And in the annals of 'hit send then immediately notice mistake' we have entry number 1,379 :P This subject should read [PATCH v2 0/4] obviously, like the patches that reply to it in-thread. Apologies for that! Other than that, series should be fine :>) Cheers, Lorenzo On Wed, May 21, 2025 at 07:20:27PM +0100, Lorenzo Stoakes wrote: > 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 >
