Re: [PATCH v4 08/14] mm/gup: grab head page refcount once for group of subpages

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Sep 29, 2021 at 12:50:15PM +0100, Joao Martins wrote:

> > If the get_dev_pagemap has to remain then it just means we have to
> > flush before changing pagemap pointers
> Right -- I don't think we should need it as that discussion on the other
> thread goes.
> 
> OTOH, using @pgmap might be useful to unblock gup-fast FOLL_LONGTERM
> for certain devmap types[0] (like MEMORY_DEVICE_GENERIC [device-dax]
> can support it but not MEMORY_DEVICE_FSDAX [fsdax]).

When looking at Logan's patches I think it is pretty clear to me that
page->pgmap must never be a dangling pointer if the caller has a
legitimate refcount on the page.

For instance the migrate and stuff all blindly calls
is_device_private_page() on the struct page expecting a valid
page->pgmap.

This also looks like it is happening, ie

void __put_page(struct page *page)
{
	if (is_zone_device_page(page)) {
		put_dev_pagemap(page->pgmap);

Is indeed putting the pgmap ref back when the page becomes ungettable.

This properly happens when the page refcount goes to zero and so it
should fully interlock with __page_cache_add_speculative():

	if (unlikely(!page_ref_add_unless(page, count, 0))) {

Thus, in gup.c, if we succeed at try_grab_compound_head() then
page->pgmap is a stable pointer with a valid refcount.

So, all the external pgmap stuff in gup.c is completely pointless.
try_grab_compound_head() provides us with an equivalent protection at
lower cost. Remember gup.c doesn't deref the pgmap at all.

Dan/Alistair/Felix do you see any hole in that argument??

So lets just delete it!

Jason