Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

From: Mike Rapoport <rppt@kernel.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Dave Hansen <dave.hansen@intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Joerg Roedel <jroedel@suse.de>, Ard Biesheuvel <ardb@kernel.org>,
	Andi Kleen <ak@linux.intel.com>,
	Kuppuswamy Sathyanarayanan 
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Varad Gautam <varad.gautam@suse.com>,
	Dario Faggioli <dfaggioli@suse.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	David Hildenbrand <david@redhat.com>,
	x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev,
	linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
	Mike Rapoport <rppt@linux.ibm.com>
Subject: Re: [PATCHv4 1/8] mm: Add support for unaccepted memory
Date: Wed, 13 Apr 2022 17:48:09 +0300	[thread overview]
Message-ID: <YlbiqdqCH+j7TK80@kernel.org> (raw)
In-Reply-To: <20220413114001.wdsi2xrm4btrghms@box.shutemov.name>

On Wed, Apr 13, 2022 at 02:40:01PM +0300, Kirill A. Shutemov wrote:
> On Mon, Apr 11, 2022 at 01:07:29PM +0300, Mike Rapoport wrote:
> > On Sun, Apr 10, 2022 at 11:38:08PM -0700, Dave Hansen wrote:
> > > On 4/9/22 08:54, Kirill A. Shutemov wrote:
> > > > On Fri, Apr 08, 2022 at 11:55:43AM -0700, Dave Hansen wrote:
> > > 
> > > >>>  	if (fpi_flags & FPI_TO_TAIL)
> > > >>>  		to_tail = true;
> > > >>>  	else if (is_shuffle_order(order))
> > > >>> @@ -1149,7 +1192,8 @@ static inline void __free_one_page(struct page *page,
> > > >>>  static inline bool page_expected_state(struct page *page,
> > > >>>  					unsigned long check_flags)
> > > >>>  {
> > > >>> -	if (unlikely(atomic_read(&page->_mapcount) != -1))
> > > >>> +	if (unlikely(atomic_read(&page->_mapcount) != -1) &&
> > > >>> +	    !PageUnaccepted(page))
> > > >>>  		return false;
> > > >>
> > > >> That probably deserves a comment, and maybe its own if() statement.
> > > > 
> > > > Own if does not work. PageUnaccepted() is encoded in _mapcount.
> > > > 
> > > > What about this:
> > > > 
> > > > 	/*
> > > > 	 * page->_mapcount is expected to be -1.
> > > > 	 *
> > > > 	 * There is an exception for PageUnaccepted(). The page type can be set
> > > > 	 * for pages on free list. Page types are encoded in _mapcount.
> > > > 	 *
> > > > 	 * PageUnaccepted() will get cleared in post_alloc_hook().
> > > > 	 */
> > > > 	if (unlikely((atomic_read(&page->_mapcount) | PG_unaccepted) != -1))
> > 
> > Maybe I'm missing something, but isn't this true for any PageType?
> 
> PG_buddy gets clear on remove from the free list, before the chec.
> 
> PG_offline and PG_table pages are never on free lists.

Right, this will work 'cause PageType is inverted. I still think this
condition is hard to parse and I liked the old variant with
!PageUnaccepted() better.

Maybe if we wrap the whole construct in a helper it will be less eye
hurting.

> -- 
>  Kirill A. Shutemov

-- 
Sincerely yours,
Mike.