linux-kernel - Re: [PATCH v1 13/29] mm/balloon_compaction: stop using _

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: <4d1d7aa7-15c7-4c2c-8cd7-0853dcde7940@lucifer.local>
Date: Tue, 1 Jul 2025 11:40:53 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.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 13/29] mm/balloon_compaction: stop using
 __ClearPageMovable()

On Tue, Jul 01, 2025 at 12:19:47PM +0200, David Hildenbrand wrote:
> On 01.07.25 12:03, Lorenzo Stoakes wrote:
> > On Mon, Jun 30, 2025 at 02:59:54PM +0200, David Hildenbrand wrote:
> > > We can just look at the balloon device (stored in page->private), to see
> > > if the page is still part of the balloon.
> > >
> > > As isolated balloon pages cannot get released (they are taken off the
> > > balloon list while isolated), we don't have to worry about this case in
> > > the putback and migration callback. Add a WARN_ON_ONCE for now.
> > >
> > > Signed-off-by: David Hildenbrand <david@...hat.com>
> >
> > Seems reasonable, one comment below re: comment.
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> >
> > > ---
> > >   include/linux/balloon_compaction.h |  4 +---
> > >   mm/balloon_compaction.c            | 11 +++++++++++
> > >   2 files changed, 12 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> > > index bfc6e50bd004b..9bce8e9f5018c 100644
> > > --- a/include/linux/balloon_compaction.h
> > > +++ b/include/linux/balloon_compaction.h
> > > @@ -136,10 +136,8 @@ static inline gfp_t balloon_mapping_gfp_mask(void)
> > >    */
> > >   static inline void balloon_page_finalize(struct page *page)
> > >   {
> > > -	if (IS_ENABLED(CONFIG_BALLOON_COMPACTION)) {
> > > -		__ClearPageMovable(page);
> > > +	if (IS_ENABLED(CONFIG_BALLOON_COMPACTION))
> > >   		set_page_private(page, 0);
> > > -	}
> > >   	/* PageOffline is sticky until the page is freed to the buddy. */
> > >   }
> > >
> > > diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> > > index ec176bdb8a78b..e4f1a122d786b 100644
> > > --- a/mm/balloon_compaction.c
> > > +++ b/mm/balloon_compaction.c
> > > @@ -206,6 +206,9 @@ static bool balloon_page_isolate(struct page *page, isolate_mode_t mode)
> > >   	struct balloon_dev_info *b_dev_info = balloon_page_device(page);
> > >   	unsigned long flags;
> > >
> > > +	if (!b_dev_info)
> > > +		return false;
> > > +
> > >   	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> > >   	list_del(&page->lru);
> > >   	b_dev_info->isolated_pages++;
> > > @@ -219,6 +222,10 @@ static void balloon_page_putback(struct page *page)
> > >   	struct balloon_dev_info *b_dev_info = balloon_page_device(page);
> > >   	unsigned long flags;
> > >
> > > +	/* Isolated balloon pages cannot get deflated. */
> > > +	if (WARN_ON_ONCE(!b_dev_info))
> > > +		return;
> > > +
> > >   	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> > >   	list_add(&page->lru, &b_dev_info->pages);
> > >   	b_dev_info->isolated_pages--;
> > > @@ -234,6 +241,10 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
> > >   	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > >   	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
> > >
> > > +	/* Isolated balloon pages cannot get deflated. */
> >
> > Hm do you mean migrated?
>
> Well, they can get migrated, obviously :)

Right yeah we isolate to migrate :P

I guess I was confused by the 'get' deflated, wrongly thinking putback was doing
this (I got confused about terminology), but I see in balloon_page_dequeue() we
balloon_page_finalize() which sets page->private = NULL which is what
balloon_page_device() returns.

OK I guess this is fine... :)

An aside, unrelated tot his series: it'd be nice to use 'deflate' consistently
in this code. We do __count_vm_event(BALLOON_DEFLATE) in
balloon_page_list_dequeue() but say 'deflate' nowhere else... well before this
patch :)

>
> Deflation would be the other code path where we would remove a balloon page
> from the balloon, and invalidate page->private, suddenly seeing !b_dev_info
> here.

ACtually it's 'balloon' not b_dev_info. Kind of out of scope for this patch but
would be good to rename.

>
> But that cannot happen, as isolation takes them off the balloon list. So
> deflating the balloon cannot find them until un-isolated.

Ack.

>
> --
> Cheers,
>
> David / dhildenb
>