From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 5B00569973 for ; Tue, 12 Mar 2024 08:05:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710230729; cv=none; b=TvgoxQTWTmIN6cf49HFwxXGnPIe489DlRDhxzFz4ixYlKF1n7EfqBJW/DX6BuPMoHUTDKtHfeV5OlcmnLaMmT8PVPDyoQyzCCXM3lR77ZyKwc3dpU4/oDxDBCT5ZCnstdSt2aD/4/gA0u/sq17gWx6YJy6f7O5YMUNoscy8dBgU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710230729; c=relaxed/simple; bh=bl6AfL2X8NIjP+96i5PQItkn9CCWuVzDqSvid1nmuvo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Nx3VIINUpcCnfSZUm0YDQMxYp+UD7MwqX/rMY2STHUapz2VJTg8XlOqlc7IzH6ZxFTI+akwkeZDfoR5yzOco3rfCCiY4PR6nu6i8/F+a0cTMGr2VPppMgizYLWsSPw0qXE1Wm57LZHwOVr8WCVTGSsoBf4u/pFaMBLnYzzTZpMU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 82A041007; Tue, 12 Mar 2024 01:06:03 -0700 (PDT) Received: from [10.57.68.246] (unknown [10.57.68.246]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BC8433F73F; Tue, 12 Mar 2024 01:05:24 -0700 (PDT) Message-ID: Date: Tue, 12 Mar 2024 08:05:23 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/migrate: put dest folio on deferred split list if source was there. Content-Language: en-GB To: Matthew Wilcox , Zi Yan Cc: linux-mm@kvack.org, Andrew Morton , Yang Shi , Huang Ying , "Kirill A . Shutemov" , linux-kernel@vger.kernel.org References: <20240311195848.135067-1-zi.yan@sent.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 12/03/2024 03:45, Matthew Wilcox wrote: > On Mon, Mar 11, 2024 at 03:58:48PM -0400, Zi Yan wrote: >> @@ -1168,6 +1172,17 @@ static int migrate_folio_unmap(new_folio_t get_new_folio, >> folio_lock(src); >> } >> locked = true; >> + if (folio_test_large_rmappable(src) && I think you also need to check that the order > 1, now that we support order-1 pagecache folios? _deferred_list only exists if order > 1. >> + !list_empty(&src->_deferred_list)) { >> + struct deferred_split *ds_queue = get_deferred_split_queue(src); >> + >> + spin_lock(&ds_queue->split_queue_lock); >> + ds_queue->split_queue_len--; >> + list_del_init(&src->_deferred_list); >> + spin_unlock(&ds_queue->split_queue_lock); >> + old_page_state |= PAGE_WAS_ON_DEFERRED_LIST; >> + } > > I have a few problems with this ... > > Trivial: your whitespace is utterly broken. You can't use a single tab > for both indicating control flow change and for line-too-long. > > Slightly more important: You're checking list_empty outside the lock > (which is fine in order to avoid unnecessarily acquiring the lock), > but you need to re-check it inside the lock in case of a race. And you > didn't mark it as data_race(), so KMSAN will whinge. I've seen data_race() used around list_empty() without the lock held inconsistently (see deferred_split_folio()). What are the rules? Given that we are not doing a memory access here, I don't really understand why it is needed? list_empty() uses READ_ONCE() which I thought would be sufficient? (I've just added a similar lockless check in my swap-out series - will add data_race() if needed, but previously concluded it's not). > > 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. > > 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. This comment makes me wonder what we do in split_huge_page_to_list_to_order() if the target order is greater than 1 and the input folio is on the deferred split list. Looks like we currently just remove it from the deferred list. Is there a case for putting any output folios that are still partially mapped back on the deferred list, or splitting them to a lower order such that all output folios are fully mapped, and all unmapped portions are freed?