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Howlett" To: John Hsu =?utf-8?B?KOioseawuOe/sCk=?= Cc: Andrew Yang =?utf-8?B?KOaliuaZuuW8tyk=?= , "linux-kernel@vger.kernel.org" , Qun-wei Lin =?utf-8?B?KOael+e+pOW0tCk=?= , "linux-mm@kvack.org" , Chinwen Chang =?utf-8?B?KOW8temMpuaWhyk=?= , Kuan-Ying Lee =?utf-8?B?KOadjuWGoOepjik=?= , Casper Li =?utf-8?B?KOadjuS4reamrik=?= , "akpm@linux-foundation.org" , "yuzhao@google.com" , "maple-tree@lists.infradead.org" Subject: Re: [BUG] trigger BUG_ON in mas_store_prealloc when low memory Message-ID: <20230706185436.agobbv72o3hma43z@revolver> Mail-Followup-To: "Liam R. Howlett" , John Hsu =?utf-8?B?KOioseawuOe/sCk=?= , Andrew Yang =?utf-8?B?KOaliuaZuuW8tyk=?= , "linux-kernel@vger.kernel.org" , Qun-wei Lin =?utf-8?B?KOael+e+pOW0tCk=?= , "linux-mm@kvack.org" , Chinwen Chang =?utf-8?B?KOW8temMpuaWhyk=?= , Kuan-Ying Lee =?utf-8?B?KOadjuWGoOepjik=?= , Casper Li =?utf-8?B?KOadjuS4reamrik=?= , "akpm@linux-foundation.org" , "yuzhao@google.com" , "maple-tree@lists.infradead.org" References: <9cb8c599b1d7f9c1c300d1a334d5eb70ec4d7357.camel@mediatek.com> <20230613141114.bwbnqsdazqbmyj3u@revolver> <3b14df2fc2a7f18fe12f87a27574b7d40f2899ba.camel@mediatek.com> <20230614155826.cxcpv33hs763gyrg@revolver> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: User-Agent: NeoMutt/20220429 X-ClientProxiedBy: YT4PR01CA0450.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:10d::29) To SN6PR10MB3022.namprd10.prod.outlook.com (2603:10b6:805:d8::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email 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definitions=2023-07-06_13,2023-07-06_02,2023-05-22_02 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 phishscore=0 bulkscore=0 malwarescore=0 mlxlogscore=914 adultscore=0 spamscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2305260000 definitions=main-2307060168 X-Proofpoint-GUID: WgLvFLZySR_ii7BQH8tX5ama0sK79bes X-Proofpoint-ORIG-GUID: WgLvFLZySR_ii7BQH8tX5ama0sK79bes X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: D963BC0018 X-Stat-Signature: angx4o5eya8btah1io8eks68w5ssfpnq X-HE-Tag: 1688669684-937484 X-HE-Meta: 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 763fplSi 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Apologies for the late response. * John Hsu (=E8=A8=B1=E6=B0=B8=E7=BF=B0) [230616 05= :19]: > On Wed, 2023-06-14 at 11:58 -0400, Liam R. Howlett wrote: > > =20 > > External email : Please do not click links or open attachments until > > you have verified the sender or the content. > > * John Hsu (=E8=A8=B1=E6=B0=B8=E7=BF=B0) [2306= 14 03:06]: > > > Hi Liam, thanks for your reply. > >=20 > > Sorry, your email response with top posting is hard to follow so I > > will > > do my best to answer your questions. >=20 > Sorry for the wrong format.... >=20 > > >=20 > > >=20 > > >=20 > > > version 6.1 or 6.1.x? Which exact version (git id or version > > number) > > >=20 > > > Our environment is kernel-6.1.25-mainline-android14-5- > > gdea04bf2c398d. > >=20 > > Okay, I can have a look at 6.1.25 then. >=20 > OK, thanks. >=20 > > >=20 > > >=20 > > > This BUG_ON() is necessary since this function should _never_ run > > out of > > >=20 > > > memory; this function does not return an error code. > > mas_preallocate() > > >=20 > > > should have gotten you the memory necessary (or returned an > > -ENOMEM) > > >=20 > > > prior to the call to mas_store_prealloc(), so this is probably an > > >=20 > > > internal tree problem. > > >=20 > > > There is a tree operation being performed here. mprotect is > > merging a > > >=20 > > > vma by the looks of the call stack. Why do you think no tree > > operation > > >=20 > > > is necessary? > > >=20 > > > As you mentioned, mas_preallocate() should allocate enough node, > > but there is such functions mas_node_count() in mas_store_prealloc(). > > > In mas_node_count() checks whether the *mas* has enough nodes, and > > allocate memory for node if there was no enough nodes in mas. > >=20 > > Right, we call mas_node_count() so that both code paths are used for > > preallocations and regular mas_store()/mas_store_gfp(). It shouldn't > > take a significant amount of time to verify there is enough nodes. >=20 > Yap..., it didn't take a significant amount of time to verify whether > there is enough nodes. The problem is why the flow in mas_node_count > will alloc nodes if there was no enough nodes in mas? What I meant is that both methods use the same call path because there is not a reason to duplicate the path. After mas_preallocate() has allocated the nodes needed, the call to check if there is enough nodes will be quick. >=20 > > > I think that if mas_preallocate() allocate enough node, why we > > check the node count and allocate nodes if there was no enough nodes > > in mas in mas_node_count()? > >=20 > > We check for the above reason. > >=20 >=20 > OK..., this is one of the root cause of this BUG. The root cause is that there was not enough memory for a store operation. Regardless of if we check the allocations in the mas_store_prealloc() path or not, this would fail. If we remove the check for nodes within this path, then we would have to BUG_ON() when we run out of nodes to use or have a null pointer dereference BUG anyways. >=20 > > >=20 > > > We have seen that there may be some maple_tree operations in > > merge_vma... > >=20 > > If merge_vma() does anything, then there was an operation to the > > maple > > tree. > >=20 > > >=20 > > > Moreover, would maple_tree provides an API for assigning user's gfp > > flag for allocating node? > >=20 > > mas_preallocate() and mas_store_gfp() has gfp flags as an > > argument. In > > your call stack, it will be called in __vma_adjust() as such: > >=20 > > if (mas_preallocate(&mas, vma, GFP_KERNEL)) > > return -ENOMEM; > >=20 > > line 715 in v6.1.25 > >=20 > > > In rb_tree, we allocate vma_area_struct (rb_node is in this > > struct.) with GFP_KERNEL, and maple_tree allocate node with > > GFP_NOWAIT and __GFP_NOWARN. > >=20 > > We use GFP_KERNEL as I explained above for the VMA tree. >=20 > Got it! But the mas_node_count() always use GFP_NOWAIT and __GFP_NOWARN > in inserting tree flow. Do you consider the performance of maintaining > the structure of maple_tree? Sorry, I don't understand what you mean by 'consider the performance of maintaining the structure of maple_tree'. >=20 > > It also will drop the lock and retry with GFP_KERNEL on failure > > when not using the external lock. The mmap_lock is configured as an > > external lock. > >=20 > > > Allocation will not wait for reclaiming and compacting when there > > is no enough available memory. > > > Is there any concern for this design? > >=20 > > This has been addressed above, but let me know if I missed anything > > here. > >=20 >=20 > I think that the mas_node_count() has higher rate of triggering > BUG_ON() when allocating nodes with GFP_NOWAIT and __GFP_NOWARN. If > mas_node_count() use GFP_KERNEL as mas_preallocate() in the mmap.c, the > allocation fail rate may be lower than use GFP_NOWAIT. Which BUG_ON() are you referring to? If I was to separate the code path for mas_store_prealloc() and mas_store_gfp(), then a BUG_ON() would still need to exist and still would have been triggered.. We are in a place in the code where we should never sleep and we don't have enough memory allocated to do what was necessary. Thanks, Liam