On 7/7/25 07:27, Lorenzo Stoakes wrote: > Right now it appears that the code is relying upon the returned destination > address having bits outside PAGE_MASK to indicate whether an error value is > specified, and decrementing the increased refcount on the uffd ctx if so. > > This is not a safe means of determining an error value, so instead, be > specific. It makes far more sense to do so in a dedicated error path, so > add mremap_userfaultfd_fail() for this purpose and use this when an error > arises. > > A vm_userfaultfd_ctx is not established until we are at the point where > mremap_userfaultfd_prep() is invoked in copy_vma_and_data(), so this is a > no-op until this happens. > > That is - uffd remap notification only occurs if the VMA is actually moved > - at which point a UFFD_EVENT_REMAP event is raised. > > No errors can occur after this point currently, though it's certainly not > guaranteed this will always remain the case, and we mustn't rely on this. > > However, the reason for needing to handle this case is that, when an error > arises on a VMA move at the point of adjusting page tables, we revert this > operation, and propagate the error. > > At this point, it is not correct to raise a uffd remap event, and we must > handle it. > > This refactoring makes it abundantly clear what we are doing. > > We assume vrm->new_addr is always valid, which a prior change made the case > even for mremap() invocations which don't move the VMA, however given no > uffd context would be set up in this case it's immaterial to this change > anyway. > > No functional change intended. > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx> Guess that renders my previous nit unimportant.
