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

Jason Gunthorpe
2022-09-22T17:56:04Z

On Wed, Sep 21, 2022 at 07:17:40PM -0700, Dan Williams wrote:
> 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 that is just a new API that wrappers the set refcount = 1,
percpu refcount and maybe building appropriate compound pages too.

Eg maybe something like:

struct folio *pgmap_alloc_folios(pgmap, start, length)

And you get back maximally sized allocated folios with refcount = 1
that span the requested range.

> 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.

I don't know enough about dax, so yes very foreign :)

I'm thinking broadly about how to make pgmap usable to all the other
drivers in a safe and robust way that makes some kind of logical sense.

Jason