linux-kernel - Re: [PATCH 3/3] cxl/core: Add sysfs attribute get

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Date:   Fri, 17 Jun 2022 18:35:49 -0700
From:   Alison Schofield <alison.schofield@...el.com>
To:     "Williams, Dan J" <dan.j.williams@...el.com>
Cc:     "Weiny, Ira" <ira.weiny@...el.com>,
        "Verma, Vishal L" <vishal.l.verma@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] cxl/core: Add sysfs attribute get_poison for list
 retrieval

On Fri, Jun 17, 2022 at 06:08:52PM -0700, Dan Williams wrote:
> Alison Schofield wrote:
> > On Fri, Jun 17, 2022 at 11:42:11AM -0700, Dan Williams wrote:
> > > alison.schofield@ wrote:
> > > > From: Alison Schofield <alison.schofield@...el.com>
> > > > 
> > > > The sysfs attribute, get_poison, allows user space to request the
> > > > retrieval of a CXL devices poison list for its persistent memory.
> > > 
> > > If the device supports get poison list for volatile memory, just grab
> > > that too. With the "to be released soon" region patches userspace can
> > > trivially translate DPA addresses to media type.
> > > 
> > 
> > Dan,
> > 
> > The only way I know to discover if the device supports poison list for
> > volatile is to do the get_poison_list on the volatile range and see
> > what happens. Am I missing a capability setting somewhere?
> 
> If someone executes "echo 1 > trace_poison_list" I expect that the
> driver does:
> 
> get_poison_list(volatile_range);
> get_poison_list(pmem_range);
> 
> ...and if scanning the volatile partition ends in error then that just
> means no error records appear. When the error is "Invalid Physical
> Address" the driver can just remember that's a permanent error and never
> try again. So it's more like:
> 
> if (volatile_range_valid) {
> 	if (get_poison_list(volatile_range) == INVALID_PHYS_ADDR)
> 		volatile_range_valid = false;
> }
> get_poison_list(pmem_range);
> 
> ...but that's probably overkill since get_poison_list() is cheap. Just
> treat it like the zero error records case.

Got it!

> 
> In the to be released region provisioning patches there is a DPA
> resource tree partitioned by DPA mode type, so the poison list code
> probably wants to do something like:
> 
> down_read(&cxl_dpa_rwsem);
> for (p = cxlds->dpa_res.child; p; p = p->sibling)
> 	get_poison_list(p->start, resource_size(p));
> up_read(&cxl_dpa_rwsem);

Great ending to the week! This is going to make collecting the
poison per region much simpler than I was imagining :)