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Shutemov wrote: > On Tue, Sep 03, 2024 at 04:22:31PM -0700, Anthony Yznaga wrote: >> This patch series implements a mechanism that allows userspace >> processes to opt into sharing PTEs. It adds a new in-memory >> filesystem - msharefs. A file created on msharefs represents a >> shared region where all processes mapping that region will map >> objects within it with shared PTEs. When the file is created, >> a new host mm struct is created to hold the shared page tables >> and vmas for objects later mapped into the shared region. This >> host mm struct is associated with the file and not with a task. > Taskless mm_struct can be problematic. Like, we don't have access to it's > counters because it is not represented in /proc. For instance, there's no > way to check its smaps. Definitely needs exposure in /proc. One of the things I'm looking into is the feasibility of showing the mappings in maps/smaps/etc.. > > Also, I *think* it is immune to oom-killer because oom-killer looks for a > victim task, not mm. > I hope it is not an intended feature :P oom-killer would have to kill all sharers of an mshare region before the mshare region itself could be freed, but I'm not sure that oom-killer would be the one to free the region. An mshare region is essentially a shared memory object not unlike a tmpfs or hugetlb file. I think some higher level intelligence would have to be involved to release it if appropriate when under oom conditions. > >> When a process mmap's the shared region, a vm flag VM_SHARED_PT >> is added to the vma. On page fault the vma is checked for the >> presence of the VM_SHARED_PT flag. > I think it is wrong approach. > > Instead of spaying VM_SHARED_PT checks across core-mm, we need to add a > generic hooks that can be used by mshare and hugetlb. And remove > is_vm_hugetlb_page() check from core-mm along the way. > > BTW, is_vm_hugetlb_page() callsites seem to be the indicator to check if > mshare has to do something differently there. I feel you miss a lot of > such cases. Good point about is_vm_hugetlb_page(). I'll review the callsites (there are only ~60 of them :-). Thanks, Anthony