On 06/15, Srikar Dronamraju wrote: > > > > > > + > > > > > + /* 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 Failed to Connect. > 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. No. My main concern is that I do not understand why do we need an extra cow. This is fine, I am not vm expert. But I think it is not fine that you can't explain why your code needs it ;) What this 'get_user_pages do the "real" cow for us' actually means? It does not do cow for us, __replace_page() does the actual/final cow. It re-installs the modified copy of the page returned by get_user_pages() at the same pte. > > 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 Hmm... How can mprotect make a difference? This mapping should be read only, and we are not going to do pte_mkwrite. > So I am okay to not force cow through get_user_pages. I am okay either way ;) But, imho, if we use FOLL_WRITE|FOLL_FORCE then it would be nice to document why it this needed. Oleg. -- 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>