[sorry to reply so late, on vacation too, and my mailing system has some kind of problem] >[sorry, was on vacation last week] >> Why use flags if they both conditions are mutually exclusive? > > Just to make the return value of break_ksm_pmd_entry() more expressive and > understandable. because break_ksm_pmd_entry have three types of returned > values (0, 1, 2). > It adds confusion. Just simplify it please. So I think it's good to add a enum value of 0 listed here as suggested by Claudio Imbrenda. > >> MADV_UNMERGEABLE -> unmerge_ksm_pages() will never unshare the shared >> zeropage? I thought the patch description mentions that that is one of >> the goals? > > No, MADV_UNMERGEABLE will trigger KSM to unshare the shared zeropages in the > context of "get_next_rmap_item() -> unshare_zero_pages(), but not directly in the > context of " madvise()-> unmerge_ksm_pages() ". The reason for this is to avoid > increasing long delays of madvise() calling on unsharing zero pages. > >Why do we care and make this case special? Yeah, the code seems a bit special, but it is a helpless way and best choice, because the action of unsharing zero-pages is too complex and CPU consuming because checking whether the page we get is actually placed by KSM or not is not a easy thing in the context of unmerge_ksm_pages. In experiment, unsharing zero-pages in the context of unmerge_ksm_pages cause user' madvise() spend 5 times the time than the way of the current patch. So let's leave it as it is now. I will add a (short) explanation of when and why the new unshare_zero_page flag should be used. Sincerely. Xu Xin
