In August 2005, Dave McCracken sent out a patch which implemented shared page tables (http://lkml.iu.edu/hypermail/linux/kernel/0508.3/1623.html) based on 2.6.13. He also wrote two OLS papers about the topic (https://landley.net/kdocs/ols/2003/ols2003-pages-315-320.pdf and https://www.landley.net/kdocs/ols/2006/ols2006v2-pages-125-130.pdf), the second of which was published after his patch submission. This patch was discussed for a few days. It was not accepted. There were several comments about technical issues (about a typo, some questions about locking, how to search the vmas, whether one must iterate through all of the vmas) which no doubt could be fixed, and in fact Dave indicated that he would eventually provide a revised patch which fixed these problems. AFAICT this never occurred. However, there were also questions about whether sharing page tables would provide any significant benefit. Specifically, there were concerns about whether the patch would improve performance at all (Dave indicated a 3% improvement on some "large benchmarks"), especially once another change (the test at at the beginning of copy_page_range() which prevents page table copies in some cases) was merged (d992895ba2, which has been in the kernel since 2.6.14). It was also suggested that the use of randomize_vm_space might also make shared page tables uninteresting, though that objection appeared to be addressed. Isn't Linux kernel archaeology fun :-) 13 years have elapsed. Given the many changes in the kernel since the original patch submission, I'd appreciate your insight into the following questions: * Is there (still?) a need for shared page tables (and if not, why not?). * If one were to resume work on this, is there any reason why one shouldn't start with Dave's 2.6.13 patch (plus fixes to the known bugs in it) and forward port it to the tip, rather than starting from scratch? Thanks. Larry Bassel
