Message-ID: <0482f13b-c922-4f54-b32e-bb1dc27b44c3@redhat.com>
Date: Tue, 1 Jul 2025 12:10:51 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 virtualization@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Jerrin Shaji George <jerrin.shaji-george@...adcom.com>,
 Arnd Bergmann <arnd@...db.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Zi Yan <ziy@...dia.com>, Matthew Brost <matthew.brost@...el.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Minchan Kim <minchan@...nel.org>,
 Sergey Senozhatsky <senozhatsky@...omium.org>,
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
 Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>,
 Peter Xu <peterx@...hat.com>, Xu Xin <xu.xin16@....com.cn>,
 Chengming Zhou <chengming.zhou@...ux.dev>, Miaohe Lin
 <linmiaohe@...wei.com>, Naoya Horiguchi <nao.horiguchi@...il.com>,
 Oscar Salvador <osalvador@...e.de>, Rik van Riel <riel@...riel.com>,
 Harry Yoo <harry.yoo@...cle.com>, Qi Zheng <zhengqi.arch@...edance.com>,
 Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH v1 09/29] mm/migrate: factor out movable_ops page handling
 into migrate_movable_ops_page()

On 01.07.25 11:24, David Hildenbrand wrote:
> 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.

But reading the docs again:

"This routine need only be called for page cache pages which can 
potentially ever be mapped into the address space of a user process."

So, not required IIUC. I'll clarify in the patch description.

-- 
Cheers,

David / dhildenb

Powered by blists - more mailing lists