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Date:   Wed, 20 Apr 2022 14:57:39 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     lipeifeng@oppo.com
Cc:     peifeng55@gmail.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, 21cnbao@gmail.com,
        zhangshiming@oppo.com
Subject: Re: [PATCH] mm: modify the method to search addr in unmapped_area
Message-Id: <20220420145739.425c01603a6f63e550e556ed@linux-foundation.org>
In-Reply-To: <20220420084039.1431-1-lipeifeng@oppo.com>
References: <20220420084039.1431-1-lipeifeng@oppo.com>
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On Wed, 20 Apr 2022 16:40:39 +0800 lipeifeng@oppo.com wrote:

> The old method will firstly find the space in len(info->length
> + info->align_mask), and get address at the desired alignment.
> 
> Sometime, addr  would be failed if there are enough
> addr space in kernel by above method, e.g., you can't get a
> addr sized in 1Mbytes, align_mask 1Mbytes successfully although
> there are still (2M-1)bytes space in kernel.
> 
> This patch would fix thr problem above by the new method: find the
> space in info->length and judge if at the desired info->align_mask
> at the same time.
> 
> Do a simple test in TIF_32BIT with unmapped_area:
> - Try to take addr (size:1M align:2M) until allocation fails;
> - Try to take addr (size:1M align:1M) and account how to space can
> be alloced successfully.
> 
> Before optimization: alloced 0     bytes.
> After  optimization: alloced 1.9+G bytes.

Thanks.

Unfortunately this part of the code is undergoing a lot of change
lately.  How serious is this problem?  Please tell us how often the
problem is being observed, under what circumstances, etc.