Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement.

Srikar Dronamraju
2011-06-15T09:03:44Z

> > > > +
> > > > + /* 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