linux-kernel - Re: [PATCH v1 20/29] mm: convert "movable" flag in page->mapping to a page flag

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <e76b6a9d-b887-4790-a2fd-032648be99ff@redhat.com>
Date: Wed, 2 Jul 2025 17:25:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Harry Yoo <harry.yoo@...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>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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>,
 Qi Zheng <zhengqi.arch@...edance.com>, Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH v1 20/29] mm: convert "movable" flag in page->mapping to a
 page flag

On 02.07.25 15:01, Harry Yoo wrote:
> On Wed, Jul 02, 2025 at 02:01:33PM +0200, David Hildenbrand wrote:
>> On 02.07.25 13:54, Harry Yoo wrote:
>>> On Mon, Jun 30, 2025 at 03:00:01PM +0200, David Hildenbrand wrote:
>>>> Instead, let's use a page flag. As the page flag can result in
>>>> false-positives, glue it to the page types for which we
>>>> support/implement movable_ops page migration.
>>>>
>>>> The flag reused by PageMovableOps() might be sued by other pages, so
>>>> warning in case it is set in page_has_movable_ops() might result in
>>>> false-positive warnings.
>>>>
>>>> Reviewed-by: Zi Yan <ziy@...dia.com>
>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>> ---
>>>
>>> LGTM,
>>> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
>>>
>>> With a question: is there any reason to change the page flag
>>> operations to use atomic bit ops?
>>
>> As we have the page lock in there, it's complicated. I thought about this
>> when writing that code, and was not able to convince myself that it is safe.
>>
>> But that was when I was prototyping and reshuffling patches, and we would
>> still have code that would clear the flag.
>   
>> Given that we only allow setting the flag, it might be okay to use the
>> non-atomic variant as long as there can be nobody racing with us when
>> modifying flags. Especially trying to lock the folio concurrently is the big
>> problem.
>>
>> In isolate_movable_ops_page(), there is a comment about checking the flag
>> before grabbing the page lock, so that should be handled.
> 
> Right.
> 
>> I'll have to check some other cases in balloon/zsmalloc code.
> 
> Okay, it's totally fine to go with atomic version and then
> switching back to non atomic ops when we're sure it's safe.
> 

I'll definitely do the following:

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 8b23a74963feb..5f2b570735852 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1145,9 +1145,11 @@ PAGEFLAG(Isolated, isolated, PF_ANY);
   * the flag might be used in other context for other pages. Always use
   * page_has_movable_ops() instead.
   */
-PAGEFLAG(MovableOps, movable_ops, PF_NO_TAIL);
+TESTPAGEFLAG(MovableOps, movable_ops, PF_NO_TAIL);
+SETPAGEFLAG(MovableOps, movable_ops, PF_NO_TAIL);
  #else /* !CONFIG_MIGRATION */
-PAGEFLAG_FALSE(MovableOps, movable_ops);
+TESTPAGEFLAG_FALSE(MovableOps, movable_ops);
+SETPAGEFLAG_NOOP(MovableOps, movable_ops);
  #endif /* CONFIG_MIGRATION */

  /**

Because the flag must not get cleared.

There is no __SETPAGEFLAG_NOOP yet, unfortunately.
-- 
Cheers,

David / dhildenb