linux-kernel - [PATCH v1 2/6] PCI/P2PDMA: Use a buffer on the stack for collecting the acs list

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Message-Id: <20210610160609.28447-3-logang@deltatee.com>
Date:   Thu, 10 Jun 2021 10:06:05 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        Bjorn Helgaas <helgaas@...nel.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>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>
Subject: [PATCH v1 2/6] PCI/P2PDMA: Use a buffer on the stack for collecting the acs list

In order to call the calc_map_type_and_dist_warn() function from
a dma_map operation, the function must not sleep. The only reason
it sleeps is to allocate memory for the seq_buf to print a verbose
warning telling the user how to disable ACS for that path.

Instead of allocating the memory with kmalloc, allocate it on
the stack with a smaller buffer. A 128B buffer is enough to print
10 pci device names. A system with 10 bridge ports between two devices
that have ACS enabled would be unusually large, so this should
still be a reasonable limit.

This also allows cleaning up the awkward (and broken) return with
-ENOMEM which contradicts the return type and the caller was
not prepared for.

Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
---
 drivers/pci/p2pdma.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
index 6f90e9812f6e..3a5fb63c5f2c 100644
--- a/drivers/pci/p2pdma.c
+++ b/drivers/pci/p2pdma.c
@@ -500,11 +500,10 @@ calc_map_type_and_dist_warn(struct pci_dev *provider, struct pci_dev *client,
 {
 	struct seq_buf acs_list;
 	bool acs_redirects;
+	char buf[128];
 	int ret;
 
-	seq_buf_init(&acs_list, kmalloc(PAGE_SIZE, GFP_KERNEL), PAGE_SIZE);
-	if (!acs_list.buffer)
-		return -ENOMEM;
+	seq_buf_init(&acs_list, buf, sizeof(buf));
 
 	ret = calc_map_type_and_dist(provider, client, dist, &acs_redirects,
 				     &acs_list);
@@ -522,8 +521,6 @@ calc_map_type_and_dist_warn(struct pci_dev *provider, struct pci_dev *client,
 			 pci_name(provider));
 	}
 
-	kfree(acs_list.buffer);
-
 	return ret;
 }
 
-- 
2.20.1