Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Dec 02, 2020 at 08:34:32PM -0500, Pavel Tatashin wrote:

> > Either here or perhaps even lower down the call chain when the page is
> > captured, similar to how GUP fast would detect it. (how is that done
> > anyhow?)
> 
> Ah, thank you for pointing this out. I think I need to address it here:
> 
> https://soleen.com/source/xref/linux/mm/gup.c?r=96e1fac1#94
> 
> static __maybe_unused struct page *try_grab_compound_head()
>               if (unlikely(flags & FOLL_LONGTERM) &&  is_migrate_cma_page(page))
>                    return NULL;
> 
> I need to change is_migrate_cma_page() to all migratable pages. Will
> study, and send an update with this fix.

Yes, missing the two flows is a common error :(

Looking at this code some more.. How is it even correct?

1633  				if (!isolate_lru_page(head)) {
1634  					list_add_tail(&head->lru, &cma_page_list);

Here we are only running under the read side of the mmap sem so multiple
GUPs can be calling that sequence in parallel. I don't see any
obvious exclusion that will prevent corruption of head->lru. The first
GUP thread to do isolate_lru_page() will ClearPageLRU() and the second
GUP thread will be a NOP for isolate_lru_page().

They will both race list_add_tail and other list ops. That is not OK.

> What I meant is the users of the interface do it incrementally not in
> large chunks. For example:
> 
> vfio_pin_pages_remote
>    vaddr_get_pfn
>         ret = pin_user_pages_remote(mm, vaddr, 1, flags |
> FOLL_LONGTERM, page, NULL, NULL);
> 1 -> pin only one pages at a time

I don't know why vfio does this, it is why it so ridiculously slow at
least.

Jason