Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator

[David Hildenbrand]

On 26.01.23 23:15, Al Viro wrote:
> On Mon, Jan 23, 2023 at 02:24:13PM +0100, David Hildenbrand wrote:
> > On 23.01.23 14:19, David Howells wrote:
> > > David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >
> > > > Switching from FOLL_GET to FOLL_PIN was in the works by John H. Not sure what
> > > > the status is. Interestingly, Documentation/core-api/pin_user_pages.rst
> > > > already documents that "CASE 1: Direct IO (DIO)" uses FOLL_PIN ... which does,
> > > > unfortunately, no reflect reality yet.
> > >
> > > Yeah - I just came across that.
> > >
> > > Should iov_iter.c then switch entirely to using pin_user_pages(), rather than
> > > get_user_pages()?  In which case my patches only need keep track of
> > > pinned/not-pinned and never "got".
> >
> > That would be the ideal case: whenever intending to access page content, use
> > FOLL_PIN instead of FOLL_GET.
> >
> > The issue that John was trying to sort out was that there are plenty of
> > callsites that do a simple put_page() instead of calling unpin_user_page().
> > IIRC, handling that correctly in existing code -- what was pinned must be
> > released via unpin_user_page() -- was the biggest workitem.
> >
> > Not sure how that relates to your work here (that's why I was asking): if
> > you could avoid FOLL_GET, that would be great :)
>
> Take a good look at iter_to_pipe().  It does *not* need to pin anything
> (we have an ITER_SOURCE there); with this approach it will.  And it
> will stuff those pinned references into a pipe, where they can sit
> indefinitely.
>
> IOW, I don't believe it's a usable approach.

Not sure what makes you believe that FOLL_GET is any better for this 
long-term pinning, I'd like to learn about that.

As raised already somewhere in the whole discussion by me, the right way 
to take such a long-term ping as vmsplice() does is to use 
FOLL_PIN|FOLL_LONGTERM. As also raised, that will fix the last remaining 
vmsplice()+hugetlb COW issue as tested by the cow.c vm selftest and make 
sure to migrate that memory off of MIGRATE_MOVABLE/CMA memory where we 
cannot tolerate to have long-term unmovable memory sitting around.

--
Thanks,

David / dhildenb