linux-kernel - Re: [PATCH v5 27/32] PCI/P2PDMA: Convert to printbuf

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Date:   Mon, 8 Aug 2022 14:42:03 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, pmladek@...e.com,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH v5 27/32] PCI/P2PDMA: Convert to printbuf

On 8/8/22 13:51, Bjorn Helgaas wrote:
> I don't object to the patch, but it would be nice if the commit log
> hinted at what the advantage is.  I assume it's faster/safer/better in
> some way, but I have no idea what.

Printbufs have some additional features over seq_buf but they're not 
used here. The main one you might be interested in is heap allocation: 
that means no need to statically allocate buffers on the stack and no 
need to calculate the buffer size, printbufs will reallocate as necessary.

I generally haven't been converting code to use that unless it's obvious 
that we're in a context where it's safe to allocate memory and can deal 
with allocation failures.

I notice that in calc_map_type_and_dist() you're using xa_store() which 
can fail, but you're not checking for that or returning errors properly 
:) perhaps a fix for that could also switch to using printbuf in 
heap-allocation mode.

> Also, cpu_show_meltdown() doesn't appear in this patch, so maybe
> that's relevant to some other patch but not this one?

Whoops, was copying the commit message from another patch, yeah.n