lkml.org 
[lkml]   [2009]   [Apr]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [RFC][PATCH v3 4/6] aio: Don't inherit aio ring memory at fork
Date
Hi!

> KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> writes:
>
> > AIO folks, Am I missing anything?
> >
> > ===============
> > Subject: [RFC][PATCH] aio: Don't inherit aio ring memory at fork
> >
> > Currently, mm_struct::ioctx_list member isn't copyed at fork. IOW aio context don't inherit at fork.
> > but only ring memory inherited. that's strange.
> >
> > This patch mark DONTFORK to ring-memory too.
>
> Well, given that clearly nobody relies on io contexts being copied to
> the child, I think it's okay to make this change. I think the current
> behaviour violates the principal of least surprise, but I'm having a
> hard time getting upset about that. ;)

ok.
So, Can I get your Acked-by?

> > In addition, This patch has good side effect. it also fix
> > "get_user_pages() vs fork" problem.
>
> Hmm, I don't follow you, here. As I understand it, the get_user_pages
> vs. fork problem has to do with the pages used for the actual I/O, not
> the pages used to store the completion data. So, could you elaborate a
> bit on what you mean by the above statement?

No.

The problem is, get_user_pages() increment page_count only.
but VM page-fault logic don't care page_count. (it only care page::_mapcount)
Then, fork and pagefault can change virtual-physical relationship although
get_user_pages() is called.

drawback worst aio scenario here
-----------------------------------------------------------------------
io_setup() and gup inc page_count

fork inc mapcount
and make write-protect to pte

write ring from userland(*) page fault and
COW break.
parent process get copyed page and
child get original page owner-ship.

kmap and memcpy from kernel change child page. (it mean data lost)

(*) Is this happend?

MADV_DONTFORK or down_read(mmap_sem) or down_read(mm_pinned_sem)
or copy-at-fork mecanism(=Nick/Andrea patch) solve it.



> > I think "man fork" also sould be changed. it only say
> >
> > * The child does not inherit outstanding asynchronous I/O operations from
> > its parent (aio_read(3), aio_write(3)).
> > but aio_context_t (return value of io_setup(2)) also don't inherit in current implementaion.
>
> I can certainly make that change, as I have other changes I need to push
> to Michael, anyway.

thanks.





\
 
 \ /
  Last update: 2009-04-15 02:59    [W:0.111 / U:0.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site