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Date: Tue, 11 May 2021 12:05:26 -0400
From: Don Dutile <ddutile@...hat.com>
To: John Hubbard <jhubbard@...dia.com>,
Logan Gunthorpe <logang@...tatee.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>,
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 5/1/21 11:58 PM, John Hubbard wrote:
> On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
>> In order to call upstream_bridge_distance_warn() from a dma_map function,
>> it must not sleep. The only reason it does sleep is to allocate the seqbuf
>> to print which devices are within the ACS path.
>>
>> Switch the kmalloc call to use a passed in gfp_mask and don't print that
>> message if the buffer fails to be allocated.
>>
>> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
>> Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>
>> ---
>> drivers/pci/p2pdma.c | 21 +++++++++++----------
>> 1 file changed, 11 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index 196382630363..bd89437faf06 100644
>> --- a/drivers/pci/p2pdma.c
>> +++ b/drivers/pci/p2pdma.c
>> @@ -267,7 +267,7 @@ static int pci_bridge_has_acs_redir(struct pci_dev *pdev)
>> static void seq_buf_print_bus_devfn(struct seq_buf *buf, struct pci_dev *pdev)
>> {
>> - if (!buf)
>> + if (!buf || !buf->buffer)
>
> This is not great, sort of from an overall design point of view, even though
> it makes the rest of the patch work. See below for other ideas, that will
> avoid the need for this sort of odd point fix.
>
+1.
In fact, I didn't see how the kmalloc was changed... you refactored the code to pass-in the
GFP_KERNEL that was originally hard-coded into upstream_bridge_distance_warn();
I don't see how that avoided the kmalloc() call.
in fact, I also see you lost a failed kmalloc() check, so it seems to have taken a step back.
>> return;
>> seq_buf_printf(buf, "%s;", pci_name(pdev));
>> @@ -495,25 +495,26 @@ upstream_bridge_distance(struct pci_dev *provider, struct pci_dev *client,
>> static enum pci_p2pdma_map_type
>> upstream_bridge_distance_warn(struct pci_dev *provider, struct pci_dev *client,
>> - int *dist)
>> + int *dist, gfp_t gfp_mask)
>> {
>> struct seq_buf acs_list;
>> bool acs_redirects;
>> int ret;
>> - seq_buf_init(&acs_list, kmalloc(PAGE_SIZE, GFP_KERNEL), PAGE_SIZE);
>> - if (!acs_list.buffer)
>> - return -ENOMEM;
>
> 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.
>
> Furthermore, the call sites can then decide for themselves which GFP
> flags, GFP_ATOMIC or GFP_KERNEL or whatever they want for kmalloc().
>
agree, good proposal to avoid a sleep due to 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).
>
> Really, these functions should all be doing "0 for success, -ERRNO for
> failure, and pass other values, including results, in the arg list".
>
WFM!
>
>> + seq_buf_init(&acs_list, kmalloc(PAGE_SIZE, gfp_mask), PAGE_SIZE);
>> ret = upstream_bridge_distance(provider, client, dist, &acs_redirects,
>> &acs_list);
>> if (acs_redirects) {
>> pci_warn(client, "ACS redirect is set between the client and provider (%s)\n",
>> pci_name(provider));
>> - /* Drop final semicolon */
>> - acs_list.buffer[acs_list.len-1] = 0;
>> - pci_warn(client, "to disable ACS redirect for this path, add the kernel parameter: pci=disable_acs_redir=%s\n",
>> - acs_list.buffer);
>> +
>> + if (acs_list.buffer) {
>> + /* Drop final semicolon */
>> + acs_list.buffer[acs_list.len - 1] = 0;
>> + pci_warn(client, "to disable ACS redirect for this path, add the kernel parameter: pci=disable_acs_redir=%s\n",
>> + acs_list.buffer);
>> + }
>> }
>> if (ret == PCI_P2PDMA_MAP_NOT_SUPPORTED) {
>> @@ -566,7 +567,7 @@ int pci_p2pdma_distance_many(struct pci_dev *provider, struct device **clients,
>> if (verbose)
>> ret = upstream_bridge_distance_warn(provider,
>> - pci_client, &distance);
>> + pci_client, &distance, GFP_KERNEL);
>> else
>> ret = upstream_bridge_distance(provider, pci_client,
>> &distance, NULL, NULL);
>>
>
> thanks,
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