Re: [PATCH v2 10/18] fsdax: Manage pgmap references at entry insertion and deletion

[Dan Williams <dan.j.williams@intel.com>]

Jason Gunthorpe wrote:
> On Wed, Sep 21, 2022 at 05:14:34PM -0700, Dan Williams wrote:
> 
> > > Indeed, you could reasonably put such a liveness test at the moment
> > > every driver takes a 0 refcount struct page and turns it into a 1
> > > refcount struct page.
> > 
> > I could do it with a flag, but the reason to have pgmap->ref managed at
> > the page->_refcount 0 -> 1 and 1 -> 0 transitions is so at the end of
> > time memunmap_pages() can look at the one counter rather than scanning
> > and rescanning all the pages to see when they go to final idle.
> 
> That makes some sense too, but the logical way to do that is to put some
> counter along the page_free() path, and establish a 'make a page not
> free' path that does the other side.
> 
> ie it should not be in DAX code, it should be all in common pgmap
> code. The pgmap should never be freed while any page->refcount != 0
> and that should be an intrinsic property of pgmap, not relying on
> external parties.

I just do not know where to put such intrinsics since there is nothing
today that requires going through the pgmap object to discover the pfn
and 'allocate' the page.

I think you may be asking to unify dax_direct_access() with pgmap
management where all dax_direct_access() users are required to take a
page reference if the pfn it returns is going to be used outside of
dax_read_lock().

In other words make dax_direct_access() the 'allocation' event that pins
the pgmap? I might be speaking a foreign language if you're not familiar
with the relationship of 'struct dax_device' to 'struct dev_pagemap'
instances. This is not the first time I have considered making them one
in the same.

> Though I suspect if we were to look at performance it is probably
> better to scan the memory on the unlikely case of pgmap removal than
> to put more code in hot paths to keep track of refcounts.. It doesn't
> need rescanning, just one sweep where it waits on every non-zero page
> to become zero.

True, on the way down nothing should be elevating page references, just
waiting for the last one to drain. I am just not sure that pgmap removal
is that unlikely going forward with things like the dax_kmem driver and
CXL Dynamic Capacity Devices where tearing down DAX devices happens.
Perhaps something to revisit if the pgmap percpu_ref ever shows up in
profiles.