From: Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
To: David Hildenbrand <david@redhat.com>,
Yang Shi <shy828301@gmail.com>, Peter Xu <peterx@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.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>
Subject: Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast
Date: Mon, 5 Sep 2022 14:11:07 +0530 [thread overview]
Message-ID: <64faaca6-bbcf-bbfb-66c8-b8e80fcdc6bd@linux.ibm.com> (raw)
In-Reply-To: <8e1aaa19-0d28-3381-3ec6-920a474f5f3f@redhat.com>
On 9/5/22 2:08 PM, David Hildenbrand wrote:
> On 04.09.22 18:52, Aneesh Kumar K V wrote:
>> On 9/2/22 12:02 PM, David Hildenbrand wrote:
>>> On 01.09.22 20:35, Yang Shi wrote:
>>>> On Thu, Sep 1, 2022 at 11:07 AM Peter Xu <peterx@redhat.com> wrote:
>>>>>
>>>>> On Thu, Sep 01, 2022 at 10:50:48AM -0700, Yang Shi wrote:
>>>>>> Yeah, because THP collapse does copy the data before clearing pte. If
>>>>>> we want to remove pmdp_collapse_flush() by just clearing pmd, we
>>>>>> should clear *AND* flush pte before copying the data IIRC.
>>>>>
>>>>> Yes tlb flush is still needed. IIUC the generic pmdp_collapse_flush() will
>>>>> still be working (with the pte level flushing there) but it should just
>>>>> start to work for all archs, so potentially we could drop the arch-specific
>>>>> pmdp_collapse_flush()s, mostly the ppc impl.
>>>>
>>>> I'm don't know why powperpc needs to have its specific
>>>> pmdp_collapse_flush() in the first place, not only the mandatory IPI
>>>> broadcast, but also the specific implementation of pmd tlb flush. But
>>>> anyway the IPI broadcast could be removed at least IMO.
>>>>
>>>
>>> pmdp_collapse_flush() is overwritten on book3s only. It either translates
>>> to radix__pmdp_collapse_flush() or hash__pmdp_collapse_flush().
>>>
>>>
>>> radix__pmdp_collapse_flush() has a comment explaining the situation:
>>>
>>>
>>> + /*
>>> + * pmdp collapse_flush need to ensure that there are no parallel gup
>>> + * walk after this call. This is needed so that we can have stable
>>> + * page ref count when collapsing a page. We don't allow a collapse page
>>> + * if we have gup taken on the page. We can ensure that by sending IPI
>>> + * because gup walk happens with IRQ disabled.
>>> + */
>>>
>>>
>>> The comment for hash__pmdp_collapse_flush() is a bit more involved:
>>>
>>> /*
>>> * Wait for all pending hash_page to finish. This is needed
>>> * in case of subpage collapse. When we collapse normal pages
>>> * to hugepage, we first clear the pmd, then invalidate all
>>> * the PTE entries. The assumption here is that any low level
>>> * page fault will see a none pmd and take the slow path that
>>> * will wait on mmap_lock. But we could very well be in a
>>> * hash_page with local ptep pointer value. Such a hash page
>>> * can result in adding new HPTE entries for normal subpages.
>>> * That means we could be modifying the page content as we
>>> * copy them to a huge page. So wait for parallel hash_page
>>> * to finish before invalidating HPTE entries. We can do this
>>> * by sending an IPI to all the cpus and executing a dummy
>>> * function there.
>>> */
>>>
>>> I'm not sure if that implies that the IPI is needed for some other hash-magic.
>>>
>>> Maybe Aneesh can clarify.
>>>
>>
>> We still need the IPI for the hash. Another reason for architecture to override that
>> function is to help them use the right page size when flushing the TLB.
>
> Thanks for clarifying. So the radix variant wouldn't need the IPI anymore, once GUP-fast is handled differently, correct?
>
yes. With this patch https://lkml.kernel.org/r/20220901222707.477402-1-shy828301@gmail.com we can remove the
serialize_against_pte_lookup(vma->vm_mm); in radix__pmdp_collapse_flush()
-aneesh
next prev parent reply other threads:[~2022-09-05 8:42 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-01 7:21 [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast David Hildenbrand
2022-09-01 14:37 ` Kirill A . Shutemov
2022-09-01 16:12 ` Jason Gunthorpe
2022-09-01 16:30 ` David Hildenbrand
2022-09-01 16:28 ` Peter Xu
2022-09-01 16:34 ` David Hildenbrand
2022-09-01 16:40 ` Peter Xu
2022-09-01 16:46 ` David Hildenbrand
2022-09-01 17:41 ` Peter Xu
2022-09-01 17:44 ` David Hildenbrand
2022-09-01 17:50 ` Yang Shi
2022-09-01 18:07 ` Peter Xu
2022-09-01 18:35 ` Yang Shi
2022-09-02 6:32 ` David Hildenbrand
2022-09-02 13:53 ` Peter Xu
2022-09-02 15:37 ` Yang Shi
2022-09-04 16:52 ` Aneesh Kumar K V
2022-09-05 8:38 ` David Hildenbrand
2022-09-05 8:41 ` Aneesh Kumar K V [this message]
2022-09-04 16:49 ` Aneesh Kumar K V
2022-09-05 8:02 ` David Hildenbrand
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