On 21/03/2018 23:46, Matthew Wilcox wrote: > On Wed, Mar 21, 2018 at 02:45:44PM -0700, Yang Shi wrote: >> Marking vma as deleted sounds good. The problem for my current approach is >> the concurrent page fault may succeed if it access the not yet unmapped >> section. Marking deleted vma could tell page fault the vma is not valid >> anymore, then return SIGSEGV. >> >>> does not care; munmap will need to wait for the existing munmap operation >> >> Why mmap doesn't care? How about MAP_FIXED? It may fail unexpectedly, right? > > The other thing about MAP_FIXED that we'll need to handle is unmapping > conflicts atomically. Say a program has a 200GB mapping and then > mmap(MAP_FIXED) another 200GB region on top of it. So I think page faults > are also going to have to wait for deleted vmas (then retry the fault) > rather than immediately raising SIGSEGV. Regarding the page fault, why not relying on the PTE locking ? When munmap() will unset the PTE it will have to held the PTE lock, so this will serialize the access. If the page fault occurs before the mmap(MAP_FIXED), the page mapped will be removed when mmap(MAP_FIXED) would do the cleanup. Fair enough.
