From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 A9D1629347C; Tue, 17 Jun 2025 22:58:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750201138; cv=none; b=BRc0EjaBvrAE+OAf2mimKGeuzvBktIdLMjN2kPx/6oWHXdIfPSQBkN0AaJZMLrWFdLEYIz6X59x3djeQTIH/gl7ICU6mjk3F8xXzfebCq9iQbZkNyHnpZ6ZHhyXppZCRajGlisUUXEKQv3/qOlmGt0RIOo+da/vRdSPwT/biDjA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750201138; c=relaxed/simple; bh=Om8rU9t5dO6lsVh6MDdI4iXYo/zn/c0NhqCeM4oAPP4=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=AsXrjqdjKC3VbQlvFZQGOhLW2fYdp0qhyMmBS8jDAKT2TUc13dqvwuw53VaUWzX5yLmvDU4gs7zJoWenTHHhv/XFHrW6Oa5Lu6z6A51KRz7R7/zdnG7lNOydU686cBNqE9840hYjK92s9jNNnLuWpxoIxOoytSBFVA4f4oGRd2Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=IxN6J0bq; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="IxN6J0bq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B0424C4CEE3; Tue, 17 Jun 2025 22:58:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1750201138; bh=Om8rU9t5dO6lsVh6MDdI4iXYo/zn/c0NhqCeM4oAPP4=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=IxN6J0bqVg7BJzBziBGOF397Zd96J+vvx3GuCJn5LqjbbZjt3Gj2osEU4aL/xU2ei 8WlruCNvsN48JduOcNc6N7qH95OxfU10NC7WoOBRDqRkiYur+BXwSveUNNy21vlbrt mOhtYMjMTuQOLLZ7So9QyrYFMsqHLQtY4vPT50aE= Date: Tue, 17 Jun 2025 15:58:57 -0700 From: Andrew Morton To: Kairui Song Cc: Kairui Song , linux-mm@kvack.org, Hugh Dickins , Baolin Wang , Matthew Wilcox , Kemeng Shi , Chris Li , Nhat Pham , Baoquan He , Barry Song , linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 1/4] mm/shmem, swap: improve cached mTHP handling and fix potential hung Message-Id: <20250617155857.589c3e700b06af7dff085166@linux-foundation.org> In-Reply-To: <20250617183503.10527-2-ryncsn@gmail.com> References: <20250617183503.10527-1-ryncsn@gmail.com> <20250617183503.10527-2-ryncsn@gmail.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) 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-Transfer-Encoding: 7bit On Wed, 18 Jun 2025 02:35:00 +0800 Kairui Song wrote: > From: Kairui Song > > The current swap-in code assumes that, when a swap entry in shmem > mapping is order 0, its cached folios (if present) must be order 0 > too, which turns out not always correct. > > The problem is shmem_split_large_entry is called before verifying the > folio will eventually be swapped in, one possible race is: > > CPU1 CPU2 > shmem_swapin_folio > /* swap in of order > 0 swap entry S1 */ > folio = swap_cache_get_folio > /* folio = NULL */ > order = xa_get_order > /* order > 0 */ > folio = shmem_swap_alloc_folio > /* mTHP alloc failure, folio = NULL */ > <... Interrupted ...> > shmem_swapin_folio > /* S1 is swapped in */ > shmem_writeout > /* S1 is swapped out, folio cached */ > shmem_split_large_entry(..., S1) > /* S1 is split, but the folio covering it has order > 0 now */ > > Now any following swapin of S1 will hang: `xa_get_order` returns 0, > and folio lookup will return a folio with order > 0. The > `xa_get_order(&mapping->i_pages, index) != folio_order(folio)` will > always return false causing swap-in to return -EEXIST. > > And this looks fragile. So fix this up by allowing seeing a larger folio > in swap cache, and check the whole shmem mapping range covered by the > swapin have the right swap value upon inserting the folio. And drop > the redundant tree walks before the insertion. > > This will actually improve the performance, as it avoided two redundant > Xarray tree walks in the hot path, and the only side effect is that in > the failure path, shmem may redundantly reallocate a few folios > causing temporary slight memory pressure. > > And worth noting, it may seems the order and value check before > inserting might help reducing the lock contention, which is not true. > The swap cache layer ensures raced swapin will either see a swap cache > folio or failed to do a swapin (we have SWAP_HAS_CACHE bit even if > swap cache is bypassed), so holding the folio lock and checking the > folio flag is already good enough for avoiding the lock contention. > The chance that a folio passes the swap entry value check but the > shmem mapping slot has changed should be very low. > > Cc: stable@vger.kernel.org > Fixes: 058313515d5a ("mm: shmem: fix potential data corruption during shmem swapin") > Fixes: 809bc86517cc ("mm: shmem: support large folio swap out") The Fixes: tells -stable maintainers (and others) which kernel versions need the fix. So having two Fixes: against different kernel versions is very confusing! Are we recommending that kernels which contain 809bc86517cc but not 058313515d5a be patched?