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linux-mm@kvack.org, linux-kernel@vger.kernel.org, michael.roth@amd.com, dev.jain@arm.com, vannapurve@google.com Subject: Re: [RFC PATCH v3 0/2] Fix storing in XArray check_split tests Date: Wed, 01 Apr 2026 09:53:30 -0400 X-Mailer: MailMate (2.0r6290) Message-ID: <79ED6AAE-96A0-4934-9A24-BB12F3D45F90@nvidia.com> In-Reply-To: References: <6ada27e1-8f85-4b85-8c25-bc9207b2624d@kernel.org> <87FD20AF-55B6-4129-9AED-E2E854593883@nvidia.com> Content-Type: text/plain X-ClientProxiedBy: BL1PR13CA0188.namprd13.prod.outlook.com (2603:10b6:208:2be::13) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|DS7PR12MB6335:EE_ X-MS-Office365-Filtering-Correlation-Id: ebe70b95-987d-480c-40a2-08de8ff6112a X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|7416014|366016|18002099003|22082099003|56012099003; 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gzY9UQbwb36+N4Rf7oPSxeuwDI2jxCQc1zW0f4LhYFS9/LJRyWzDQqZJRjc9cOrHWasfVsI+27LLsrINJsOYmPPYZESRpKk+URSgARevOHeGA3s2A2f0e/Zi01UgHTJpO1OVF2KJkM8uJ2zQtv/4c4noBqxgrq30tsfhNqXbsP4COA+qRf9s35FTjtuOiIVtaDuqc5koHNQsz9FgWwZCc6tCsvaCVvJTRWkJYeK5SLkrHcbLlojNY1+NZxWYSQQ3xFFTjO4+IVa8PxkbVqZYVQx/kcn3kbiCzr31eV0U5/Oxnp+hRj7k/4lXeFMDFcn3QGSUmgHo5IC4MM0Y2sV61vVOZsdIY5lu+VKEbfEIvP49VxjhHAfxZYRuCidE5KGrcXqgfyEUEJIbK7AfFBRXid0fhIhUKd5DIjlQ2wS8MpGs4YBY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 1 Apr 2026, at 3:32, David Hildenbrand (Arm) wrote: > 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? The conclusion is that if user wants to erase (or xas_store(NULL)) an index that is in the middle of a multi-index entry, they need to split that multi-index first then do the erase (or xas_store(NULL)). Because it is documented in xa_erase() (or xas_store(NULL)) that it erases all indices of a multi-index entry[1] and requiring xa_erase() (or xas_store(NULL)) to split a multi-index entry and erase the specified index only is too much due to potential memory allocations during multi-index entry split process. [1] https://elixir.bootlin.com/linux/v6.19.10/source/lib/xarray.c#L1640 > > (I can try getting access to the recording) Best Regards, Yan, Zi