Re: [aarcange@redhat.com: [PATCH] fork vs gup(-fast) fix]

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Mar 11, 2009 at 01:33:17PM -0700, Linus Torvalds wrote:
> Btw, if we don't do that, then there are better alternatives. One is:
> 
>  - fork already always takes the write lock on mmap_sem (and f*ck no, I 
>    doubt anybody will ever care one whit how "parallel" you can do forks 
>    from threads, so I don't think this is an issue)
> 
>  - Just make the rule be that people who use get_user_pages() always 
>    have to have the read-lock on mmap_sem until they've used the pages.

How do you handle pages where gup already returned and I/O still in
flight? Forcing gup-fast to be called with mmap_sem already hold (like
gup used to require) only avoids the need of changes in gup-fast
AFAICT. You'll still get pages that are pinned and calling gup-fast
under mmap_sem (no matter if read or even write mode) won't make a
difference, still those pages will be pinned while fork runs and with
dma going to them (by O_DIRECT or some driver using gup, as long as
PageReserved isn't set on them).

> We already take the read-lock for the lookup (well, not for the gup, but 
> for all the slow cases), but I'm saying that we could go one step further 
> - just read-lock over the _whole_ O_DIRECT read or write. That way you 
> literally protect against concurrent fork()s.

Releasing the mmap_sem read mode in the irq-completion handler context
should be possible, however fork will end up throttled blocking for
I/O which isn't very nice behavior. BTW, direct-io.c is a total mess,
I couldn't even figure out where to release those locks in the I/O
completion handlers when I tried something like this with PG_lock
instead of the mmap_sem...  Eventually I gave it up because this isn't
just about O_DIRECT but all gup users have this trouble with fork.

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