Re: [PATCH 2/2] mm: memfd_luo: always dirty all folios

[Pratyush Yadav]

Hi Mike,

On Wed, Feb 25 2026, Mike Rapoport wrote:

> On Mon, Feb 23, 2026 at 06:39:29PM +0100, Pratyush Yadav wrote:
[...]
>> -		if (folio_test_dirty(folio))
>> -			flags |= MEMFD_LUO_FOLIO_DIRTY;
>> +		/*
>> +		 * A dirty folio is one which has been written to. A clean folio
>> +		 * is its opposite. Since a clean folio does not carry user
>> +		 * data, it can be freed by page reclaim under memory pressure.
>> +		 *
>> +		 * Saving the dirty flag at prepare() time doesn't work since it
>> +		 * can change later. Saving it at freeze() also won't work
>> +		 * because the dirty bit is normally synced at unmap and there
>> +		 * might still be a mapping of the file at freeze().
>> +		 *
>> +		 * To see why this is a problem, say a folio is clean at
>> +		 * preserve, but gets dirtied later. The pfolio flags will mark
>> +		 * it as clean. After retrieve, the next kernel might try to
>> +		 * reclaim this folio under memory pressure, losing user data.
>> +		 *
>> +		 * Unconditionally mark it dirty to avoid this problem. This
>> +		 * comes at the cost of making clean folios un-reclaimable after
>> +		 * live update.
>> +		 */
>
> Can we make the comment here shorter to only contain the gist of the issue?

Is this any better? Or should I try to make it shorter still?

	/*
	 * Tracking the dirty flag of the folio is difficult since it is
	 * normally synced at unmap and there might still be mappings of
	 * the file alive.
	 *
	 * Not tracking it correctly can cause a dirty folio to be
	 * restored as clean after KHO. The next kernel might then try
	 * to reclaim it, losing user data.
	 *
	 * Unconditionally mark the folio dirty to avoid this. This
	 * comes at the cost of making clean folios un-reclaimable.
	 */

[...]

-- 
Regards,
Pratyush Yadav