linux-kernel - Re: [RFC PATCH v2 04/11] PCI/P2PDMA: Introduce pci_p2pdma_should_map_bus() and pci_p2pdma_bus

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Date:   Wed, 24 Mar 2021 19:34:53 +0100
From:   Christian König <christian.koenig@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Logan Gunthorpe <logang@...tatee.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Ira Weiny <iweiny@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>
Subject: Re: [RFC PATCH v2 04/11] PCI/P2PDMA: Introduce
 pci_p2pdma_should_map_bus() and pci_p2pdma_bus_offset()

Am 24.03.21 um 18:21 schrieb Jason Gunthorpe:
> On Mon, Mar 15, 2021 at 10:27:08AM -0600, Logan Gunthorpe wrote:
>
>> In this case the WARN_ON is just to guard against misuse of the
>> function. It should never happen unless a developer changes the code in
>> a way that is incorrect. So I think that's the correct use of WARN_ON.
>> Though I might change it to WARN and return, that seems safer.
> Right, WARN_ON and return is the right pattern for an assertion that
> must never happen:
>
>    if (WARN_ON(foo))
>        return -1
>
> Linus wants assertions like this to be able to recover. People runing
> the 'panic on warn' mode want the kernel to stop if it detects an
> internal malfunction.

The only justification I can see for a "panic on warn" is to prevent 
further data loss or warn early about a crash.

We only use a BUG_ON() when the alternative would be to corrupt something.

Christian.

>
> Jason