Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast

[David Hildenbrand]

On 04.09.22 18:49, Aneesh Kumar K V wrote:
> On 9/1/22 10:04 PM, David Hildenbrand wrote:
> > On 01.09.22 18:28, Peter Xu wrote:
> > > On Thu, Sep 01, 2022 at 09:21:19AM +0200, David Hildenbrand wrote:
> > > > commit 4b471e8898c3 ("mm, thp: remove infrastructure for handling splitting
> > > > PMDs") didn't remove all details about the THP split requirements for
> > > > RCU GUP-fast.
> > > >
> > > > IPI broeadcasts on THP split are no longer required.
> > > >
> > > > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > > Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
> > > > Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> > > > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > > > Cc: Jerome Marchand <jmarchan@xxxxxxxxxx>
> > > > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > > > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > > > Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > > > Cc: Peter Xu <peterx@xxxxxxxxxx>
> > > > Cc: Yang Shi <shy828301@xxxxxxxxx>
> > > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > > > ---
> > > >   mm/gup.c | 5 ++---
> > > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/mm/gup.c b/mm/gup.c
> > > > index 5abdaf487460..cfe71f422787 100644
> > > > --- a/mm/gup.c
> > > > +++ b/mm/gup.c
> > > > @@ -2309,9 +2309,8 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
> > > >    *
> > > >    * Another way to achieve this is to batch up page table containing pages
> > > >    * belonging to more than one mm_user, then rcu_sched a callback to free those
> > > > - * pages. Disabling interrupts will allow the fast_gup walker to both block
> > > > - * the rcu_sched callback, and an IPI that we broadcast for splitting THPs
> > > > - * (which is a relatively rare event). The code below adopts this strategy.
> > > > + * pages. Disabling interrupts will allow the fast_gup walker to block the
> > > > + * rcu_sched callback.
> > >
> > > This is the comment for fast-gup in general but not only for thp split.
> >
> > "an IPI that we broadcast for splitting THP" is about splitting THP.
> >
> > > I can understand that we don't need IPI for thp split, but isn't the IPIs
> > > still needed for thp collapse (aka pmdp_collapse_flush)?
> >
> > That was, unfortunately, never documented -- and as discussed in the
> > other thread, arm64 doesn't do that IPI before collapse and might need
> > fixing. We'll most probably end up getting rid of that
> > (undocumented/forgotten) IPI requirement and fix it in GUP-fast by
> > re-rechecking if the PMD changed.
>
> Can you point to the other thread ?

Sure

see

https://lkml.kernel.org/r/CAHbLzkqeDAnCdt3q4E2RZw64QEzVaO_pseR3VaoHUhB+rZFcZQ@xxxxxxxxxxxxxx

and a resulting patch from that discussion

https://lkml.kernel.org/r/20220901222707.477402-1-shy828301@xxxxxxxxx

--
Thanks,

David / dhildenb