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 D75E915BF for ; Tue, 12 Mar 2024 16:38:38 +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=1710261520; cv=none; b=YhekNo71qQC0uEFk3zrPloNtg9w5s0nXvoGnmttrD90a6cXL2QF365UuECNUex81U5UMoT8e6hEVSdtIY9Of/ffJnY7gETvWdtQm6+Myt7phfc9BrhKlIgM1Ok3L59MPt65XC7jz+byWWskHqco5AvHBp20FC//Ncxkv7HHKx6Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710261520; c=relaxed/simple; bh=2Ney3Be8zQ+ahSZrL4rbdZbA6XT2or+RZEEolylCT6g=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=MeG50liojE1JROWyK6l5Io4ncDUYkLYrNU2uHtXXWa4RalNFZDpP2CERv+Nn6KS4mUSW7x2+/NcIe68iy/YHM7q9MDEYQf/X+byC0YjTG3A7kNM1OaPGmL+mLrVyfpjEQ16lxTFATnCAXSRjc9uD/vFaw+5+h9kfLJ7BZe+XRL4= 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=jYe8NXlh; 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="jYe8NXlh" 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=8LxRhGuwXJFpATs069eW/+reuCwEH54tH10y+l407N4=; b=jYe8NXlha4EYzdWWFia+M3+OgZ zcOr25Z8IhZ7NogxmHSvhJBS4mlncMVYqEA4ZTB0ctlMv62PpDh+A4O2uExc+OQyuSYVQLnxXZQFn vHmjzdp/5IwVV3oRbpUX5P9gMOmUfw0rFRIfJh6DofHPQ0Milb7vIGn/QehrzX3eTD2dmN+20PqCR x0d6t0Y8A9Eb1cIJok8pYcxtFfX63bOCfsyemI1wb4a0rkpqA0cVOUuwWX53m60MW2ElMvGow0+6X pgULWQrqUXt/XJ0LvA1AQqwDvqCw2ao2/VMK97VIZVIJJQTHnjPkC04Fq+4VhCezk/WwbWM7FD86p uJxSzQAQ==; Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1rk596-00000003RNz-1xJ1; Tue, 12 Mar 2024 16:38:32 +0000 Date: Tue, 12 Mar 2024 16:38:32 +0000 From: Matthew Wilcox To: Zi Yan Cc: linux-mm@kvack.org, Andrew Morton , Yang Shi , Huang Ying , "\"Kirill A . Shutemov\"" , Ryan Roberts , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/migrate: put dest folio on deferred split list if source was there. Message-ID: References: <20240311195848.135067-1-zi.yan@sent.com> <74AF044A-A14A-4C66-A019-70F8F138A9AB@nvidia.com> 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 Tue, Mar 12, 2024 at 11:51:13AM -0400, Zi Yan wrote: > On 12 Mar 2024, at 10:19, Matthew Wilcox wrote: > > > On Tue, Mar 12, 2024 at 10:13:16AM -0400, Zi Yan wrote: > >> On 11 Mar 2024, at 23:45, Matthew Wilcox wrote: > >>> Much more important: You're doing this with a positive refcount, which > >>> breaks the (undocumented) logic in deferred_split_scan() that a folio > >>> with a positive refcount will not be removed from the list. > >> > >> What is the issue here? I thought as long as the split_queue_lock is held, > >> it should be OK to manipulate the list. > > > > I just worked this out yesterday: > > https://lore.kernel.org/linux-mm/Ze9EFdFLXQEUVtKl@casper.infradead.org/ > > (the last chunk, starting with Ryan asking me "what about the first bug > > you found") > > Hmm, like you said a folio with a positive refcount will not be removed > from ds_queue->split_queue, it will have no chance going to the separate > list in deferred_list_scan() and list_del_init() will not corrupt > that list. You've misread it. Folios with a _zero_ refcount are not removed from the list in deferred_split_scan. Folios with a positive refcount are removed from the per-node or per-cgroup list _at which point there is an undocumented assumption_ that they will not be removed from the local list because they have a positive refcount. > So it should be safe. Or the issue is that before migration > adding a refcount, the folio is removed from ds_queue->split_queue > and put on the list in deferred_list_scan(), as a result, any manipulation > of folio->_deferred_list could corrupt the list. Basically, > !list_empty(&folio->_deferred_list) cannot tell if the folio is on > ds_queue->split_queue or another list. I am not sure about why "a positive > refcount" is related here. > > That makes me wonder whether ds_queue->split_queue_lock is also needed > for list_for_each_entry_safe() in deferred_split_scan(). Basically, > ds_queue->split_queue_lock protects folio->_deferred_list in addition to > ds_queue->split_queue. > > > > >>> Maximally important: Wer shouldn't be doing any of this! This folio is > >>> on the deferred split list. We shouldn't be migrating it as a single > >>> entity; we should be splitting it now that we're in a context where we > >>> can do the right thing and split it. Documentation/mm/transhuge.rst > >>> is clear that we don't split it straight away due to locking context. > >>> Splitting it on migration is clearly the right thing to do. > >>> > >>> If splitting fails, we should just fail the migration; splitting fails > >>> due to excess references, and if the source folio has excess references, > >>> then migration would fail too. > >> > >> You are suggesting: > >> 1. checking if the folio is on deferred split list or not > >> 2. if yes, split the folio > >> 3. if split fails, fail the migration as well. > >> > >> It sounds reasonable to me. The split folios should be migrated since > >> the before-split folio wants to be migrated. This split is not because > >> no new page cannot be allocated, thus the split folios should go > >> into ret_folios list instead of split_folios list. > > > > Yes, I'm happy for the split folios to be migrated. Bonus points if you > > want to figure out what order to split the folio to ;-) I don't think > > it's critical. > > > -- > Best Regards, > Yan, Zi