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.
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> -- Thanks, David / dhildenb
