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
