Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Received-SPF: Pass (protection.outlook.com: domain of nvidia.com designates
 216.228.112.34 as permitted sender) receiver=protection.outlook.com;
 client-ip=216.228.112.34; helo=mail.nvidia.com;
Subject: Re: [PATCH 01/16] PCI/P2PDMA: Pass gfp_mask flags to
 upstream_bridge_distance_warn()
To:     Logan Gunthorpe <logang@deltatee.com>,
        <linux-kernel@vger.kernel.org>, <linux-nvme@lists.infradead.org>,
        <linux-block@vger.kernel.org>, <linux-pci@vger.kernel.org>,
        <linux-mm@kvack.org>, <iommu@lists.linux-foundation.org>
CC:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Don Dutile <ddutile@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Jakowski Andrzej <andrzej.jakowski@intel.com>,
        Minturn Dave B <dave.b.minturn@intel.com>,
        Jason Ekstrand <jason@jlekstrand.net>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Xiong Jianxin <jianxin.xiong@intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Bjorn Helgaas <bhelgaas@google.com>
References: <20210408170123.8788-1-logang@deltatee.com>
 <20210408170123.8788-2-logang@deltatee.com>
 <d8ac4c84-1e69-d5d6-991a-7de87c569acc@nvidia.com>
 <8ea5b5b3-e10f-121a-bd2a-07db83c6da01@deltatee.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <3bced3a4-b826-46ab-3d98-d2dc6871bfe1@nvidia.com>
Date:   Mon, 3 May 2021 11:17:31 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.10.0
MIME-Version: 1.0
In-Reply-To: <8ea5b5b3-e10f-121a-bd2a-07db83c6da01@deltatee.com>
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 03 May 2021 18:17:32.7214
 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: 667131e9-b486-446f-90c4-08d90e5fb869
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.112.34];Helo=[mail.nvidia.com]
X-MS-Exchange-CrossTenant-AuthSource: DM6NAM11FT023.eop-nam11.prod.protection.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Anonymous
X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem
X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR12MB4429
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.

> 
> Also, keep in mind the result of all these functions are cached so it
> only ever happens once. So for this to matter, the user would have to do
> their first transaction between two devices exactly at the time memory
> allocations would fail.
> 
> 
>> Furthermore, the call sites can then decide for themselves which GFP
>> flags, GFP_ATOMIC or GFP_KERNEL or whatever they want for kmalloc().
>>
>> A related thing: this whole exercise would go better if there were a
>> preparatory patch or two that changed the return codes in this file to
>> something less crazy. There are too many functions that can fail, but
>> are treated as if they sort-of-mostly-would-never-fail, in the hopes of
>> using the return value directly for counting and such. This is badly
>> mistaken, and it leads developers to try to avoid returning -ENOMEM
>> (which is what we need here).
> 
> Hmm? Which functions can fail? and how?
> 

Let's defer that to the other patches, I was sort of looking ahead to
those, sorry.

thanks,
-- 
John Hubbard
NVIDIA