Message-ID: <315c5e9a-8fbf-4b9c-98b2-1ea69065af85@redhat.com>
Date: Tue, 1 Jul 2025 11:24:58 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH v1 09/29] mm/migrate: factor out movable_ops page handling
 into migrate_movable_ops_page()

On 30.06.25 19:05, Lorenzo Stoakes wrote:
> On Mon, Jun 30, 2025 at 02:59:50PM +0200, David Hildenbrand wrote:
>> Let's factor it out, simplifying the calling code.
>>
>> The assumption is that flush_dcache_page() is not required for
>> movable_ops pages: as documented for flush_dcache_folio(), it really
>> only applies when the kernel wrote to pagecache pages / pages in
>> highmem. movable_ops callbacks should be handling flushing
>> caches if ever required.
> 
> But we've enot changed this have we? The flush_dcache_folio() invocation seems
> to happen the same way now as before? Did I miss something?

I think, before this change we would have called it also for movable_ops 
pages


if (rc == MIGRATEPAGE_SUCCESS) {
	if (__folio_test_movable(src)) {
		...
	}

	...

	if (likely(!folio_is_zone_device(dst)))
		flush_dcache_folio(dst);
}

Now, we no longer do that for movable_ops pages.

For balloon pages, we're not copying anything, so we never possibly have 
to flush the dcache.

For zsmalloc, we do the copy in zs_object_copy() through kmap_local.

I think we could have HIGHMEM, so I wonder if we should just do a 
flush_dcache_page() in zs_object_copy().

At least, staring at highmem.h with memcpy_to_page(), it looks like that 
might be the right thing to do.


So likely I'll add a patch before this one that will do the 
flush_dcache_page() in there.

> 
>>
>> Note that we can now change folio_mapping_flags() to folio_test_anon()
>> to make it clearer, because movable_ops pages will never take that path.
>>
>> Reviewed-by: Zi Yan <ziy@...dia.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Have scrutinised this a lot and it seems correct to me, so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> 
>> ---
>>   mm/migrate.c | 82 ++++++++++++++++++++++++++++------------------------
>>   1 file changed, 45 insertions(+), 37 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index d97f7cd137e63..0898ddd2f661f 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -159,6 +159,45 @@ static void putback_movable_ops_page(struct page *page)
>>   	folio_put(folio);
>>   }
>>
>> +/**
>> + * migrate_movable_ops_page - migrate an isolated movable_ops page
>> + * @page: The isolated page.
>> + *
>> + * Migrate an isolated movable_ops page.
>> + *
>> + * If the src page was already released by its owner, the src page is
>> + * un-isolated (putback) and migration succeeds; the migration core will be the
>> + * owner of both pages.
>> + *
>> + * If the src page was not released by its owner and the migration was
>> + * successful, the owner of the src page and the dst page are swapped and
>> + * the src page is un-isolated.
>> + *
>> + * If migration fails, the ownership stays unmodified and the src page
>> + * remains isolated: migration may be retried later or the page can be putback.
>> + *
>> + * TODO: migration core will treat both pages as folios and lock them before
>> + * this call to unlock them after this call. Further, the folio refcounts on
>> + * src and dst are also released by migration core. These pages will not be
>> + * folios in the future, so that must be reworked.
>> + *
>> + * Returns MIGRATEPAGE_SUCCESS on success, otherwise a negative error
>> + * code.
>> + */
> 
> Love these comments you're adding!!
> 
>> +static int migrate_movable_ops_page(struct page *dst, struct page *src,
>> +		enum migrate_mode mode)
>> +{
>> +	int rc = MIGRATEPAGE_SUCCESS;
> 
> Maybe worth asserting src, dst locking?

We do have these sanity checks right now in move_to_new_folio() already. 
(next patch moves it further out)

Not sure how reasonable these sanity checks are in these internal 
helpers: E.g., after we called move_to_new_folio() we will unlock both 
folios, which should blow up if the folios wouldn't be locked.

-- 
Cheers,

David / dhildenb

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