Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked()

Nick Piggin
2009-09-27T19:22:49Z

On Sun, Sep 27, 2009 at 05:26:25PM +0100, Hugh Dickins wrote:
> On Sat, 26 Sep 2009, Andi Kleen wrote:
> > > This is a bit tricky to do right now; you have a chicken and egg
> > > problem between locking the page and pinning the inode mapping.
> >
> > One possibly simple solution would be to just allocate the page
> > locked (GFP_LOCKED). When the allocator clears the flags it already
> > modifies the state, so it could as well set the lock bit too. No
> > atomics needed. And then clearing it later is also atomic free.
>
> That's a good idea.
>
> I don't particularly like adding a GFP_LOCKED just for this, and I
> don't particularly like having to remember to unlock the thing on the
> various(?) error paths between getting the page and adding it to cache.

God no, please no more crazy branches in the page allocator.

I'm going to resubmit my patches to allow 0-ref page allocations,
so the pagecache will be able to work with those to do what we
want here.

> But it is a good idea, and if doing it that way would really close a
> race window which checking page->mapping (or whatever) cannot (I'm
> simply not sure about that), then it would seem the best way to go.

Yep, seems reasonable: the ordering is no technical burden, and a
simple comment pointing to hwpoison will keep it maintainable.