Linux Device Drivers, Third Edition, Chapter 15: Memory Mapping and DMA says get_user_pages is a low-level memory management function, with a suitably complex interface. It also requires that the mmap reader/writer semaphore for the address space be obtained in read mode before the call. As a result, calls to get_user_pages usually look something like: down_read(¤t->mm->mmap_sem); result = get_user_pages(current, current->mm, ...); up_read(¤t->mm->mmap_sem); The return value is the number of pages actually mapped, which could be fewer than the number requested (but greater than zero). but, it isn't true. mmap_sem isn't only used for vma traversal, but also prevent vs-fork race. up_read(mmap_sem) mean end of critical section, IOW after up_read() code is fork unsafe. (access_process_vm() explain proper get_user_pages() usage) Oh well, We have many wrong caller now. What is the best fix method? Nick Piggin and Andrea Arcangeli proposed to change get_user_pages() semantics as caller expected. see "[PATCH] fork vs gup(-fast) fix" thead in linux-mm but Linus NACKed it. Thus I made caller change approach patch series. it is made for discuss to compare Nick's approach. I don't hope submit it yet. Nick, This version fixed vmsplice and aio issue (you pointed). I hope to hear your opiniton ;) ChangeLog: V2 -> V3 o remove early decow logic o introduce prevent unmap logic o fix nfs-directio o fix aio o fix bio (only bandaid fix) V1 -> V2 o fix aio+dio case TODO o implement down_write_killable() o fix kvm (need?) o fix get_arg_page() (Why this function don't use mmap_sem?) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
