From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-175.mta0.migadu.com (out-175.mta0.migadu.com [91.218.175.175]) (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 035141624E0 for ; Wed, 26 Feb 2025 15:33:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.175 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740584032; cv=none; b=bOnvZd0TwRPrr8IW+C4yrDunf/5gO2tWq/qyQXUig0mZLTaNgArAbnljGw4P/PljA0DjJyvyivTZwP4Xl+ckQNzpacM6tf3YidibGIFJAvCTJgm2EpdZLapxim68Q9933AGnSOP6cg5QAbYGv+d5Q9IDWcovcObm76nqMqqoLHM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740584032; c=relaxed/simple; bh=/bB/yZLGL3DWE2PUG3M5lw3kgRGqPeayAXHhqErbi10=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=QBKCEFOCCYkCnOj0OOZFUoDJIsKL7DeApCiNZEK6egzVc/+URLpPUTA8knf2pRe5TahXz5rZlEzKuMemCaEiIisR3muCT8pxcbVUBXiK9EryCfyroftX6kkKhGfn4gTHsJpiEL6X6YVuS5vk6e+vO/TpTVv8X6iZjxLbMygS4NQ= 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=AX7MX3wM; arc=none smtp.client-ip=91.218.175.175 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="AX7MX3wM" Date: Wed, 26 Feb 2025 15:33:43 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1740584027; 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: in-reply-to:in-reply-to:references:references; bh=YghUIUtGJFDQ2JIT44YnprUkmQqQzSSO6n1OgaplZRk=; b=AX7MX3wMWgNLdA8YTvt8dSILGImJRuHBwZfP9SPk6Nd9B/IDdvnJogK8ehjKudK4AGLCH/ T4oOprYE0r6ihvzf6xnEmIstsYkgnvP2L0maEprSSu4xkQK5pvsUwcIoP2U6awEyoyHMse zjCIDxX/rtym74xSdEBw1a5mqf/+fYg= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yosry Ahmed To: Johannes Weiner Cc: Nhat Pham , akpm@linux-foundation.org, chengming.zhou@linux.dev, linux-mm@kvack.org, kernel-team@meta.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] zswap: do not crash the kernel on decompression failure Message-ID: References: <20250225213200.729056-1-nphamcs@gmail.com> <20250226045727.GB1775487@cmpxchg.org> 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: <20250226045727.GB1775487@cmpxchg.org> X-Migadu-Flow: FLOW_OUT On Tue, Feb 25, 2025 at 11:57:27PM -0500, Johannes Weiner wrote: > On Wed, Feb 26, 2025 at 03:12:35AM +0000, Yosry Ahmed wrote: > > On Tue, Feb 25, 2025 at 01:32:00PM -0800, Nhat Pham wrote: > > > Currently, we crash the kernel when a decompression failure occurs in > > > zswap (either because of memory corruption, or a bug in the compression > > > algorithm). This is overkill. We should only SIGBUS the unfortunate > > > process asking for the zswap entry on zswap load, and skip the corrupted > > > entry in zswap writeback. > > > > Some relevant observations/questions, but not really actionable for this > > patch, perhaps some future work, or more likely some incoherent > > illogical thoughts : > > > > (1) It seems like not making the folio uptodate will cause shmem faults > > to mark the swap entry as hwpoisoned, but I don't see similar handling > > for do_swap_page(). So it seems like even if we SIGBUS the process, > > other processes mapping the same page could follow in the same > > footsteps. > > It's analogous to what __end_swap_bio_read() does for block backends, > so it's hitchhiking on the standard swap protocol for read failures. Right, that's also how I got the idea when I did the same for large folios handling. > > The page sticks around if there are other users. It can get reclaimed, > but since it's not marked dirty, it won't get overwritten. Another > access will either find it in the swapcache and die on !uptodate; if > it was reclaimed, it will attempt another decompression. If all > references have been killed, zswap_invalidate() will finally drop it. > > Swapoff actually poisons the page table as well (unuse_pte). Right. My question was basically why don't we also poison the page table in do_swap_page() in this case. It's like that we never swapoff. This will cause subsequent fault attempts to return VM_FAULT_HWPOISON quickly without doing through the swapcache or decompression. Probably not a big deal, but shmem does it so maybe it'd be nice to do it for consistency. > > > (2) A hwpoisoned swap entry results in VM_FAULT_SIGBUS in some cases > > (e.g. shmem_fault() -> shmem_get_folio_gfp() -> shmem_swapin_folio()), > > even though we have VM_FAULT_HWPOISON. This patch falls under this > > bucket, but unfortunately we cannot tell for sure if it's a hwpoision or > > a decompression bug. > > Are you sure? Actual memory failure should replace the ptes of a > mapped shmem page with TTU_HWPOISON, which turns them into special > swap entries that trigger VM_FAULT_HWPOISON in do_swap_page(). I was looking at the shmem_fault() path. It seems like for this path we end up with VM_SIGBUS because shmem_swapin_folio() returns -EIO and not -EHWPOISON. This seems like something that can be easily fixed though, unless -EHWPOISON is not always correct for a diffrent reason. > > Anon swap distinguishes as long as the swapfile is there. Swapoff > installs poison markers, which are then handled the same in future > faults (VM_FAULT_HWPOISON): > > /* > * "Poisoned" here is meant in the very general sense of "future accesses are > * invalid", instead of referring very specifically to hardware memory errors. > * This marker is meant to represent any of various different causes of this. > * > * Note that, when encountered by the faulting logic, PTEs with this marker will > * result in VM_FAULT_HWPOISON and thus regardless trigger hardware memory error > * logic. If that's the case, maybe it's better for zswap in the future if we stop relying on not marking the folio uptodate, and instead propagate an error through swap_read_folio() to the callers to make sure we always return VM_FAULT_HWPOISON and install poison markers. The handling is a bit quirky and inconsistent, but it ultimately results in VM_SIGBUS or VM_FAULT_HWPOISON which I guess is fine for now. > */ > #define PTE_MARKER_POISONED BIT(1)