On Thu, Feb 16, 2023 at 10:37:36AM +0100, David Hildenbrand wrote: > On 16.02.23 10:16, Muhammad Usama Anjum wrote: > > mwriteprotect_range() errors out if [start, end) doesn't fall in one > > VMA. We are facing a use case where multiple VMAs are present in one > > range of interest. For example, the following pseudocode reproduces the > > error which we are trying to fix: > > - Allocate memory of size 16 pages with PROT_NONE with mmap > > - Register userfaultfd > > - Change protection of the first half (1 to 8 pages) of memory to > > PROT_READ | PROT_WRITE. This breaks the memory area in two VMAs. > > - Now UFFDIO_WRITEPROTECT_MODE_WP on the whole memory of 16 pages errors > > out. > > I think, in QEMU, with partial madvise()/mmap(MAP_FIXED) while handling > memory remapping during reboot to discard pages with memory errors, it would > be possible that we get multiple VMAs and could not enable uffd-wp for > background snapshots anymore. So this change makes sense to me. Any pointer for this one? > > Especially, because userfaultfd_register() seems to already properly handle > multi-VMA ranges correctly. It traverses the VMA list twice ... but also > holds the mmap lock in write mode. > > > > > This is a simple use case where user may or may not know if the memory > > area has been divided into multiple VMAs. > > > > We need an implementation which doesn't disrupt the already present > > users. So keeping things simple, stop going over all the VMAs if any one > > of the VMA hasn't been registered in WP mode. While at it, remove the > > un-needed error check as well. > > > > Reported-by: Paul Gofman <pgofman@xxxxxxxxxxxxxxx> > > Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx> > > --- > > > Acked-by: David Hildenbrand <david@xxxxxxxxxx> Acked-by: Peter Xu <peterx@xxxxxxxxxx> Thanks, -- Peter Xu
