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. This patch set stops rcu readers from seeing the temporal gap by splitting up the munmap() function into two parts. The first part prepares the vma tree for modifications by doing the necessary splits and tracks the vmas marked for removal in a side tree. The second part completes the munmapping of the vmas after the vma tree has been overwritten (either by a MAP_FIXED replacement vma or by a NULL in the munmap() case). Please note that rcu walkers will still be able to see a temporary state of split vmas that may be in the process of being removed, but the temporal gap will not be exposed. vma_start_write() are called on both parts of the split vma, so this state is detectable. RFC: https://lore.kernel.org/linux-mm/20240531163217.1584450-1-Liam.Howlett@xxxxxxxxxx/ v1: https://lore.kernel.org/linux-mm/20240611180200.711239-1-Liam.Howlett@xxxxxxxxxx/ v2: https://lore.kernel.org/all/20240625191145.3382793-1-Liam.Howlett@xxxxxxxxxx/ Changes since v2: - Introduced vms_complete_pte_clear() helper needed in case of modules mmaping over MAP_FIXED area. - Reduced arguments to unmap_region() - Fixed potential free_pgtables() range issue on first/last VMA. Tested with stress-ng --vm-sigv Liam R. Howlett (16): mm/mmap: Correctly position vma_iterator in __split_vma() mm/mmap: Introduce abort_munmap_vmas() mm/mmap: Introduce vmi_complete_munmap_vmas() mm/mmap: Extract the gathering of vmas from do_vmi_align_munmap() mm/mmap: Introduce vma_munmap_struct for use in munmap operations mm/mmap: Change munmap to use vma_munmap_struct() for accounting and surrounding vmas mm/mmap: Extract validate_mm() from vma_complete() mm/mmap: Inline munmap operation in mmap_region() mm/mmap: Expand mmap_region() munmap call mm/mmap: Reposition vma iterator in mmap_region() mm/mmap: Track start and end of munmap in vma_munmap_struct mm/mmap: Clean up unmap_region() argument list mm/mmap: Avoid zeroing vma tree in mmap_region() mm/mmap: Use PHYS_PFN in mmap_region() mm/mmap: Use vms accounted pages in mmap_region() mm/mmap: Move may_expand_vm() check in mmap_region() mm/internal.h | 25 +++ mm/mmap.c | 479 +++++++++++++++++++++++++++++++------------------- 2 files changed, 321 insertions(+), 183 deletions(-) -- 2.43.0
