linux-kernel - Re: [PATCH v1 12/29] mm/zsmalloc: stop using _

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Message-ID: <vscedd6m3cq73c5ggjjz6ndordivgeh4dmvzeok222bnderr5c@dujm4ndthtxb>
Date: Thu, 3 Jul 2025 12:22:23 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
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	John Hubbard <jhubbard@...dia.com>, Peter Xu <peterx@...hat.com>, Xu Xin <xu.xin16@....com.cn>, 
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	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v1 12/29] mm/zsmalloc: stop using __ClearPageMovable()

On (25/07/03 11:28), Sergey Senozhatsky wrote:
> > > > > >    static int zs_page_migrate(struct page *newpage, struct page *page,
> > > > > > @@ -1736,6 +1736,13 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
> > > > > >    	unsigned long old_obj, new_obj;
> > > > > >    	unsigned int obj_idx;
> > > > > > +	/*
> > > > > > +	 * TODO: nothing prevents a zspage from getting destroyed while
> > > > > > +	 * isolated: we should disallow that and defer it.
> > > > > > +	 */
> > > > > 
> > > > > Can you elaborate?
> > > > 
> > > > We can only free a zspage in free_zspage() while the page is locked.
> > > > 
> > > > After we isolated a zspage page for migration (under page lock!), we drop
> > >                        ^^ a physical page? (IOW zspage chain page?)
> > > 
> > > > the lock again, to retake the lock when trying to migrate it.
> > > > 
> > > > That means, there is a window where a zspage can be freed although the page
> > > > is isolated for migration.
> > > 
> > > I see, thanks.  Looks somewhat fragile.  Is this a new thing?
> > 
> > No, it's been like that forever. And I was surprised that only zsmalloc
> > behaves that way
> 
> Oh, that makes two of us.

I sort of wonder if zs_page_migrate() VM_BUG_ON_PAGE() removal and
zspage check addition need to be landed outside of this series, as
a zsmalloc fixup.