> 
> Alternatively we could use is_pfn_swap_entry?

It's all weird, because only device private fake swp entries are 
fake-present. For these, we might want to use PM_PRESENT, but I don't 
care enough about device private entries to handle that here in a better 
way :)

Indicating PM_SWAP for something that is not swap (migration/poison/...) 
is also a bit weird. But likely nobody cared about that for now: it's 
either present (PM_PRESENT), something else (PM_SWAP), or nothing is 
there (no bit set).

Thanks!

-- 
Cheers,

David / dhildenb