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 12:50:46 +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 06:49, 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.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Forgot to comment on something:
> 
>> @@ -1517,14 +1514,13 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
>>   			if (pmd_swp_uffd_wp(pmd))
>>   				flags |= PM_UFFD_WP;
>>   			VM_BUG_ON(!is_pmd_migration_entry(pmd));
>> -			migration = is_migration_entry(entry);
>>   			page = pfn_swap_entry_to_page(entry);
> 
> We do not really need to get the page anymore here as that is the non-present
> part.
> 
> Then we could get away without checking the flags as only page != NULL
> would mean a present pmd.
> 
> Not that we gain much as this is far from being a hot-path, but just
> saying..

I *think* we still want that for indicating PM_FILE after patch #1.

-- 
Cheers,

David / dhildenb