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 B281F27442; Wed, 1 Apr 2026 07:32:36 +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=1775028756; cv=none; b=N1Kfcu4iTQdH7kuI1g2IWKwxXb7PGKTzOTMKYBPDbSzSKyBV0zD0RbQ1hJpGz+2Y7HinT74jLSX4pCQUVl4ko+e0zyAtzpBgeQzLUjNETz66rbTHHlBkF9IY58oACFoLXHX9S+U+z0Pi/81E/DpruHH0vUAjyL8+ui3EuPtRb5E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775028756; c=relaxed/simple; bh=Xn630OcqIQHi39xngA8rdVFLMzJ6oRMn/HMdRSOCPls=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BHYkD2Hk+jEz9eBNR/JKoO8J7S3hrJznTwKja+dch2CvGO1emIiKcT0pXnbIu0AkHjc/gzYNuJDKVG7iQlzn7pOt7iIL1MjvSEasigsQZjTW1uRmbIrjxkvTft6UcG7iiYvFsxT05bnEd2imCFUPf9t9YuvzCqSOf5Im96nfavg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=humF71eb; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="humF71eb" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 63542C4CEF7; Wed, 1 Apr 2026 07:32:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775028756; bh=Xn630OcqIQHi39xngA8rdVFLMzJ6oRMn/HMdRSOCPls=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=humF71ebabeqOf720kG9Kp3v1uYNN24B/eb+zf0MwE0EGHGt2SYsVKdjAeLFIYQPE JpKTQti9qdA1UD0aJ2wjmmtixB9bFuBuNiNW0fwF3ilqHrSx2t8VLzRyRIbw6bmN2c saJYL9J6ElfuXkNpR6MQZlw5JCLTibOual5FmZzEu5pm/AbinbE1hM/XAFIdXV+6lp ry3tOJfVitrM3Lyzuapcby8OwKWlDVY+ikszsrQmDUWK8FIDdhxWVxq5Q8e1n+TcQb aqgJswJUKR+MLwPFaGtVmpizs2Uk+dbXHjxKIPqVsgEKItt6A0x3cOseQpYN4SmQs0 PcSyJjFVX2qNQ== Message-ID: Date: Wed, 1 Apr 2026 09:32:30 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v3 0/2] Fix storing in XArray check_split tests To: Zi Yan Cc: Ackerley Tng , willy@infradead.org, akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, michael.roth@amd.com, dev.jain@arm.com, vannapurve@google.com References: <6ada27e1-8f85-4b85-8c25-bc9207b2624d@kernel.org> <87FD20AF-55B6-4129-9AED-E2E854593883@nvidia.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/16/26 17:49, Zi Yan wrote: > On 16 Mar 2026, at 12:23, David Hildenbrand (Arm) wrote: > >> On 2/23/26 08:34, Ackerley Tng wrote: >>> Hi, >>> >>> I hit an assertion while making some modifications to >>> lib/test_xarray.c [1] and I believe this is the fix. >>> >>> In check_split, the tests split the XArray node and then store values >>> after the split to verify that splitting worked. While storing and >>> retrieval works as expected, the node's metadata, specifically >>> node->nr_values, is not updated correctly. >>> >>> This led to the assertion being hit in [1], since the storing process >>> did not increment node->nr_values sufficiently, while the erasing >>> process assumed the fully-incremented node->nr_values state. >>> >>> Would like to check my understanding on these: >>> >>> 1. In the multi-index xarray world, is node->nr_values definitely the >>> total number of values *and siblings* in the node? >>> >>> 2. IIUC xas_store() has significantly different behavior when entry is >>> NULL vs non-NULL: when entry is NULL, xas_store() does not make >>> assumptions on the number of siblings and erases all the way till >>> the next non-sibling entry. This sounds fair to me, but it's also >>> kind of surprising that it is differently handled when entry is >>> non-NULL, where xas_store() respects xas->xa_sibs. >>> >>> 3. If xas_store() is dependent on its caller to set up xas correctly >>> (also sounds fair), then there are places where xas_store() is >>> used, like replace_page_cache_folio() or >>> migrate_huge_page_move_mapping(), where xas is set up assuming 0 >>> order pages. Are those buggy? >> >> Zi, do you have any familiarity with that code and could help? > > Not much. But I used lib/test_xarray.c to did a test: > > 1. initialize an xarray with order 6 and set entry to 0, > > 2. add a new xas like XA_STATE(xas0, xa, 0); > 3. xas_store(&xas0, xa_mk_value(32)); > > 4. add a new xas like XA_STATE(xas0, xa, 16); > 5. xas_store(&xas0, xa_mk_value(48)); > > The outcome is that xas_store() does not change xarray structure, > namely the orders are preserved. No issue is present. > > After 2 and 3, the xarray is still order 6, but its 0-63 entries (all order-6) > are changed from 0 to 32. > After 4 and 5, the xarray is still order 6, but its 0-63 entries > are changed from 32 to 48. > > I will need to dig into the code more to explain how xas_store() works. Zi, we discussed this topic with Willy in the THP cabal call. I did not get all the details, do you remember our conclusion? (I can try getting access to the recording) -- Cheers, David