From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta0.migadu.com (out-179.mta0.migadu.com [91.218.175.179]) (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 5BC6018636D for ; Mon, 25 Mar 2024 09:46:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1711359994; cv=none; b=fV64gERmPCr5UlQyveAI0lDHTQZa3YR+gf6ICSvqpjie07qVp+jUHjjKkBjpVujSxw4xQjy7bj/D9f7wIcfwwDCbOtE+y74flSPU+/BIzDsEqJLWswzeDgIrVMzxmQf74nkp/kiIbn+yYB9InVIaW0cXd6fAzSh++Oa5+0e9hHo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1711359994; c=relaxed/simple; bh=1iD9rwO2v6dLL4gbGaYi5aDf6QxpcpHrmKpLpxt28nw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kqLTGzcRvfXVWNr9P98La8gs0GqzTNVfjyNqTJ3ErgL/4zLYT2/uKerNeDJnhbetUfJGOYP5bXk3HNAkfE05IMpzRzrAVSeLwSd6Ri047JGrqlOFi5Zu1Gf9UycWKQta8yFUmmNFpDDz9xJr1K4FNFbjfsJ8e2YZ5jn5f4FoXPk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=TDnhqOlC; arc=none smtp.client-ip=91.218.175.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="TDnhqOlC" Message-ID: <339639f3-4334-44a8-a811-d20b3c578f74@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1711359990; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=3kW6CpvNLcQN5MKehXI/yVTzkUIvW6nXEzppExY7bng=; b=TDnhqOlCJfkHT730fvX2kvpKxlweFV/uEXDJpP84aMp90lyehm7owaGbrJ+jekWYRCPeG4 28hTF/JVa/AjbHIbA+xixil1gG5rENt2vG0wlofv2lGI2y3QOt2UTGSi6NwFIkh8Rbv2Zz wY861xo6ZuEhsRS4oHE4k/lDMJwCCQM= Date: Mon, 25 Mar 2024 17:46:21 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH] mm: zswap: fix data loss on SWP_SYNCHRONOUS_IO devices Content-Language: en-US To: Yosry Ahmed Cc: Barry Song <21cnbao@gmail.com>, Johannes Weiner , Andrew Morton , Zhongkun He , Chengming Zhou , Chris Li , Nhat Pham , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kairui Song References: <20240324210447.956973-1-hannes@cmpxchg.org> <1e7ce417-b9dd-4d62-9f54-0adf1ccdae35@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Chengming Zhou In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2024/3/25 17:40, Yosry Ahmed wrote: > On Mon, Mar 25, 2024 at 2:22 AM Chengming Zhou wrote: >> >> On 2024/3/25 16:38, Yosry Ahmed wrote: >>> On Mon, Mar 25, 2024 at 12:33 AM Chengming Zhou >>> wrote: >>>> >>>> On 2024/3/25 15:06, Yosry Ahmed wrote: >>>>> On Sun, Mar 24, 2024 at 9:54 PM Barry Song <21cnbao@gmail.com> wrote: >>>>>> >>>>>> On Mon, Mar 25, 2024 at 10:23 AM Yosry Ahmed wrote: >>>>>>> >>>>>>> On Sun, Mar 24, 2024 at 2:04 PM Johannes Weiner wrote: >>>>>>>> >>>>>>>> Zhongkun He reports data corruption when combining zswap with zram. >>>>>>>> >>>>>>>> The issue is the exclusive loads we're doing in zswap. They assume >>>>>>>> that all reads are going into the swapcache, which can assume >>>>>>>> authoritative ownership of the data and so the zswap copy can go. >>>>>>>> >>>>>>>> However, zram files are marked SWP_SYNCHRONOUS_IO, and faults will try >>>>>>>> to bypass the swapcache. This results in an optimistic read of the >>>>>>>> swap data into a page that will be dismissed if the fault fails due to >>>>>>>> races. In this case, zswap mustn't drop its authoritative copy. >>>>>>>> >>>>>>>> Link: https://lore.kernel.org/all/CACSyD1N+dUvsu8=zV9P691B9bVq33erwOXNTmEaUbi9DrDeJzw@mail.gmail.com/ >>>>>>>> Reported-by: Zhongkun He >>>>>>>> Fixes: b9c91c43412f ("mm: zswap: support exclusive loads") >>>>>>>> Cc: stable@vger.kernel.org [6.5+] >>>>>>>> Signed-off-by: Johannes Weiner >>>>>>>> Tested-by: Zhongkun He >>>>>> >>>>>> Acked-by: Barry Song >>>>>> >>>>>>> >>>>>>> Do we also want to mention somewhere (commit log or comment) that >>>>>>> keeping the entry in the tree is fine because we are still protected >>>>>>> from concurrent loads/invalidations/writeback by swapcache_prepare() >>>>>>> setting SWAP_HAS_CACHE or so? >>>>>> >>>>>> It seems that Kairui's patch comprehensively addresses the issue at hand. >>>>>> Johannes's solution, on the other hand, appears to align zswap behavior >>>>>> more closely with that of a traditional swap device, only releasing an entry >>>>>> when the corresponding swap slot is freed, particularly in the sync-io case. >>>>> >>>>> It actually worked out quite well that Kairui's fix landed shortly >>>>> before this bug was reported, as this fix wouldn't have been possible >>>>> without it as far as I can tell. >>>>> >>>>>> >>>>>> Johannes' patch has inspired me to consider whether zRAM could achieve >>>>>> a comparable outcome by immediately releasing objects in swap cache >>>>>> scenarios. When I have the opportunity, I plan to experiment with zRAM. >>>>> >>>>> That would be interesting. I am curious if it would be as >>>>> straightforward in zram to just mark the folio as dirty in this case >>>>> like zswap does, given its implementation as a block device. >>>>> >>>> >>>> This makes me wonder who is responsible for marking folio dirty in this swapcache >>>> bypass case? Should we call folio_mark_dirty() after the swap_read_folio()? >>> >>> In shrink_folio_list(), we try to add anonymous folios to the >>> swapcache if they are not there before checking if they are dirty. >>> add_to_swap() calls folio_mark_dirty(), so this should take care of >> >> Right, thanks for your clarification, so should be no problem here. >> Although it was a fix just for MADV_FREE case. >> >>> it. There is an interesting comment there though. It says that PTE >>> should be dirty, so unmapping the folio should have already marked it >>> as dirty by the time we are adding it to the swapcache, except for the >>> MADV_FREE case. >> >> It seems to say the folio will be dirtied when unmap later, supposing the >> PTE is dirty. > > Oh yeah it could mean that the folio will be dirted later. > >> >>> >>> However, I think we actually unmap the folio after we add it to the >>> swapcache in shrink_folio_list(). Also, I don't immediately see why >>> the PTE would be dirty. In do_swap_page(), making the PTE dirty seems >> >> If all anon pages on LRU list are faulted by write, it should be true. >> We could just use the zero page if faulted by read, right? > > This applies for the initial fault that creates the folio, but this is > a swap fault. It could be a read fault and in that case we still need > to make the folio dirty because it's not in the swapcache and we need > to write it out if it's reclaimed, right? Yes, IMHO I think it should be marked as dirty here. But it should be no problem with that unconditional folio_mark_dirty() in add_to_swap(). Not sure if there are other issues. > >> >>> to be conditional on the fault being a write fault, but I didn't look >>> thoroughly, maybe I missed it. It is also possible that the comment is >>> just outdated. >> >> Yeah, dirty is only marked on write fault. >> >> Thanks.