On Thu, Feb 24, 2022 at 01:26:13PM +0100, David Hildenbrand wrote: > Whenever GUP currently ends up taking a R/O pin on an anonymous page that > might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault > on the page table entry will end up replacing the mapped anonymous page > due to COW, resulting in the GUP pin no longer being consistent with the > page actually mapped into the page table. > > The possible ways to deal with this situation are: > (1) Ignore and pin -- what we do right now. > (2) Fail to pin -- which would be rather surprising to callers and > could break user space. > (3) Trigger unsharing and pin the now exclusive page -- reliable R/O > pins. How does this mesh with the common FOLL_FORCE|FOLL_WRITE|FOLL_PIN pattern used for requesting read access? Can they be converted to just FOLL_WRITE|FOLL_PIN after this? Jason
