From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 C48B53FBB3 for ; Tue, 1 Apr 2025 13:20:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743513602; cv=none; b=cBfQySdVXOJzlyZkgqIeBu9linyunCzC0pkjlhvdRAsrfpbADuRKOYBiRG+0iSjEtSREh2pljQAuq1/RiAwxzrFgWEyI+zHg673DyKjd7AS5ZRBhIj8z4o49OvEm3x44lNOBNmi0ls3k8ZfSK+RtavcHmGp8nG1pg5v3tXC3xvI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743513602; c=relaxed/simple; bh=9wSJvejkq5NxPrQKr0Jhsiek8qkHChwl57pzJsxs7qo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Ab1iV8TmiG/Em0jgt8oY8geaVWLm49rDxPFZcP0j+0l9pj/49E8KlkbMyW5jRoG8yRNj8zJwhEI+UFuwCiTlE0DvMOkXvy51nydWSVlDVS15oKImfuQaX520S6EdbG47KmtUutuREYmQDio9G3Rq1f/BfbFs0MQVsLj2LbqRhQw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YIaDl7Mv; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="YIaDl7Mv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DDE0CC4CEE4; Tue, 1 Apr 2025 13:19:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1743513602; bh=9wSJvejkq5NxPrQKr0Jhsiek8qkHChwl57pzJsxs7qo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=YIaDl7MvazjElWMG6+MRQRAKFy4pTiPH2Y5kKB+bwOeQGRBx28iNTxNhm+kBGiDtp 4YKO8ZQVxGwwSlju1t7NgbfrcAPwjIo8gvofNVbWgUvO0GBU0+rwWFjhkT3apkAmJ4 uY6pQqwyoDvqNlDYRZ6RCPga2q3TC6ojEw3OP+LuTxjk5AxZLRDTvJDoDvb6gOWS6h jWngKU0OhKMznLLv+XUMAgOMR4RZ/zSX670RxahSYlQ+eSNB6tA5K2+nNlGWzX4fN9 HG0WNS17ib8DRY6FK3ZVY7oTA0kM7Eb5cnhK7WDEDKBVJRdkqT7xn73igeNcfTzWSk KgnRbPM3oyH6A== Date: Tue, 1 Apr 2025 16:19:54 +0300 From: Mike Rapoport To: David Woodhouse Cc: Andrew Morton , "Sauerwein, David" , Anshuman Khandual , Ard Biesheuvel , Catalin Marinas , David Hildenbrand , Marc Zyngier , Mark Rutland , Mike Rapoport , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v4 2/4] memblock: update initialization of reserved pages Message-ID: References: <20210511100550.28178-1-rppt@kernel.org> <20210511100550.28178-3-rppt@kernel.org> <9f33c0b4517eaf5f36c515b92bdcb6170a4a576a.camel@infradead.org> <1787b97c267b53127c60a61419d99751f8a66b1a.camel@infradead.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1787b97c267b53127c60a61419d99751f8a66b1a.camel@infradead.org> On Tue, Apr 01, 2025 at 12:50:33PM +0100, David Woodhouse wrote: > On Tue, 2025-04-01 at 14:33 +0300, Mike Rapoport wrote: > > On Mon, Mar 31, 2025 at 04:13:56PM +0100, David Woodhouse wrote: > > > On Mon, 2025-03-31 at 17:50 +0300, Mike Rapoport wrote: > > > > On Mon, Mar 31, 2025 at 01:50:33PM +0100, David Woodhouse wrote: > > > > > On Tue, 2021-05-11 at 13:05 +0300, Mike Rapoport wrote: > > > > > > > > > > On platforms with large NOMAP regions (e.g. which are actually reserved > > > > > for guest memory to keep it out of the Linux address map and allow for > > > > > kexec-based live update of the hypervisor), this pointless loop ends up > > > > > taking a significant amount of time which is visible as guest steal > > > > > time during the live update. > > > > > > > > > > Can reserve_bootmem_region() skip the loop *completely* if no PFN in > > > > > the range from start to end is valid? Or tweak the loop itself to have > > > > > an 'else' case which skips to the next valid PFN? Something like > > > > > > > > > >  for(...) { > > > > >     if (pfn_valid(start_pfn)) { > > > > >        ... > > > > >     } else { > > > > >        start_pfn = next_valid_pfn(start_pfn); > > > > >     } > > > > >  } > > > > > > > > My understanding is that you have large reserved NOMAP ranges that don't > > > > appear as memory at all, so no memory map for them is created and so > > > > pfn_valid() is false for pfns in those ranges. > > > > > > > > If this is the case one way indeed would be to make > > > > reserve_bootmem_region() skip ranges with no valid pfns. > > > > > > > > Another way could be to memblock_reserved_mark_noinit() such ranges and > > > > then reserve_bootmem_region() won't even get called, but that would require > > > > firmware to pass that information somehow. > > > > > > I was thinking along these lines (not even build tested)... > > > > > > I don't much like the (unsigned long)-1 part. I might make the helper > > > 'static inline bool first_valid_pfn (unsigned long *pfn)' and return > > > success or failure. But that's an implementation detail. > > > > > > index 6d1fb6162ac1..edd27ba3e908 100644 > > > --- a/include/asm-generic/memory_model.h > > > +++ b/include/asm-generic/memory_model.h > > > @@ -29,8 +29,43 @@ static inline int pfn_valid(unsigned long pfn) > > >         return pfn >= pfn_offset && (pfn - pfn_offset) < max_mapnr; > > >  } > > >  #define pfn_valid pfn_valid > > > + > > > +static inline unsigned long first_valid_pfn(unsigned long pfn) > > > +{ > > > +       /* avoid include hell */ > > > +       extern unsigned long max_mapnr; > > > +       unsigned long pfn_offset = ARCH_PFN_OFFSET; > > > + > > > +       if (pfn < pfn_offset) > > > +               return pfn_offset; > > > + > > > +       if ((pfn - pfn_offset) < max_mapnr) > > > +               return pfn; > > > + > > > +       return (unsigned long)(-1); > > > +} > > > > This seems about right for FLATMEM. For SPARSEMEM it would be something > > along these lines (I kept dubious -1): > > Thanks. Is that right even with CONFIG_SPARSEMEM_VMEMMAP? It seems that > it's possible for pfn_valid() to be false for a given *page*, but there > may still be valid pages in the remainder of the same section in that > case? Right, it might after memory hot-remove. At boot the entire section either valid or not. > I think it should only skip to the next section if the current section > doesn't exist at all, not just when pfn_section_valid() return false? Yeah, when pfn_section_valid() returns false it should itereate pfns until the end of the section and check if they are valid. > I also wasn't sure how to cope with the rcu_read_lock_sched() that > happens in pfn_valid(). What's that protecting against? Does it mean > that by the time pfn_valid() returns true, that might not be the > correct answer any more? That's protecting against kfree_rcu() in section_deactivate() so even if the answer is still correct, later access to apparently valid struct page may blow up :/ > > static inline unsigned long first_valid_pfn(unsigned long pfn) > > { > > unsigned long nr = pfn_to_section_nr(pfn); > > > > do { > > if (pfn_valid(pfn)) > > return pfn; > > pfn = section_nr_to_pfn(nr++); > > } while (nr < NR_MEM_SECTIONS); > > > > return (unsigned long)-1; > > } -- Sincerely yours, Mike.