On 5/14/21 5:31 AM, Peter Xu wrote: > Hi, Mike, > > On Thu, May 13, 2021 at 09:02:15PM -0700, Mike Kravetz wrote: > > [...] > >> I am also concerned with the semantics of this approach and what happens >> when a fault races with the userfaultfd copy. Previously I asked Peter >> if we could/should use a page found in the cache for the copy. His >> answer was as follows: >> >> AFAICT that's the expected behavior, and it need to be like that so as to avoid >> silent data corruption (if the page cache existed, it means the page is not >> "missing" at all, then it does not suite for a UFFDIO_COPY as it's only used >> for uffd page missing case). > > I didn't follow the rest discussion in depth yet... but just to mention that > the above answer was for the question whether we can "update the page in the > page cache", rather than "use a page found in the page cache". > > I think reuse the page should be fine, however it'll definitely break existing > user interface (as it'll expect -EEXIST for now - we have kselftest covers > that), meanwhile I don't see why the -EEXIST bothers a lot: it still tells the > user that this page was filled in already. Normally it was filled in by > another UFFDIO_COPY (as we could have multiple uffd service threads) along with > a valid pte, then this userspace thread can simply skip this message as it > means the event has been handled by some other servicing thread. > > (This also reminded me that there won't be a chance of UFFDIO_COPY race on page > no page fault at least, since no page fault will always go into the uffd > missing handling rather than filling in the page cache for a VM_UFFD_MISSING > vma; while mmap read lock should guarantee VM_UFFD_MISSING be persistent) Perhaps I am missing something. Since this is a shared mapping, can we not have a 'regular' mapping to the same range that is uffd registered? And, that regular mappings could fault and race with the uffd copy code? -- Mike Kravetz
