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Shutemov" , Ryan Roberts , Hugh Dickins , David Hildenbrand , Yang Shi , Miaohe Lin , Kefeng Wang , Yu Zhao , John Hubbard , linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v8 1/8] xarray: add xas_try_split() to split a multi-index entry Date: Wed, 26 Feb 2025 15:58:28 -0500 X-Mailer: MailMate (2.0r6222) Message-ID: <17AC91C3-4660-4559-A25F-EC64589AD7E7@nvidia.com> In-Reply-To: <3cda74d5-ae82-4f7b-844f-b14e146dc60a@linux.alibaba.com> References: <20250218235012.1542225-1-ziy@nvidia.com> <20250218235012.1542225-2-ziy@nvidia.com> <8FC0353E-EB2D-4C4C-B96D-8BEC06FE77F7@nvidia.com> <3cda74d5-ae82-4f7b-844f-b14e146dc60a@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BY3PR05CA0018.namprd05.prod.outlook.com (2603:10b6:a03:254::23) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|SJ5PPF01781787B:EE_ X-MS-Office365-Filtering-Correlation-Id: 18dc35a2-a1fd-4b01-0309-08dd56a8544a X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014; 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They cost the same number of xa_node as the >>>> to-be-split slots. For example, to split an order-9 entry, which take= s >>>> 2^(9-6)=3D8 slots, assuming XA_CHUNK_SHIFT is 6 (!CONFIG_BASE_SMALL), = 8 >>>> xa_node are needed. Instead xas_try_split() is intended to be used >>>> iteratively to split the order-9 entry into 2 order-8 entries, then sp= lit >>>> one order-8 entry, based on the given index, to 2 order-7 entries, ...= , >>>> and split one order-1 entry to 2 order-0 entries. When splitting the >>>> order-6 entry and a new xa_node is needed, xas_try_split() will try to >>>> allocate one if possible. As a result, xas_try_split() would only nee= d >>>> one xa_node instead of 8. >>>> >>>> When a new xa_node is needed during the split, xas_try_split() can try= to >>>> allocate one but no more. -ENOMEM will be return if a node cannot be >>>> allocated. -EINVAL will be return if a sibling node is split or casca= de >>>> split happens, where two or more new nodes are needed, and these are n= ot >>>> supported by xas_try_split(). >>>> >>>> xas_split_alloc() and xas_split() split an order-9 to order-0: >>>> >>>> --------------------------------- >>>> | | | | | | | | | >>>> | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | >>>> | | | | | | | | | >>>> --------------------------------- >>>> | | | | >>>> ------- --- --- ------- >>>> | | ... | | >>>> V V V V >>>> ----------- ----------- ----------- ----------- >>>> | xa_node | | xa_node | ... | xa_node | | xa_node | >>>> ----------- ----------- ----------- ----------- >>>> >>>> xas_try_split() splits an order-9 to order-0: >>>> --------------------------------- >>>> | | | | | | | | | >>>> | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | >>>> | | | | | | | | | >>>> --------------------------------- >>>> | >>>> | >>>> V >>>> ----------- >>>> | xa_node | >>>> ----------- >>>> >>>> Signed-off-by: Zi Yan >>>> Cc: Baolin Wang >>>> Cc: David Hildenbrand >>>> Cc: Hugh Dickins >>>> Cc: John Hubbard >>>> Cc: Kefeng Wang >>>> Cc: Kirill A. Shuemov >>>> Cc: Miaohe Lin >>>> Cc: Matthew Wilcox >>>> Cc: Ryan Roberts >>>> Cc: Yang Shi >>>> Cc: Yu Zhao >>>> Cc: Zi Yan >>>> --- >>>> Documentation/core-api/xarray.rst | 14 ++- >>>> include/linux/xarray.h | 7 ++ >>>> lib/test_xarray.c | 47 ++++++++++ >>>> lib/xarray.c | 138 ++++++++++++++++++++++++++-= --- >>>> tools/testing/radix-tree/Makefile | 1 + >>>> 5 files changed, 190 insertions(+), 17 deletions(-) >>>> >>>> diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-ap= i/xarray.rst >>>> index f6a3eef4fe7f..c6c91cbd0c3c 100644 >>>> --- a/Documentation/core-api/xarray.rst >>>> +++ b/Documentation/core-api/xarray.rst >>>> @@ -489,7 +489,19 @@ Storing ``NULL`` into any index of a multi-index = entry will set the >>>> entry at every index to ``NULL`` and dissolve the tie. A multi-ind= ex >>>> entry can be split into entries occupying smaller ranges by calling >>>> xas_split_alloc() without the xa_lock held, followed by taking the = lock >>>> -and calling xas_split(). >>>> +and calling xas_split() or calling xas_try_split() with xa_lock. The >>>> +difference between xas_split_alloc()+xas_split() and xas_try_alloc() = is >>>> +that xas_split_alloc() + xas_split() split the entry from the origina= l >>>> +order to the new order in one shot uniformly, whereas xas_try_split() >>>> +iteratively splits the entry containing the index non-uniformly. >>>> +For example, to split an order-9 entry, which takes 2^(9-6)=3D8 slots= , >>>> +assuming ``XA_CHUNK_SHIFT`` is 6, xas_split_alloc() + xas_split() nee= d >>>> +8 xa_node. xas_try_split() splits the order-9 entry into >>>> +2 order-8 entries, then split one order-8 entry, based on the given i= ndex, >>>> +to 2 order-7 entries, ..., and split one order-1 entry to 2 order-0 e= ntries. >>>> +When splitting the order-6 entry and a new xa_node is needed, xas_try= _split() >>>> +will try to allocate one if possible. As a result, xas_try_split() wo= uld only >>>> +need 1 xa_node instead of 8. >>>> Functions and structures >>>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D >>>> diff --git a/include/linux/xarray.h b/include/linux/xarray.h >>>> index 0b618ec04115..9eb8c7425090 100644 >>>> --- a/include/linux/xarray.h >>>> +++ b/include/linux/xarray.h >>>> @@ -1555,6 +1555,8 @@ int xa_get_order(struct xarray *, unsigned long = index); >>>> int xas_get_order(struct xa_state *xas); >>>> void xas_split(struct xa_state *, void *entry, unsigned int order); >>>> void xas_split_alloc(struct xa_state *, void *entry, unsigned int o= rder, gfp_t); >>>> +void xas_try_split(struct xa_state *xas, void *entry, unsigned int or= der, >>>> + gfp_t gfp); >>>> #else >>>> static inline int xa_get_order(struct xarray *xa, unsigned long ind= ex) >>>> { >>>> @@ -1576,6 +1578,11 @@ static inline void xas_split_alloc(struct xa_st= ate *xas, void *entry, >>>> unsigned int order, gfp_t gfp) >>>> { >>>> } >>>> + >>>> +static inline void xas_try_split(struct xa_state *xas, void *entry, >>>> + unsigned int order, gfp_t gfp) >>>> +{ >>>> +} >>>> #endif >>>> /** >>> >>> [snip] >>> >>>> diff --git a/lib/xarray.c b/lib/xarray.c >>>> index 116e9286c64e..b9a63d7fbd58 100644 >>>> --- a/lib/xarray.c >>>> +++ b/lib/xarray.c >>>> @@ -1007,6 +1007,31 @@ static void node_set_marks(struct xa_node *node= , unsigned int offset, >>>> } >>>> } >>>> +static struct xa_node *__xas_alloc_node_for_split(struct xa_state *= xas, >>>> + void *entry, gfp_t gfp) >>>> +{ >>>> + unsigned int i; >>>> + void *sibling =3D NULL; >>>> + struct xa_node *node; >>>> + unsigned int mask =3D xas->xa_sibs; >>>> + >>>> + node =3D kmem_cache_alloc_lru(radix_tree_node_cachep, xas->xa_lru, g= fp); >>>> + if (!node) >>>> + return NULL; >>>> + node->array =3D xas->xa; >>>> + for (i =3D 0; i < XA_CHUNK_SIZE; i++) { >>>> + if ((i & mask) =3D=3D 0) { >>>> + RCU_INIT_POINTER(node->slots[i], entry); >>>> + sibling =3D xa_mk_sibling(i); >>>> + } else { >>>> + RCU_INIT_POINTER(node->slots[i], sibling); >>>> + } >>>> + } >>>> + RCU_INIT_POINTER(node->parent, xas->xa_alloc); >>>> + >>>> + return node; >>>> +} >>>> + >>>> /** >>>> * xas_split_alloc() - Allocate memory for splitting an entry. >>>> * @xas: XArray operation state. >>>> @@ -1025,7 +1050,6 @@ void xas_split_alloc(struct xa_state *xas, void = *entry, unsigned int order, >>>> gfp_t gfp) >>>> { >>>> unsigned int sibs =3D (1 << (order % XA_CHUNK_SHIFT)) - 1; >>>> - unsigned int mask =3D xas->xa_sibs; >>>> /* XXX: no support for splitting really large entries yet */ >>>> if (WARN_ON(xas->xa_shift + 2 * XA_CHUNK_SHIFT <=3D order)) >>>> @@ -1034,23 +1058,9 @@ void xas_split_alloc(struct xa_state *xas, void= *entry, unsigned int order, >>>> return; >>>> do { >>>> - unsigned int i; >>>> - void *sibling =3D NULL; >>>> - struct xa_node *node; >>>> - >>>> - node =3D kmem_cache_alloc_lru(radix_tree_node_cachep, xas->xa_lru, = gfp); >>>> + struct xa_node *node =3D __xas_alloc_node_for_split(xas, entry, gfp= ); >>>> if (!node) >>>> goto nomem; >>>> - node->array =3D xas->xa; >>>> - for (i =3D 0; i < XA_CHUNK_SIZE; i++) { >>>> - if ((i & mask) =3D=3D 0) { >>>> - RCU_INIT_POINTER(node->slots[i], entry); >>>> - sibling =3D xa_mk_sibling(i); >>>> - } else { >>>> - RCU_INIT_POINTER(node->slots[i], sibling); >>>> - } >>>> - } >>>> - RCU_INIT_POINTER(node->parent, xas->xa_alloc); >>>> xas->xa_alloc =3D node; >>>> } while (sibs-- > 0); >>>> @@ -1122,6 +1132,102 @@ void xas_split(struct xa_state *xas, void *e= ntry, unsigned int order) >>>> xas_update(xas, node); >>>> } >>>> EXPORT_SYMBOL_GPL(xas_split); >>>> + >>>> +/** >>>> + * xas_try_split() - Try to split a multi-index entry. >>>> + * @xas: XArray operation state. >>>> + * @entry: New entry to store in the array. >>>> + * @order: Current entry order. >>>> + * @gfp: Memory allocation flags. >>>> + * >>>> + * The size of the new entries is set in @xas. The value in @entry i= s >>>> + * copied to all the replacement entries. If and only if one xa_node = needs to >>>> + * be allocated, the function will use @gfp to get one. If more xa_no= de are >>>> + * needed, the function gives EINVAL error. >>>> + * >>>> + * Context: Any context. The caller should hold the xa_lock. >>>> + */ >>>> +void xas_try_split(struct xa_state *xas, void *entry, unsigned int or= der, >>>> + gfp_t gfp) >>> >>> The xas_try_split() may sleep if =E2=80=98gfp=E2=80=99 flags permit whi= le holding the xa_lock, which can cause issues. So can we add a check for t= he =E2=80=98gfp=E2=80=99 or only use GFP_NOWAIT? >> >> You mean only allow gfp to be GFP_NOWAIT or GFP_ATOMIC? > > Yes. After discussed with Matthew, I think it is better to use GFP_NOWAIT in xas_try_split() and user can use xas_nomem() if xas_try_split() fails to allocate a xa_node. So I will remove gfp in the parameter. I also discovered a bug in xas_try_split() when a xa_node is allocated from xas_nomem(), during my refactoring. Basically, the xa_node from xas_nomem() is not initialized for split, namely node->slots is not set correctly, so using that node in xas_try_split() corrupts xarray. This bug does not affect this series, but Minimize xa_node allocation during xarry split series. I will send out new versions of both series. Best Regards, Yan, Zi