Re: [PATCH v2 16/18] mm/memremap_pages: Support initializing pages to a zero reference count

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 15, 2022 at 08:36:43PM -0700, Dan Williams wrote:
> The initial memremap_pages() implementation inherited the
> __init_single_page() default of pages starting life with an elevated
> reference count. This originally allowed for the page->pgmap pointer to
> alias with the storage for page->lru since a page was only allowed to be
> on an lru list when its reference count was zero.
> 
> Since then, 'struct page' definition cleanups have arranged for
> dedicated space for the ZONE_DEVICE page metadata, and the
> MEMORY_DEVICE_{PRIVATE,COHERENT} work has arranged for the 1 -> 0
> page->_refcount transition to route the page to free_zone_device_page()
> and not the core-mm page-free. With those cleanups in place and with
> filesystem-dax and device-dax now converted to take and drop references
> at map and truncate time, it is possible to start MEMORY_DEVICE_FS_DAX
> and MEMORY_DEVICE_GENERIC reference counts at 0.
> 
> MEMORY_DEVICE_{PRIVATE,COHERENT} still expect that their ZONE_DEVICE
> pages start life at _refcount 1, so make that the default if
> pgmap->init_mode is left at zero.

I'm shocked to read this - how does it make any sense?

dev_pagemap_ops->page_free() is only called on the 1->0 transition, so
any driver which implements it must be expecting pages to have a 0
refcount.

Looking around everything but only fsdax_pagemap_ops implements
page_free()

So, how does it work? Surely the instant the page map is created all
the pages must be considered 'free', and after page_free() is called I
would also expect the page to be considered free.

How on earth can a free'd page have both a 0 and 1 refcount??

eg look at the simple hmm_test, it threads pages on to the
mdevice->free_pages list immediately after memremap_pages and then
again inside page_free() - it is completely wrong that they would have
different refcounts while on the free_pages list.

I would expect that after the page is removed from the free_pages list
it will have its recount set to 1 to make it non-free then it will go
through the migration.

Alistair how should the refcounting be working here in hmm_test?

Jason