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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Sep 30, 2021 at 06:54:05PM +0100, Joao Martins wrote:
> On 9/30/21 04:01, Alistair Popple wrote:
> > On Thursday, 30 September 2021 5:34:05 AM AEST Jason Gunthorpe wrote:
> >> 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??
> > 
> > As background note that device pages are currently considered free when
> > refcount == 1 but the pgmap reference is dropped when the refcount transitions
> > 1->0. The final pgmap reference is typically dropped when a driver calls
> > memunmap_pages() and put_page() drops the last page reference:
> > 
> > void memunmap_pages(struct dev_pagemap *pgmap)
> > {
> >         unsigned long pfn;
> >         int i;
> > 
> >         dev_pagemap_kill(pgmap);
> >         for (i = 0; i < pgmap->nr_range; i++)
> >                 for_each_device_pfn(pfn, pgmap, i)
> >                         put_page(pfn_to_page(pfn));
> >         dev_pagemap_cleanup(pgmap);
> > 
> > If there are still pgmap references dev_pagemap_cleanup(pgmap) will block until
> > the final reference is dropped. So I think your argument holds at least for
> > DEVICE_PRIVATE and DEVICE_GENERIC. DEVICE_FS_DAX defines it's own pagemap
> > cleanup but I can't see why the same argument wouldn't hold there - if a page
> > has a valid refcount it must have a reference on the pagemap too.
>
> IIUC Dan's reasoning was that fsdax wasn't able to deal with
> surprise removal [1] so his patches were to ensure fsdax (or the
> pmem block device) poisons/kills the pages as a way to notify
> filesystem/dm that the page was to be kept unmapped:

Sure, but that has nothing to do with GUP, that is between the
filesytem and fsdax
 
> But if fsdax doesn't wait for all the pgmap references[*] on its
> pagemap cleanup callback then what's the pgmap ref in
> __gup_device_huge() pairs/protects us up against that is specific to
> fsdax?

It does wait for refs

It sets the pgmap.ref to:

	pmem->pgmap.ref = &q->q_usage_counter;

And that ref is incr'd by the struct page lifetime - the unincr is in
__put_page() above

fsdax_pagemap_ops does pmem_pagemap_kill() which calls
blk_freeze_queue_start() which does percpu_ref_kill(). Then the
pmem_pagemap_cleanup() eventually does blk_mq_freeze_queue_wait()
which will sleep until the prefcpu ref reaches zero.

In other words fsdax cannot pass cleanup while a struct page exists
with a non-zero refcount, which answers Alistair's question about how
fsdax's cleanup work.

Jason