Messages in this thread |  | | Subject | Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned | From | John Hubbard <> | Date | Mon, 21 Sep 2020 16:53:38 -0700 |
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On 9/21/20 2:17 PM, Peter Xu wrote: > (Commit message collected from Jason Gunthorpe) > > Reduce the chance of false positive from page_maybe_dma_pinned() by keeping
Not yet, it doesn't. :) More:
> track if the mm_struct has ever been used with pin_user_pages(). mm_structs > that have never been passed to pin_user_pages() cannot have a positive > page_maybe_dma_pinned() by definition. This allows cases that might drive up > the page ref_count to avoid any penalty from handling dma_pinned pages. > > Due to complexities with unpining this trivial version is a permanent sticky > bit, future work will be needed to make this a counter.
How about this instead:
Subsequent patches intend to reduce the chance of false positives from page_maybe_dma_pinned(), by also considering whether or not a page has even been part of an mm struct that has ever had pin_user_pages*() applied to any of its pages.
In order to allow that, provide a boolean value (even though it's not implemented exactly as a boolean type) within the mm struct, that is simply set once and never cleared. This will suffice for an early, rough implementation that fixes a few problems.
Future work is planned, to provide a more sophisticated solution, likely involving a counter, and *not* involving something that is set and never cleared.
> > Suggested-by: Jason Gunthorpe <jgg@ziepe.ca> > Signed-off-by: Peter Xu <peterx@redhat.com> > --- > include/linux/mm_types.h | 10 ++++++++++ > kernel/fork.c | 1 + > mm/gup.c | 6 ++++++ > 3 files changed, 17 insertions(+) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 496c3ff97cce..6f291f8b74c6 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -441,6 +441,16 @@ struct mm_struct { > #endif > int map_count; /* number of VMAs */ > > + /** > + * @has_pinned: Whether this mm has pinned any pages. This can > + * be either replaced in the future by @pinned_vm when it > + * becomes stable, or grow into a counter on its own. We're > + * aggresive on this bit now - even if the pinned pages were > + * unpinned later on, we'll still keep this bit set for the > + * lifecycle of this mm just for simplicity. > + */ > + int has_pinned;
I think this would be elegant as an atomic_t, and using atomic_set() and atomic_read(), which seem even more self-documenting that what you have here.
But it's admittedly a cosmetic point, combined with my perennial fear that I'm missing something when I look at a READ_ONCE()/WRITE_ONCE() pair. :)
It's completely OK to just ignore this comment, but I didn't want to completely miss the opportunity to make it a tiny bit cleaner to the reader.
thanks, -- John Hubbard NVIDIA
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