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From: Uladzislau Rezki <urezki@gmail.com>
Date: Thu, 13 Jun 2024 13:28:00 +0200
To: Baoquan He <bhe@redhat.com>
Cc: Uladzislau Rezki <urezki@gmail.com>,
	Zhaoyang Huang <huangzhaoyang@gmail.com>,
	"zhaoyang.huang" <zhaoyang.huang@unisoc.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	hailong liu <hailong.liu@oppo.com>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	steve.kang@unisoc.com
Subject: Re: [Resend PATCHv4 1/1] mm: fix incorrect vbq reference in
 purge_fragmented_block
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On Thu, Jun 13, 2024 at 04:41:34PM +0800, Baoquan He wrote:
> On 06/12/24 at 01:27pm, Uladzislau Rezki wrote:
> > On Wed, Jun 12, 2024 at 10:00:14AM +0800, Zhaoyang Huang wrote:
> > > On Wed, Jun 12, 2024 at 2:16 AM Uladzislau Rezki <urezki@gmail.com> wrote:
> > > >
> > > > >
> > > > > Sorry to bother you again. Are there any other comments or new patch
> > > > > on this which block some test cases of ANDROID that only accept ACKed
> > > > > one on its tree.
> > > > >
> > > > I have just returned from vacation. Give me some time to review your
> > > > patch. Meanwhile, do you have a reproducer? So i would like to see how
> > > > i can trigger an issue that is in question.
> > > This bug arises from an system wide android test which has been
> > > reported by many vendors. Keep mount/unmount an erofs partition is
> > > supposed to be a simple reproducer. IMO, the logic defect is obvious
> > > enough to be found by code review.
> > >
> > Baoquan, any objection about this v4?
> > 
> > Your proposal about inserting a new vmap-block based on it belongs
> > to, i.e. not per-this-cpu, should fix an issue. The problem is that
> > such way does __not__ pre-load a current CPU what is not good.
> 
> With my understand, when we start handling to insert vb to vbq->xa and
> vbq->free, the vmap_area allocation has been done, it doesn't impact the
> CPU preloading when adding it into which CPU's vbq->free, does it? 
> 
> Not sure if I miss anything about the CPU preloading.
> 
Like explained below in this email-thread:

vb_alloc() inserts a new block _not_ on this CPU. This CPU tries to
allocate one more time and its free_list is empty(because on a prev.
step a block has been inserted into another CPU-block-queue), thus
it allocates a new block one more time and which is inserted most
likely on a next zone/CPU. And so on.

See:

<snip vb_alloc>
...
        rcu_read_lock();
	vbq = raw_cpu_ptr(&vmap_block_queue); <- Here it is correctly accessing this CPU 
	list_for_each_entry_rcu(vb, &vbq->free, free_list) {
		unsigned long pages_off;
...
<snip vb_alloc>

<snip new_vmap_block>
...
       vbq = addr_to_vbq(va->va_start); <- Here we insert based on hashing, i.e. not to this CPU-block-queue
       spin_lock(&vbq->lock);
       list_add_tail_rcu(&vb->free_list, &vbq->free);
       spin_unlock(&vbq->lock);
...
<snip new_vmap_block>

Thanks!

--
Uladzislau Rezki