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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 5 Aug 2022 16:09:52 +0200
Message-ID: <CAK8P3a2hyhzjYqeL1LY7WziDjXQJasg3jFe83eErzKgbfP-a3w@mail.gmail.com>
Subject: Re: [PATCH 4/4] sample/reserved_mem: Introduce a sample of struct
 page and dio support to no-map rmem
To:     Li Chen <me@linux.beauty>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Li Chen <lchen@ambarella.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        DTML <devicetree@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>
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On Thu, Aug 4, 2022 at 12:07 PM Li Chen <me@linux.beauty> wrote:

> Apart from our cases, I heard there are some other cases where cma_alloc
>  failed even non-cma system memory has enough memory because pages in
> CMA memory are pinned and cannot move out of CMA. There are some fixes like
> 1. move these memory out of CMA before pinned
> 2. only allow non-long-time pinned memory allocation from CMA.
>
> But these two solutions are not merged into the mainline yet.

Right, I think this has come up before, not sure why it wasn't implemented.
My feeling is that 2. cannot work because you don't know if memory will be
pinned in the future at the time of allocation, but 1. should be doable.

A possible alternative here would be to avoid the pinning. In most workloads
it should not be possible to pin a large number of pages, but I assume there
is a good reason to do so here.

        Arnd