Re: [RFC][PATCH v3 4/6] aio: Don't inherit aio ring memory at fork

Hi!

KOSAKI Motohiro [off-list ref] writes:

quoted

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?

quoted

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.

quoted

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.