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 9536F7F7FC for ; Thu, 3 Apr 2025 14:10:12 +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=1743689412; cv=none; b=TYm8alCi8lIP4dFCrdtYPlqnRH1aOjTPqWYTRdzH/xEPtbHZs/CV8VjoMclUDUC+8q1xs+1j/2sHtw7xW8VcvMcLl+IXMDuRwYv78terUXT7AQM4XtWllMaozOTa1a6flSvBiY4v9tXyQSGHJM8QR/J2yB/GHqqtolLQ1VrhHbI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743689412; c=relaxed/simple; bh=HehmzWN07oImSDzOXjOMFAC9bLomTxZFRkiMNjxI5FQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=icmOizhuEMDttNTeeCWfAS11uEOhxveKpMxD39UgfVGu2XlRivD5ryrLVna3I5kJ1jqKM0NLcPYF2JYVYHR8PJQEGYHKfbtrYTCwRnfs+MUyupUC/Sf97WXzi/loe2W0EC8R15lMExyA2rETQOuDhBv2FDZETMl8alRRR8FJr0A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=p4tCPFYY; 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="p4tCPFYY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 96A22C4CEE3; Thu, 3 Apr 2025 14:10:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1743689412; bh=HehmzWN07oImSDzOXjOMFAC9bLomTxZFRkiMNjxI5FQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=p4tCPFYYYPGoaZWejEYhyx1y7zD3cp3NTUeFqKkYekF1dCNx8KDC4P0g66yoeO4fj IwQeRmrNwV+NXJciR0mgdgA6w6dLUogofUeFKcS6brgieJ4VEzsExBZsOaZYHtc5fR sLRe6uQRvYTJw2G5EZMRtQ8sydPfG4I8GEmaP/Gm2jeQtdppNQCAGEDSHnU9vhK7Hq SFXipEe7jLpRvLbkwRKX3pJWCXfRBYqfkk9EXc8forpXVlHdDUfdZ3Vo4TcIpD8vk9 ui6dBY5E6bgi5+Ffe916I7e/DwfPLsM9iBqNRXH9PJO1hX3BTGEisqGFtMeFiPRumM 2X0cBdR7DNuXg== Date: Thu, 3 Apr 2025 17:10:03 +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: [RFC PATCH 3/3] mm: Implement for_each_valid_pfn() for CONFIG_SPARSEMEM Message-ID: References: <20250402201841.3245371-1-dwmw2@infradead.org> <20250402201841.3245371-3-dwmw2@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=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Apr 03, 2025 at 08:07:22AM +0100, David Woodhouse wrote: > On Thu, 2025-04-03 at 09:24 +0300, Mike Rapoport wrote: > > with a small nit below > > > > > +static inline bool first_valid_pfn(unsigned long *p_pfn) > > > +{ > > > + unsigned long pfn = *p_pfn; > > > + unsigned long nr = pfn_to_section_nr(pfn); > > > + struct mem_section *ms; > > > + bool ret = false; > > > + > > > + ms = __pfn_to_section(pfn); > > > + > > > + rcu_read_lock_sched(); > > > + > > > + while (!ret && nr <= __highest_present_section_nr) { > > > > This could be just for(;;), we anyway break when ret becomes true or we get > > past last present section. > > True for the 'ret' part but not *nicely* for the last present section. > If the original pfn is higher than the last present section it could > trigger that check before entering the loop. > > Yes, in that case 'ms' will be NULL, valid_section(NULL) is false and > you're right that it'll make it through to the check in the loop > without crashing. So it would currently be harmless, but I didn't like > it. It's relying on the loop not to do the wrong thing with an input > which is arguably invalid. > > I'll see if I can make it neater. I may drop the 'ret' variable > completely and just turn the match clause into unlock-and-return-true. > I *like* having a single unlock site. But I think I like simpler loop > code more than that. > > FWIW I think the check for (PHYS_PFN(PFN_PHYS(pfn)) != pfn) at the > start of pfn_valid() a few lines above is similarly redundant. Because > if the high bits are set in the PFN then pfn_to_section_nr(pfn) is > surely going to be higher than NR_MEM_SECTIONS and it'll get thrown out > at the very next check, won't it? I believe the check for (PHYS_PFN(PFN_PHYS(pfn)) != pfn) got to the generic version from arm64::pfn_valid() that historically supported both FLATMEM and SPARSEMEM. I can't think of a configuration in which (PHYS_PFN(PFN_PHYS(pfn)) != pfn) and pfn_to_section_nr(pfn) won't be higher than NR_MEM_SECTIONS, but with all variants that arm64 has for PAGE_SHIFT and ARM64_PA_BITS I could miss something. > I care because I didn't bother to duplicate that 'redundant' check in > my first_valid_pfn(), so if there's a reason for it that I'm missing, I > should take a closer look. > > I'm also missing the reason why the FLATMEM code in memory_model.h does > 'unsigned long pfn_offset = ARCH_PFN_OFFSET' and then uses its local > pfn_offset variable, instead of just using ARCH_PFN_OFFSET directly as > I do in the FLATMEM for_each_valid_pfn() macro. Don't remember now, but I surely had some $REASON for that :) -- Sincerely yours, Mike.