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

[Peter Xu]

On Thu, Sep 01, 2022 at 06:34:41PM +0200, 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.

Ah OK.  Shall we still keep some "IPI broadcast" information here if we're
modifying it?  Otherwise it gives a feeling that none needs the IPIs.

It can be dropped later if you want to rework the thp collapse side and
finally remove IPI dependency on fast-gup, but so far it seems to me it's
still needed.  Or just drop this patch until that rework happens?

> 
> > 
> > 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.

Yeah from an initial thought that looks valid to me.  It'll also allow
pmdp_collapse_flush() to be dropped too, am I right?

-- 
Peter Xu