linux-kernel - Re: [PATCH 1/1] dax: add PROBE_PREFER

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Message-ID: <68f9a61ccc6b1_10e9100e5@dwillia2-mobl4.notmuch>
Date: Wed, 22 Oct 2025 20:50:52 -0700
From: <dan.j.williams@...el.com>
To: Michał Cłapiński <mclapinski@...gle.com>,
	<dan.j.williams@...el.com>
CC: Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang
	<dave.jiang@...el.com>, <nvdimm@...ts.linux.dev>,
	<linux-cxl@...r.kernel.org>, Pasha Tatashin <pasha.tatashin@...een.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] dax: add PROBE_PREFER_ASYNCHRONOUS to the pmem driver

Michał Cłapiński wrote:
[..]
> > However, I believe that falls back to synchronous probing if the driver
> > is loaded after the device has already arrived. Is that the case you are
> > hitting?
> 
> Yes. I use all pmem/devdax modules built into the kernel so loading
> them is in the critical path for kernel boot.
> I use memmap= with devdax. So first, the pmem device is created
> asynchronously, which means loading the nd_e820 module is always fast.
> But then, the dax_pmem driver is loaded. If the dax device has not yet
> been created by the async code, then loading this module is also fast.
> But if the dax device has already been created, then attaching it to
> the dax_pmem driver will be synchronous and on the critical boot path.
> 
> For thousands of dax devices, this increases the boot time by more
> than a second. With the patch it takes ~10ms.
> 
> > I am ok with this in concept, but if we do this it should be done for
> > all dax drivers, not just dax_pmem.
> 
> Will do in v2.

Sounds good, include that detail above and I'll ack / poke Ira to pick
it up.