linux-kernel - Re: [PATCH 01/16] PCI/P2PDMA: Pass gfp_mask flags to upstream_bridge_distance

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Date:   Mon, 3 May 2021 12:20:38 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     John Hubbard <jhubbard@...dia.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
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....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>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 01/16] PCI/P2PDMA: Pass gfp_mask flags to
 upstream_bridge_distance_warn()



On 2021-05-03 12:17 p.m., John Hubbard wrote:
> On 5/3/21 8:57 AM, Logan Gunthorpe wrote:
>>
>>
>> On 2021-05-01 9:58 p.m., John Hubbard wrote:
>>> Another odd thing: this used to check for memory failure and just give
>>> up, and now it doesn't. Yes, I realize that it all still works at the
>>> moment, but this is quirky and we shouldn't stop here.
>>>
>>> Instead, a cleaner approach would be to push the memory allocation
>>> slightly higher up the call stack, out to the
>>> pci_p2pdma_distance_many(). So pci_p2pdma_distance_many() should make
>>> the kmalloc() call, and fail out if it can't get a page for the seq_buf
>>> buffer. Then you don't have to do all this odd stuff.
>>
>> I don't really agree with this assessment. If kmalloc fails to
>> initialize the seq_buf() (which should be very rare), the only thing
>> that is lost is the one warning print that tells the user the command
>> line parameter needed disable the ACS. Everything else works fine,
>> nothing else can fail. I don't see the need to add extra complexity just
>> so the code errors out in no-mem instead of just skipping the one,
>> slightly more informative, warning line.
> 
> That's the thing: memory failure should be exceedingly rare for this.
> Therefore, just fail out entirely (which I don't expect we'll likely
> ever see), instead of doing all this weird stuff to try to continue
> on if you cannot allocate a single page. If you are in that case, the
> system is not in a state that is going to run your dma p2p setup well
> anyway.
> 
> I think it's *less* complexity to allocate up front, fail early if
> allocation fails, and then not have to deal with these really odd
> quirks at the lower levels.
>

I don't see how it's all that weird. We're skipping a warning if we
can't allocate memory to calculate part of the message. It's really not
necessary. If the memory really can't be allocated then something else
will fail, but we really don't need to fail here because we couldn't
print a verbose warning message.

Logan