Re: [PATCH v3 13/16] mm/gup: disallow follow_page(FOLL_PIN)

[Vlastimil Babka]

On 3/29/22 18:04, David Hildenbrand wrote:
> We want to change the way we handle R/O pins on anonymous pages that
> might be shared: if we detect a possibly shared anonymous page --
> mapped R/O and not !PageAnonExclusive() -- we want to trigger unsharing
> via a page fault, resulting in an exclusive anonymous page that can be
> pinned reliably without getting replaced via COW on the next write
> fault.
> 
> However, the required page fault will be problematic for follow_page():
> in contrast to ordinary GUP, follow_page() doesn't trigger faults
> internally. So we would have to end up failing a R/O pin via
> follow_page(), although there is something mapped R/O into the page
> table, which might be rather surprising.
> 
> We don't seem to have follow_page(FOLL_PIN) users, and it's a purely
> internal MM function. Let's just make our life easier and the semantics of
> follow_page() clearer by just disallowing FOLL_PIN for follow_page()
> completely.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>