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Date:   Tue, 8 Aug 2023 16:00:57 -0400
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     John Hsu =?utf-8?B?KOioseawuOe/sCk=?= <John.Hsu@mediatek.com>
Cc:     Andrew Yang =?utf-8?B?KOaliuaZuuW8tyk=?= 
        <Andrew.Yang@mediatek.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Qun-wei Lin =?utf-8?B?KOael+e+pOW0tCk=?= 
        <Qun-wei.Lin@mediatek.com>,
        "surenb@google.com" <surenb@google.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Chinwen Chang =?utf-8?B?KOW8temMpuaWhyk=?= 
        <chinwen.chang@mediatek.com>,
        Kuan-Ying Lee =?utf-8?B?KOadjuWGoOepjik=?= 
        <Kuan-Ying.Lee@mediatek.com>,
        Casper Li =?utf-8?B?KOadjuS4reamrik=?= <casper.li@mediatek.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "yuzhao@google.com" <yuzhao@google.com>,
        "maple-tree@lists.infradead.org" <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" <Liam.Howlett@Oracle.com>,
        John Hsu =?utf-8?B?KOioseawuOe/sCk=?= <John.Hsu@mediatek.com>,
        Andrew Yang =?utf-8?B?KOaliuaZuuW8tyk=?= <Andrew.Yang@mediatek.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Qun-wei Lin =?utf-8?B?KOael+e+pOW0tCk=?= <Qun-wei.Lin@mediatek.com>,
        "surenb@google.com" <surenb@google.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Chinwen Chang =?utf-8?B?KOW8temMpuaWhyk=?= <chinwen.chang@mediatek.com>,
        Kuan-Ying Lee =?utf-8?B?KOadjuWGoOepjik=?= <Kuan-Ying.Lee@mediatek.com>,
        Casper Li =?utf-8?B?KOadjuS4reamrik=?= <casper.li@mediatek.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "yuzhao@google.com" <yuzhao@google.com>,
        "maple-tree@lists.infradead.org" <maple-tree@lists.infradead.org>
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* John Hsu (=E8=A8=B1=E6=B0=B8=E7=BF=B0) <John.Hsu@mediatek.com> [230807 05=
:55]:
> On Wed, 2023-07-19 at 14:51 -0400, Liam R. 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/