> > > > + > > > > + /* Read the page with vaddr into memory */ > > > > + ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 1, 1, &old_page, &vma); > > > > > > Sorry if this was already discussed... But why we are using FOLL_WRITE here? > > > We are not going to write into this page, and this provokes the unnecessary > > > cow, no? > > > > Yes, We are not going to write to the page returned by get_user_pages > > but a copy of that page. > > Yes I see. But the page returned by get_user_pages(write => 1) is already > a cow'ed copy (this mapping should be read-only). > > > The idea was if we cow the page then we dont > > need to cow it at the replace_page time > > Yes, replace_page() shouldn't cow. > > > and since get_user_pages knows > > the right way to cow the page, we dont have to write another routine to > > cow the page. > > Confused. write_opcode() allocs another page and does memcpy. This is > correct, but I don't understand the first cow. > we decided on get_user_pages(FOLL_WRITE|FOLL_FORCE) based on discussions in these threads https://lkml.org/lkml/2010/4/23/327 and https://lkml.org/lkml/2010/5/12/119 Summary of those two sub-threads as I understand was to have get_user_pages do the "real" cow for us. If I understand correctly, your concern is on the extra overhead added by the get_user_pages. Other than that is there any side-effect of we forcing the cow through get_user_pages. > > I am still not clear on your concern. > > Probably I missed something... but could you please explain why we can't > > - ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 1, 1, &old_page, &vma); > + ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 0, 0, &old_page, &vma); > > ? I tried the code with this change and it works for regular cases. I am not sure if it affects cases where programs do mprotect So I am okay to not force cow through get_user_pages. > > > > Also. This is called under down_read(mmap_sem), can't we race with > > > access_process_vm() modifying the same memory? > > > > Yes, we could be racing with access_process_vm on the same memory. > > > > Do we have any other option other than making write_opcode/read_opcode > > being called under down_write(mmap_sem)? > > I dunno. Probably we can simply ignore this issue, there are other ways > to modify this memory. > Okay. -- Thanks and Regards Srikar -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>