Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator

[David Hildenbrand <david@redhat.com>]

On 27.01.23 00:56, David Howells wrote:
> Al says that pinning a page (ie. FOLL_PIN) could cause a deadlock if a page is
> vmspliced into a pipe with the pipe holding a pin on it because pinned pages
> are removed from all page tables.  Is this actually the case?  I can't see
> offhand where in mm/gup.c it does this.

Pinning a page is mostly taking a "special" reference on the page, 
indicating to the system that the page maybe pinned. For an ordinary 
order-0 page, this is increasing the refcount by 1024 instead of 1.

In addition, we'll do some COW-unsharing magic depending on the page 
type (e.g., anon vs. fike-backed), and FOLL_LONGTERM. So if the page is 
mapped R/O only and we want to pin it R/O (!FOLL_WRITE), we might 
replace it in the page table by a different page via a fault 
(FAULT_FLAG_UNSHARE).

Last but not least, with FOLL_LONGTERM we will make sure to migrate the 
target page off of MIGRATE_MOVABLE/CMA memory where the unmovable page 
(while pinned) could otherwise cause trouble (e.g., blocking memory 
hotunplug). So again, we'd replace it in the page tale by a different 
page via a fault.

In all cases, the page won't be unmapped from the page table.

-- 
Thanks,

David / dhildenb