Motivation In the current kernel it is impossible to merge two anonymous VMAs if one of them was moved. That is because VMA's page offset is set according to the virtual address where it was created and in order to merge two VMAs page offsets need to follow up. Another problem when merging two VMA's is their anon_vma. In current kernel these anon_vmas have to be the one and the same. Otherwise merge is again not allowed. There are several places from which vma_merge() is called and therefore several use cases that might profit from this upgrade. These include mmap (that fills a hole between two VMAs), mremap (that moves VMA next to another one or again perfectly fills a hole), mprotect (that modifies protection and allows merging with a neighbor) and brk (that expands VMA so that it is adjacent to a neighbor). Missed merge opportunities increase the number of VMAs of a process and in some cases can cause problems when a max count is reached. Solution Following series of these patches solves the first problem with page offsets by updating them when the VMA is moved to a different virtual address (patch 2). As for the second problem merging of VMAs with different anon_vma is allowed (patch 3). Patch 1 refactors function vma_merge and makes it easier to understand and also allows relatively seamless tracing of successful merges introduced by the patch 4. Limitations For both problems solution works only for VMAs that do not share physical pages with other processes (usually child or parent processes). This is checked by looking at anon_vma of the respective VMA. The reason why it is not possible or at least not easy to accomplish is that each physical page has a pointer to anon_vma and page offset. And when this physical page is shared we cannot simply change these parameters without affecting all of the VMAs mapping this physical page. Good thing is that this case amounts only for about 1-3% of all merges (measured on jemalloc (0%), redis (2.7%) and kcbench (1.2%) tests) that fail to merge in the current kernel. Measuring also shows slight increase in running time, jemalloc (0.3%), redis (1%), kcbench (1%). More extensive data can be viewed at https://home.alabanda.cz/share/results.png This series of patches and documentation of the related code will be part of my master's thesis. This patch series is based on tag v5.17-rc4. This is a second version including minor changes that arose from the first RFC like formatting. Speed and failed merge percentage data are also included. Jakub Matěna (4): mm: refactor of vma_merge() mm: adjust page offset in mremap mm: enable merging of VMAs with different anon_vmas mm: add tracing for VMA merges include/linux/rmap.h | 17 ++- include/trace/events/mmap.h | 83 +++++++++++++++ mm/internal.h | 12 +++ mm/mmap.c | 206 ++++++++++++++++++++++++------------ mm/rmap.c | 77 ++++++++++++++ 5 files changed, 325 insertions(+), 70 deletions(-) -- 2.34.1
