Messages in this thread | | | Date | Thu, 1 Sep 2022 18:34:41 +0200 | Subject | Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast | From | David Hildenbrand <> |
| |
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@linux.intel.com> >> Cc: Sasha Levin <sasha.levin@oracle.com> >> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> >> Cc: Vlastimil Babka <vbabka@suse.cz> >> Cc: Jerome Marchand <jmarchan@redhat.com> >> Cc: Andrea Arcangeli <aarcange@redhat.com> >> Cc: Hugh Dickins <hughd@google.com> >> Cc: Jason Gunthorpe <jgg@nvidia.com> >> Cc: John Hubbard <jhubbard@nvidia.com> >> Cc: Peter Xu <peterx@redhat.com> >> Cc: Yang Shi <shy828301@gmail.com> >> Signed-off-by: David Hildenbrand <david@redhat.com> >> --- >> 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.
Thanks!
-- Thanks,
David / dhildenb
| |