> Martin Maletinsky wrote: > > > > Hello Joe, > > > > Thanks again for your answer. I have one more question regarding a statement you make in your answer: > > > > Joseph A Knapka wrote: > > > > > > > > In this case, A and B will always share page mappings, even > > > when writing to copy-on-write pages, and C will get its own > > > copy if/when it attempts to write the page. Here's why: > > > > > > In do_fork(), we call copy_mm(), which, in the case of > > > CLONE_VM being set, just copies the entire mm context > > > for the new thread, -without- marking anything copy-on-write. > > > So thread A and thread B both have non-COW mappings. > > > > > > When Thread B forks process C, C gets an entirely new MM > > > context, with -no- user-space PTEs. So in order to write > > > A+B's virtual page, C takes a "not-present" page fault.regards > > > > Why is it that C will have no user-space PTEs in it's MM. I thought at a fork call, the user-space PTEs are copied from the parent's MM into the childs MM, and write protected in both > > MMs. > > I concluded that from the following call sequence (in 2.4.10): > > do_fork() > > -> copy_mm() > > --> dup_mmap() [note that since C is created by a call to fork(), CLONE_VM is *not* set, when creating C] > > ---> copy_page_range() [here the PTEs are copied for one vmarea; both PTEs (child and parent) are write protected at line 256, if the conditions to make them COW are fullfilled]. > > > > My understanding was therefore that all physical pages backing A&B's page mapping would now be shared among A&B and C, and write access to this pages (by either A/B or C would cause a > > copy-on-write). > > > > Did I misunderstand somthing in your answer or in the source code? > > No, you're right. I didn't read dup_mmap() carefully. Now I don't > understand it either :-( Could you explain the problem here? Now, pages are write protected, and B (ie, A+B) writes to the page containing the buffer to which A has initiated read(), gets a new physical page to which old contents are copied and old virtual address of the buffer(in A+B address space)are mapped, read() resumes and copy_to_user() copies the remaining data to this new page, and read() completes normally with the buffer filled with data. I am not really getting the problem here, although am not looking at the code at the moment. Can anyone explain to me the hitch. regards sourav > > -- Joe > # "You know how many remote castles there are along the > # gorges? You can't MOVE for remote castles!" - Lu Tze re. Uberwald > # Linux MM docs: > http://home.earthlink.net/~jknapka/linux-mm/vmoutline.html > - > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > IRC Channel: irc.openprojects.net / #kernelnewbies > Web Page: http://www.kernelnewbies.org/ > - Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ IRC Channel: irc.openprojects.net / #kernelnewbies Web Page: http://www.kernelnewbies.org/
