From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 71FFE25A63B; Mon, 27 Jan 2025 09:53:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737971620; cv=none; b=bEoLsQ/FPykBYTy8jOcnzU4G//oPO63arG1kzraVPZCIdzAYxRxUmCwI1AwKzMxB7NiG4LfZ2MGsgiyWSWYaLFXd919QyncqanfDk9vfuz0Kr8yn9k4uwzKZzuJNdB8VAycptytsISZRwcxYmg9oWMHOih36GhVWGJ6GrcRrptM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737971620; c=relaxed/simple; bh=MP1tghh+sFjWsQmfTpDYmczVqGWed510wFZkOZ+SZmQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=efytYfmLVtHMW6kbvWhuphymInD4WJZhbGE7ktKZrEdTfIa0giEtuc0Ilzhd0w0dyS3m1nlJjK+G7a0fjObeXVgK/0ljB2UC/b+KTEg3G+DHEnNVGfwJxlwYrmkBN1/FqMuVOutMrdzO3fuTcjm8+7KopTegzRMQdo0y7elsE8s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=dhY+XGFE; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="dhY+XGFE" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=kJGDu6W10NCF/1kHbq9MWN63eWI9W/iOmx8WRJAXCOg=; b=dhY+XGFEy3eopIVBJHf10Av56B 6Xkmnx1VQvr6mQp7+jp3RcTV/RFe97oKO5DY+XOrgyGdWgFlJ6bwGastDUA4lSbZljoQWzF9GzDdd xm8pCO5G0bGL+L8FJRJhlkJBkMKlQWddJI21h8Eh0rIj28c0MmyG84AnELKnar+HC+GxjBj/m1z9o qX9x0Gy+cLh7WCU9U40xBMSFZCZCjOSR/TB56jz+6PnWOSF56uKUndBupWk3JvG8Oq0Z7Zwg13uBn Y0tkphhi4yjewRGnBmBLHdrhb1KQE7IbCwPOyCY9uiSti3xN32asvrV9OJw+hKG5vUqliT7Az9f51 MI2ptaTw==; Received: from 77-249-17-89.cable.dynamic.v4.ziggo.nl ([77.249.17.89] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98 #2 (Red Hat Linux)) id 1tcLoC-00000009EhB-2INM; Mon, 27 Jan 2025 09:53:32 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 945463004DE; Mon, 27 Jan 2025 10:53:31 +0100 (CET) Date: Mon, 27 Jan 2025 10:53:31 +0100 From: Peter Zijlstra To: Hugh Dickins Cc: Jann Horn , Roman Gushchin , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , linux-arch@vger.kernel.org Subject: Re: [PATCH v2] mmu_gather: move tlb flush for VM_PFNMAP/VM_MIXEDMAP vmas into free_pgtables() Message-ID: <20250127095331.GB16742@noisy.programming.kicks-ass.net> References: <20250122232716.1321171-1-roman.gushchin@linux.dev> <20250123214531.GA969@noisy.programming.kicks-ass.net> <26cd41c2-b8b6-0c1d-c36d-28f2f9f369be@google.com> <20250124083139.GB13226@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-arch@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 Sun, Jan 26, 2025 at 06:34:48PM -0800, Hugh Dickins wrote: > > If we've flushed the relevant PFNs earlier, for whatever reason, > > batching, or the arch has !MERGE_VMAS or whatever, then we do not need > > to flush again. So clearing vma_pfn in __tlb_reset_range() is the right > > place. > > Yes, Roman moved to clearing vma_pfn in __tlb_reset_range() in his v3: > we are all in agreement on that. Ah, I had not seen v3 yet. > > Similarly, if we don't ever actually free/unlink the PFN vma, we also > > don't care. > > I cannot think of a case in which we arrive at free_pgtables(), but do not > unlink the vma(s) which caused vma_pfn to be set. If there is such a case, > it's not worth optimizing for; Yeah, I suppose it doesn't happen. But I figured why assume stuff. > and wrong to check just the first vma in the > list (don't look only at the stable commit 895428ee124a which Roman cited - > it had to be fixed by 891f03f688de afterwards). Duh, yeah, so tlb_free_vma() wants to be inside the vma loop of free_pgtables(). > Personally, I prefer code inline in free_pgtables() which shows what's > going on, as Roman did in v1, rather than struggling to devise a > self-explanatory function name for something over there in tlb.h. > > But I may be in a minority on that, and his tlb_flush_mmu_pfnmap() > is much more to the point than tlb_free_vma(). I prefer a function over the in-line thing such that mmu-gather is more or less self contained. So my concern is/was maintainability of all this; tlb_flush_mmu_pfnmap() tells me nothing about when this function should be called. Otoh tlb_free_vma() tell me this should be called when we're freeing VMAs -- much harder to misplace etc. If we have hooks placed at, and named after, natural events in the lifetime of things, placement is 'obvious'. Another possible name might be tlb_free_pgtables(), indicating we're about to start freeing pagetables -- but it would need to assert !tlb->freed_tables, and I'm not sure this is a constraint worth imposing. It would bring pain if someone wanted to mix freeing pages and page-tables. And we already have vma based hooks, so I much prefer adding one more of those. This is about funky VMAs after all.