From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CD5C63D9543 for ; Mon, 3 Aug 2026 09:19:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785748750; cv=none; b=mbZwrPqDJqtSpZt64iI1sBZNUg4ZZ2m1NqZLSGvq/TluDBoqLpC/ZD5AunOl+gMOtidKCnrtevTZFNHeDfGyuKpbf2d++wDt4nOifgQZ3qqePc4WgfeUgjV3dXLDdAJvya0yyWA6NEXgXeumu3JhMRsGofxnR3qXG+OuTh5T5FY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785748750; c=relaxed/simple; bh=W5ypENlx3yxsg+hnBw9QXKXcfMN78U8u3lvaLkeymps=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=m8DMbchZVo6w4AoG6UdoOfLQEdZ2jNCB7sfsUKnNS0A7YNo4TD0B16YkqUBD06Lp/Ufk5q4jpsex38sitjGqVknJMiCCir+uIiRf/Iuf04/MC+a5omE88Szr4cdoDEeEggPBpUtCmpB4SNRQAUIC4e9+myT+KSRE9I/SuOteMV8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SoVb+i+B; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SoVb+i+B" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BB2AE1F000E9; Mon, 3 Aug 2026 09:19:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785748745; bh=5NLzFDn/DIDYnKuFtRjjdVsbfvke595vqQGCR+hV60w=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=SoVb+i+BWfxBsK9hIBTWcdEhCUza72DRdfmxdWPqDZaXCntv5d2pXvlMMihf4dJ8C tVuX/WqtRM/X50+M2IyYy9p3p26rzqBS7Nb2IsI56aoiXDbrJ+9N/kfQokcenLrJDX 0YXrahvVbN34EjchyxMxNyuBtu9Ef0qAtmAU7H5OpEbXkxrAfQ4bZ+a/GqN59ScdqF NIdDtypnnVFWMtzJjEAqmN0/9TpnfDjl4WdpZuJD9tS+/R96bO0MOtqa8BJ4dsCwZ1 7oDtMd3y8erfmlm9Xm3IcbQL0Db3nOuiP51EU7fZ6h/IAIO6+H5rI3VHvh29EDmuG4 U7wCXvN1fjr+w== Message-ID: Date: Mon, 3 Aug 2026 11:18:58 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 21/26] mm/page_alloc: implement FREETYPE_UNMAPPED allocations Content-Language: en-US To: Brendan Jackman , Borislav Petkov , Dave Hansen , Peter Zijlstra , Andrew Morton , David Hildenbrand , Mike Rapoport , Wei Xu , Johannes Weiner , Zi Yan , Lorenzo Stoakes Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, x86@kernel.org, Sumit Garg , Will Deacon , rientjes@google.com, "Kalyazin, Nikita" , patrick.roy@linux.dev, "Itazuri, Takahiro" , Andy Lutomirski , David Kaplan , Thomas Gleixner , Yosry Ahmed , Patrick Bellasi , Reiji Watanabe , Sean Christopherson References: <20260726-page_alloc-unmapped-v3-0-6f5729aa9832@google.com> <20260726-page_alloc-unmapped-v3-21-6f5729aa9832@google.com> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/27/26 00:22, Brendan Jackman wrote: > Currently FREETYPE_UNMAPPED allocs will always fail because, although the > lists exist to hold them, there is no way to actually create an unmapped > page block. This commit adds one, and also the logic to map it back > again when that's needed. > > Doing this at pageblock granularity ensures that the pageblock flags can > be used to infer which freetype a page belongs to. It also provides nice > batching of TLB flushes, and also avoids creating too much unnecessary > TLB fragmentation in the physmap. > > There are some functional requirements for flipping a block: > > - Unmapping requires a TLB shootdown, meaning IRQs must be enabled. > > - Updating the pagetables might require allocating a pagetable to break > down a huge page. This would deadlock if the zone lock was held. > > This makes allocations that need to change sensitivity _somewhat_ > similar to those that need to fallback to a different migratetype. But, > the locking requirements mean that this can't just be squashed into the > existing "fallback" allocator logic, instead a new allocator path just > for this purpose is needed. > > The new path is assumed to be much cheaper than the really heavyweight > stuff like compaction and reclaim. But at present it is treated as less > desirable than the mobility-related "fallback" and "stealing" logic. > This might turn out to need revision (in particular, maybe it's a > problem that __rmqueue_steal(), which causes fragmentation, happens > before __rmqueue_direct_map()), but that should be treated as a subsequent > optimisation project. > > Adding alloc_flags to gfp_freetype() requires moving it to > mm/page_alloc.h so it can refer to ALLOC_UNMAPPED. It was already only > used in internal mm code. > > Now that unmapped pageblocks actually exist, exclude them from > migration. Migrating unmapped pages via the mermap should be possible > but that's something to be added later when needed. > > Signed-off-by: Brendan Jackman Reviewed-by: Vlastimil Babka (SUSE) A nit below: > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -34,6 +34,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -1001,6 +1002,26 @@ static void change_pageblock_range(struct page *pageblock_page, > } > } > > +/* > + * Can pages of these two freetypes be combined into a single higher-order free > + * page? > + */ > +static inline bool can_merge_freetypes(freetype_t a, freetype_t b) > +{ > + if (freetypes_equal(a, b)) > + return true; > + > + if (!migratetype_is_mergeable(free_to_migratetype(a)) || > + !migratetype_is_mergeable(free_to_migratetype(b))) > + return false; > + > + /* > + * Mustn't "just" merge pages with different freetype flags, changing > + * those requires updating pagetables. > + */ > + return freetype_flags(a) == freetype_flags(b); > +} > + > /* > * Freeing function for a buddy system allocator. > * > @@ -1069,9 +1090,7 @@ static inline void __free_one_page(struct page *page, > buddy_ft = get_pfnblock_freetype(buddy, buddy_pfn); > buddy_mt = free_to_migratetype(buddy_ft); There's a comment above this that should probably be moved to can_merge_freetypes() as well?