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

From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Sasha Levin <sasha.levin@oracle.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Jerome Marchand <jmarchan@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Hugh Dickins <hughd@google.com>, Jason Gunthorpe <jgg@nvidia.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Yang Shi <shy828301@gmail.com>
Subject: Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast
Date: Thu, 1 Sep 2022 18:34:41 +0200	[thread overview]
Message-ID: <fa0bb4b1-3edd-eb5a-7ad6-dff785d88d8f@redhat.com> (raw)
In-Reply-To: <YxDdycTur733hMgt@xz-m1.local>

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