* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> [240626 21:28]: > On Wed, 26 Jun 2024 21:15:18 -0400 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote: > > > * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> [240626 16:59]: > > > On Tue, 25 Jun 2024 15:11:30 -0400 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote: > > > > > > > It is now possible to walk the vma tree using the rcu read locks and is > > > > beneficial to do so to reduce lock contention. Doing so while a > > > > MAP_FIXED mapping is executing means that a reader may see a gap in the > > > > vma tree that should never logically exist - and does not when using the > > > > mmap lock in read mode. The temporal gap exists because mmap_region() > > > > calls munmap() prior to installing the new mapping. > > > > > > What are the consequences when this race hits? IOW, why do we need to > > > change anything? > > > > > > > In the (near) future, we want to walk the vma tree to produce > > /proc/<pid>/maps. Without this change we will see the temporal gap and > > expose it to the user. This series was initially sent to Suren as part > > of his patch set. > > > > We also have the new interface for an ioctl request to a vma at or above > > an address. I had highlighted that an rcu reader would be ideal, but > > proved too difficult at this time. These patches by Andrii are currently > > not using the rcu reading method as this and a per-vma locking > > clarification are needed. > > > > Since there were two users for this code, I decided to send it out > > before the other patches. > > OK, thanks. We're approaching -rc6 and things are a bit sketchy so I'm > inclined to hold this off until the next cycle, unless there's urgency? > There is no urgency. I'm more than happy to hold off merging to get a full cycle of testing. Thanks, Liam
