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: Sun, 4 Sep 2022 22:22:16 +0530 [thread overview]
Message-ID: <e87bad0c-47fa-5e6d-9c43-f2f66da32a76@linux.ibm.com> (raw)
In-Reply-To: <2368d91f-8442-076f-f33a-64b51b44825c@redhat.com>
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.
-aneesh
next prev parent reply other threads:[~2022-09-04 16:52 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 [this message]
2022-09-05 8:38 ` David Hildenbrand
2022-09-05 8:41 ` Aneesh Kumar K V
2022-09-04 16:49 ` Aneesh Kumar K V
2022-09-05 8:02 ` David Hildenbrand
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