linux-kernel - Re: [RFC PATCH 01/15] PCI/P2PDMA: Don't sleep in upstream_bridge_distance

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Date:   Mon, 9 Nov 2020 10:12:06 +0100
From:   Christoph Hellwig <hch@....de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     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>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....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>
Subject: Re: [RFC PATCH 01/15] PCI/P2PDMA: Don't sleep in
 upstream_bridge_distance_warn()

On Fri, Nov 06, 2020 at 10:00:22AM -0700, Logan Gunthorpe wrote:
> In order to call this function from a dma_map function, it must not sleep.
> The only reason it does sleep so to allocate the seqbuf to print
> which devices are within the ACS path.
> 
> Switch the kmalloc call to use GFP_NOWAIT and simply not print that
> message if the buffer fails to be allocated.

Please pass in the actual gfp_t.  Especially from an I/O path
GFP_NOWAIT is not the right gfp_t anyway, you probably want GFP_ATOMIC
there.  But also for the path where we can sleep we should allow that.