linux-kernel - Re: [PATCH v2] mm: cma: indefinitely retry allocations in cma

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Message-ID: <20200928073907.GA29322@infradead.org>
Date:   Mon, 28 Sep 2020 08:39:07 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Chris Goldsworthy <cgoldswo@...eaurora.org>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pratikp@...eaurora.org, pdaly@...eaurora.org,
        sudraja@...eaurora.org, iamjoonsoo.kim@....com,
        linux-arm-msm-owner@...r.kernel.org,
        Vinayak Menon <vinmenon@...eaurora.org>,
        linux-kernel-owner@...r.kernel.org
Subject: Re: [PATCH v2] mm: cma: indefinitely retry allocations in cma_alloc

On Tue, Sep 15, 2020 at 09:53:30AM +0200, David Hildenbrand wrote:
> Two thoughts:
> 
> 1. Most (all?) alloc_contig_range() users are interested in handling
> short-term pinnings in a nice way (IOW, make the allocation succeed).
> I'd much rather want to see this being handled in a nice fashion inside
> alloc_contig_range() than having to encode endless loops in the caller.
> This means I strongly prefer something like [3] if feasible. But I can
> understand that stuff ([5]) is complicated. I have to admit that I am
> not an expert on the short term pinning described by you, and how to
> eventually fix it.

Agreed.  Also retrying forever is simply broken, and will lead to
deadlocks for the DMA calls into CMA, so with my dma-mapping hat on
I have to hard-NAK this approach.