When KSM-by-default is established using prctl(PR_SET_MEMORY_MERGE), this defaults all newly mapped VMAs to having VM_MERGEABLE set, and thus makes them available to KSM for samepage merging. It also sets VM_MERGEABLE in all existing VMAs. However this causes an issue upon mapping of new VMAs - the initial flags will never have VM_MERGEABLE set when attempting a merge with adjacent VMAs (this is set later in the mmap() logic), and adjacent VMAs will ALWAYS have VM_MERGEABLE set. This renders literally all VMAs in the virtual address space unmergeable. To avoid this, this series performs the check for PR_SET_MEMORY_MERGE far earlier in the mmap() logic, prior to the merge being attempted. However we run into a complexity with the depreciated .mmap() callback - if a driver hooks this, it might change flags thus adjusting KSM merge eligibility. This isn't a problem for brk(), where the VMA must be anonymous. However for mmap() we are conservative - if the VMA is anonymous then we can always proceed, however if not, we permit only shmem mappings and drivers which implement .mmap_prepare(). If we can't be sure of the driver changing things, then we maintain the same behaviour of performing the KSM check later in the mmap() logic (and thus losing VMA mergeability). Since the .mmap_prepare() hook is invoked prior to the KSM check, this means we can always perform the KSM check early if it is present. Over time as drivers are converted, we can do away with the later check altogether. A great many use-cases for this logic will use anonymous or shmem memory at any rate, as KSM is not supported for the page cache, and the driver outliers in question are MAP_PRIVATE mappings of these files. So this change should already cover the majority of actual KSM use-cases. v2: * Removed unnecessary ret local variable in ksm_vma_flags() as per David. * added Stefan Roesch in cc and added Fixes tag as per Andrew, David. * Propagated tags (thanks everyone!) * Removed unnecessary !CONFIG_KSM ksm_add_vma() stub from include/linux/ksm.h. * Added missing !CONFIG_KSM ksm_vma_flags() stub in include/linux/ksm.h. * After discussion with David, I've decided to defer removing the VM_SPECIAL case for KSM, we can address this in a follow-up series. * Expanded 3/4 commit message to reference KSM eligibility vs. merging and referenced future plans to permit KSM for VM_SPECIAL VMAs. v1: https://lore.kernel.org/all/cover.1747431920.git.lorenzo.stoakes@xxxxxxxxxx/ Lorenzo Stoakes (4): mm: ksm: have KSM VMA checks not require a VMA pointer mm: ksm: refer to special VMAs via VM_SPECIAL in ksm_compatible() mm: prevent KSM from completely breaking VMA merging tools/testing/selftests: add VMA merge tests for KSM merge include/linux/fs.h | 7 ++- include/linux/ksm.h | 8 +-- mm/ksm.c | 49 ++++++++++++------- mm/vma.c | 49 ++++++++++++++++++- tools/testing/selftests/mm/merge.c | 78 ++++++++++++++++++++++++++++++ 5 files changed, 167 insertions(+), 24 deletions(-) -- 2.49.0
