Re: [PATCH v4 08/10] mm/gup: limit number of gup migration failures, honor failures

[Jason Gunthorpe]

On Thu, Dec 17, 2020 at 05:02:03PM -0500, Pavel Tatashin wrote:
> Hi Jason,
> 
> Thank you for your comments. My replies below.
> 
> On Thu, Dec 17, 2020 at 3:50 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > On Thu, Dec 17, 2020 at 01:52:41PM -0500, Pavel Tatashin wrote:
> > > +/*
> > > + * Verify that there are no unpinnable (movable) pages, if so return true.
> > > + * Otherwise an unpinnable pages is found return false, and unpin all pages.
> > > + */
> > > +static bool check_and_unpin_pages(unsigned long nr_pages, struct page **pages,
> > > +                               unsigned int gup_flags)
> > > +{
> > > +     unsigned long i, step;
> > > +
> > > +     for (i = 0; i < nr_pages; i += step) {
> > > +             struct page *head = compound_head(pages[i]);
> > > +
> > > +             step = compound_nr(head) - (pages[i] - head);
> >
> > You can't assume that all of a compound head is in the pages array,
> > this assumption would only work inside the page walkers if the page
> > was found in a PMD or something.
> 
> I am not sure I understand your comment. The compound head is not
> taken from the pages array, and not assumed to be in it. It is exactly
> the same logic as that we currently have:
> https://soleen.com/source/xref/linux/mm/gup.c?r=a00cda3f#1565

Oh, that existing logic is wrong too :( Another bug.

You can't skip pages in the pages[] array under the assumption they
are contiguous. ie the i+=step is wrong.

> >
> > > +     if (gup_flags & FOLL_PIN) {
> > > +             unpin_user_pages(pages, nr_pages);
> >
> > So we throw everything away? Why? That isn't how the old algorithm worked
> 
> It is exactly like the old algorithm worked: if there are pages to be
> migrated (not pinnable pages) we unpinned everything.
> See here:
> https://soleen.com/source/xref/linux/mm/gup.c?r=a00cda3f#1603

Hmm, OK, but I'm not sure that is great either

> cleaner, and handle errors. We must unpin everything because if we
> fail, no pages should stay pinned, and also if we migrated some pages,
> the pages array must be updated, so we need to call
> __get_user_pages_locked() pin and repopulated pages array.

However the page can't be unpinned until it is put on the LRU (and I'm
hoping that the LRU is enough of a 'lock' to make that safe, no idea)

> > I don't like this at all. It shouldn't be so flakey
> >
> > Can you do migration without the LRU?
> 
> I do not think it is possible, we must isolate pages before migration.

I don't like this at all :( Lots of stuff relies on GUP, introducing a
random flakiness like this not good.

Jason