Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 31 May 2021 20:24:07 -1000 | Subject | Re: Sealed memfd & no-fault mmap |
| |
On Mon, May 31, 2021 at 11:13 AM Ming Lin <minggr@gmail.com> wrote: > > OK, I borrowed code from filemap_xip.c and implemented this behavior.
I think that "unmap page" is too complicated and fragile.
The only page that could possibly validly be unmapped is a stale zero page, but that code in shmem_unmap_nofault_page() seems to try to handle other cases too (ie that whole page_remove_rmap() - afaik a zero page has no rmap).
I get the feeling that the simpler thing to do is to just say "if you use MAP_NOSIGBUS, and you access pages that don't have a backing store, you will get zero pages, and they will NOT BE SYNCHRONIZED with the backing store possibly later being updated".
IOW, just document the fact that a MAP_NOSIGBUS mapping isn't coherent wrt shmem contents that are expanded and filled in later.
Don't try to "fix" it - because any user that uses MAP_NOSIGBUS had better just accept that it's not compatible with expanding the shmem backing store later.
Keep it simple and stupid. Hmm?
Linus
| |