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Tue, 8 Aug 2023 20:01:00 +0000 Received: from SN6PR10MB3022.namprd10.prod.outlook.com ([fe80::26d3:6f41:6415:8c35]) by SN6PR10MB3022.namprd10.prod.outlook.com ([fe80::26d3:6f41:6415:8c35%3]) with mapi id 15.20.6652.026; Tue, 8 Aug 2023 20:01:00 +0000 Date: Tue, 8 Aug 2023 16:00:57 -0400 From: "Liam R. 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=?= , "surenb@google.com" , "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: <20230808200057.p3pqwkumghqbdvbi@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=?= , "surenb@google.com" , "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: <20230613141114.bwbnqsdazqbmyj3u@revolver> <3b14df2fc2a7f18fe12f87a27574b7d40f2899ba.camel@mediatek.com> <20230614155826.cxcpv33hs763gyrg@revolver> <20230706185436.agobbv72o3hma43z@revolver> <3507fdacf001c49f4c73f2952257beabfe9e1a53.camel@mediatek.com> <20230710142425.t5vhrqkjd7ugbmad@revolver> <20230719185135.nyun5fvcuv3ydku4@revolver> <981aac92a51c4e2cfd2ca83d11742e5741e60285.camel@mediatek.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: 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 DbuY2D4z 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Howlett wrote: ... > > > As I know, following is rb_tree flow in 5.15.186: > > >=20 > > > ... > > > mmap_write_lock_killable(mm) > > > ... > > > do_mmap() > > > ... > > > mmap_region() > > > ... > > > vm_area_alloc(mm) > > > ... > > > mmap_write_unlock(mm) > > >=20 > > > vm_area_alloc is in the mmap_lock hoding period. > > > It seems that the flow would sleep here in rb_tree flow. > > > If I miss anything, please tell me, thanks! > >=20 > > Before the mmap_write_unlock(mm) in the above sequence, the > > i_mmap_lock_write(), anon_vma_lock_write(), and/or the > > flush_dcache_mmap_lock() may be taken. Check __vma_adjust(). > >=20 > > The insertion into the tree needs to hold some subset of these locks. > > The rb-tree insert did not allocate within these locks, but the maple > > tree would need to allocate within these locks to insert into the > > tree. > > This is why the preallocation exists and why it is necessary. > >=20 >=20 > Yap, preallocation is necessary. anon_vma_lock_write() and > flush_dcache_mmap_lock() hold the lock and manipulate rb_tree. I think > that there is no maple tree manipulations during the lock holding > period. Is there any future work in this section? __vma_adjust() does modify the maple tree during the lock holding section through vma_mas_store() in 6.1. Prior to 6.1, there is no maple tree. ... > > There are also config options to debug the tree operations, but they > > do > > not detect the redundant write issues. Perhaps I can look at adding > > support for detecting redundant writes, but that will not be > > backported > > to a stable kernel. > >=20 >=20 > The sufficient test cases of maple tree ensure the function work well. > But the redundant operations (alloc node, free node, tree > manipulations) of maple_tree are not easy to detect (e.g. the case > reported this time and mas_preallocate() allocates redundant nodes with > the worst case). >=20 > The detecting redundant writes mechanism may help the developers to > find out the problems easier. Hope it can be establised successfully!! When I went to add this, I had found I already added it here [1]. This operation was not caught by MA_STATE_PREALLOC because there are two writes before a mas_destroy(), so there may be nodes left which avoid the warning. I'll look at improving this situation. Thanks, Liam [1] https://lore.kernel.org/linux-mm/20220722160546.1478722-2-Liam.Howlett@= oracle.com/