From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 34AE0156669; Wed, 18 Dec 2024 09:41:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734514892; cv=none; b=UHc6dC+d1EErjiIgcPmZouzsLVSypk+RNChegqTbIjNbBUEenB2m5naicb0sHSCQRt/o3OBgErkkYe2UuxUr92DdlTLGrbrOrEvCqTAiZa2eHPyNRGoFk5SiApCFp5ePKUTcjJ7QaWMObHNminHx2RxEzxIYLRNYJK3/0AzgFaE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734514892; c=relaxed/simple; bh=13QAhbRjan08XBeFEhfpBy/Dr9HlX8T3FrQVI7PlHYE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=qZNa6J7cYNQOYMqTBvBC7wmB8HSyYI/Tu7F2Wt0tqxmjEvi0+adfYj0DKtIKOY3TzPzkz+g1azItBrJl4tkNJsiwxPESTSvXbHt1fwjPW/1bPhAzqByBVqW64d/T1qaYfJzyNtdxuID6fokv/00UxtcoFI1sLOHTqKEOA37OgQM= 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=FqvimX+6; arc=none smtp.client-ip=90.155.92.199 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="FqvimX+6" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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=+fAmL7GJf78nGDkmx//Bq8X/X+N3cQCfDHep37xE3Ik=; b=FqvimX+6+e0AWVocIqyU2abwr7 YLayjW0MU500m5v+sbMVzluwXVJd7sWBk+g2/I3RTTSNM3X9rCz1oMyqJoRGWrUXBgtWacOTW5ce+ dlyiidEz9YmObS2/QYrYGydGQsSIVpehi6f5WRhNvy5hRozxRrtNzLqWSD1TQJz5MGQn7rcslupXk 3Nb+MO7B1kOIImsv0yLHzHutIEz17zT+HQBFN90Xnw3Ykz4RWbvvgY5xkiESQiShExuIwH2eq2ZdG xTbV5Uwbc+OYKpFgnGUqIIW9dIqHn9jG0UoTl5SKNdZFYJBhwwF9jX7hJEaS16btKGUC8QV3PayOK LXLB2G7g==; Received: from 77-249-17-89.cable.dynamic.v4.ziggo.nl ([77.249.17.89] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98 #2 (Red Hat Linux)) id 1tNqYD-00000005DVr-0ty3; Wed, 18 Dec 2024 09:41:05 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 5B9253002A0; Wed, 18 Dec 2024 10:41:04 +0100 (CET) Date: Wed, 18 Dec 2024 10:41:04 +0100 From: Peter Zijlstra To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, willy@infradead.org, liam.howlett@oracle.com, lorenzo.stoakes@oracle.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mjguzik@gmail.com, oliver.sang@intel.com, mgorman@techsingularity.net, david@redhat.com, peterx@redhat.com, oleg@redhat.com, dave@stgolabs.net, paulmck@kernel.org, brauner@kernel.org, dhowells@redhat.com, hdanton@sina.com, hughd@google.com, lokeshgidra@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, klarasmodin@gmail.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count Message-ID: <20241218094104.GC2354@noisy.programming.kicks-ass.net> References: <20241216192419.2970941-1-surenb@google.com> <20241216192419.2970941-11-surenb@google.com> <20241216213753.GD9803@noisy.programming.kicks-ass.net> <20241217103035.GD11133@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-doc@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 Tue, Dec 17, 2024 at 08:27:46AM -0800, Suren Baghdasaryan wrote: > > So I just replied there, and no, I don't think it makes sense. Just put > > the kmem_cache_free() in vma_refcount_put(), to be done on 0. > > That's very appealing indeed and makes things much simpler. The > problem I see with that is the case when we detach a vma from the tree > to isolate it, then do some cleanup and only then free it. That's done > in vms_gather_munmap_vmas() here: > https://elixir.bootlin.com/linux/v6.12.5/source/mm/vma.c#L1240 and we > even might reattach detached vmas back: > https://elixir.bootlin.com/linux/v6.12.5/source/mm/vma.c#L1312. IOW, > detached state is not final and we can't destroy the object that > reached this state. Urgh, so that's the munmap() path, but arguably when that fails, the map stays in place. I think this means you're marking detached too soon; you should only mark detached once you reach the point of no return. That said, once you've reached the point of no return; and are about to go remove the page-tables, you very much want to ensure a lack of concurrency. So perhaps waiting for out-standing readers at this point isn't crazy. Also, I'm having a very hard time reading this maple tree stuff :/ Afaict vms_gather_munmap_vmas() only adds the VMAs to be removed to a second tree, it does not in fact unlink them from the mm yet. AFAICT it's vma_iter_clear_gfp() that actually wipes the vmas from the mm -- and that being able to fail is mind boggling and I suppose is what gives rise to much of this insanity :/ Anyway, I would expect remove_vma() to be the one that marks it detached (it's already unreachable through vma_lookup() at this point) and there you should wait for concurrent readers to bugger off. > We could change states to: 0=unused (we can free > the object), 1=detached, 2=attached, etc. but then vma_start_read() > should do something like refcount_inc_more_than_one() instead of > refcount_inc_not_zero(). Would you be ok with such an approach? Urgh, I would strongly suggest ditching refcount_t if we go this route. The thing is; refcount_t should remain a 'simple' straight forward interface and not allow people to do the wrong thing. Its not meant to be the kitchen sink -- we have atomic_t for that. Anyway, the more common scheme at that point is using -1 for 'free', I think folio->_mapcount uses that even. For that see: atomic_add_negative*(). > > Additionally, having vma_end_write() would allow you to put a lockdep > > annotation in vma_{start,end}_write() -- which was I think the original > > reason I proposed it a while back, that and having improved clarity when > > reading the code, since explicitly marking the end of a section is > > helpful. > > The vma->vmlock_dep_map is tracking vma->vm_refcnt, not the > vma->vm_lock_seq (similar to how today vma->vm_lock has its lockdep > tracking that rw_semaphore). If I implement vma_end_write() then it > will simply be something like: > > void vma_end_write(vma) > { > vma_assert_write_locked(vma); > vma->vm_lock_seq = UINT_MAX; > } > > so, vmlock_dep_map would not be involved. That's just weird; why would you not track vma_{start,end}_write() with the exclusive side of the 'rwsem' dep_map ? > If you want to track vma->vm_lock_seq with a separate lockdep, that > would be more complicated. Specifically for vma_end_write_all() that > would require us to call rwsem_release() on all locked vmas, however > we currently do not track individual locked vmas. vma_end_write_all() > allows us not to worry about tracking them, knowing that once we do > mmap_write_unlock() they all will get unlocked with one increment of > mm->mm_lock_seq. If your suggestion is to replace vma_end_write_all() > with vma_end_write() and unlock vmas individually across the mm code, > that would be a sizable effort. If that is indeed your ultimate goal, > I can do that as a separate project: introduce vma_end_write(), > gradually add them in required places (not yet sure how complex that > would be), then retire vma_end_write_all() and add a lockdep for > vma->vm_lock_seq. Yeah, so ultimately I think it would be clearer if you explicitly mark the point where the vma modification is 'done'. But I don't suppose we have to do that here.