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Date:   Tue, 13 Jun 2023 10:11:14 -0400
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     John Hsu =?utf-8?B?KOioseawuOe/sCk=?= <John.Hsu@mediatek.com>
Cc:     "maple-tree@lists.infradead.org" <maple-tree@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Yang =?utf-8?B?KOaliuaZuuW8tyk=?= 
        <Andrew.Yang@mediatek.com>,
        Qun-wei Lin =?utf-8?B?KOael+e+pOW0tCk=?= 
        <Qun-wei.Lin@mediatek.com>,
        Chinwen Chang =?utf-8?B?KOW8temMpuaWhyk=?= 
        <chinwen.chang@mediatek.com>,
        Casper Li =?utf-8?B?KOadjuS4reamrik=?= <casper.li@mediatek.com>,
        Kuan-Ying Lee =?utf-8?B?KOadjuWGoOepjik=?= 
        <Kuan-Ying.Lee@mediatek.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "yuzhao@google.com" <yuzhao@google.com>
Subject: Re: [BUG] trigger BUG_ON in mas_store_prealloc when low memory
Message-ID: <20230613141114.bwbnqsdazqbmyj3u@revolver>
Mail-Followup-To: "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
        John Hsu =?utf-8?B?KOioseawuOe/sCk=?= <John.Hsu@mediatek.com>,
        "maple-tree@lists.infradead.org" <maple-tree@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Yang =?utf-8?B?KOaliuaZuuW8tyk=?= <Andrew.Yang@mediatek.com>,
        Qun-wei Lin =?utf-8?B?KOael+e+pOW0tCk=?= <Qun-wei.Lin@mediatek.com>,
        Chinwen Chang =?utf-8?B?KOW8temMpuaWhyk=?= <chinwen.chang@mediatek.com>,
        Casper Li =?utf-8?B?KOadjuS4reamrik=?= <casper.li@mediatek.com>,
        Kuan-Ying Lee =?utf-8?B?KOadjuWGoOepjik=?= <Kuan-Ying.Lee@mediatek.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "yuzhao@google.com" <yuzhao@google.com>
References: <9cb8c599b1d7f9c1c300d1a334d5eb70ec4d7357.camel@mediatek.com>
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* John Hsu (=E8=A8=B1=E6=B0=B8=E7=BF=B0) <John.Hsu@mediatek.com> [230609 04=
:37]:
> Hi reviewers and author liam.howlett@oracle.com,
> Kindly ping.
>=20

Hello!

Thanks for reporting this issue.

> We met BUG_ON in mas_store_prealloc with kernel-6.1 stress testing
> environment.

version 6.1 or 6.1.x?  Which exact version (git id or version number)

> According to coredump, BUG_ON is triggered by mas->node with error
> number -ENOMEM(0xffffffffffffffd2).
> There are some mas_node_count functions in mas_wr_store_entry, and it
> seems that mas->node may be set to error node with -ENOMEM if there was
> no enough memory spcace for maple tree operations.
> We think that return -ENOMEM instead of directly triggering BUG_ON when
> memory is not available is suitable,

This BUG_ON() is necessary since this function should _never_ run out of
memory; this function does not return an error code. mas_preallocate()
should have gotten you the memory necessary (or returned an -ENOMEM)
prior to the call to mas_store_prealloc(), so this is probably an
internal tree problem.

>because in reality the tree
> operation shouldn't be performed in this situation.

There is a tree operation being performed here.  mprotect is merging a
vma by the looks of the call stack.  Why do you think no tree operation
is necessary?

>=20
> following are the backtrace:
> mas_store_prealloc+0x23c/0x484
> vma_mas_store+0xe4/0x2d0
> __vma_adjust+0xab0/0x1470
> vma_merge+0x5b8/0x5d4
> mprotect_fixup+0x1f4/0x478
> __arm64_sys_mprotect+0x6b0/0x8f0
> invoke_syscall+0x84/0x264
> el0_svc_common+0x118/0x1f0
> do_el0_svc+0x5c/0x184
> el0_svc+0x38/0x98

I see this is arm64.  Do you have a reproducer?  If you don't have a
reproducer, I can try stress-ng on amr64 to simulate your workload using
mprotect, but I need to know the exact kernel version as this issue may
have been fixed in a later stable release.

Thanks,
Liam