linux-kernel - Re: [PATCH v2 2/3] mm: rmap: Move the cache flushing to the correct place for hugetlb PMD sharing

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Date:   Wed, 4 May 2022 10:50:33 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>
Cc:     akpm@...ux-foundation.org, almasrymina@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] mm: rmap: Move the cache flushing to the correct
 place for hugetlb PMD sharing



On 5/4/2022 2:42 AM, Mike Kravetz wrote:
> On 4/27/22 22:55, Muchun Song wrote:
>> On Wed, Apr 27, 2022 at 06:52:06PM +0800, Baolin Wang wrote:
>>> The cache level flush will always be first when changing an existing
>>> virtual–>physical mapping to a new value, since this allows us to
>>> properly handle systems whose caches are strict and require a
>>> virtual–>physical translation to exist for a virtual address. So we
>>> should move the cache flushing before huge_pmd_unshare().
>>>
>>
>> Right.
>>
>>> As Muchun pointed out[1], now the architectures whose supporting hugetlb
>>> PMD sharing have no cache flush issues in practice. But I think we
>>> should still follow the cache/TLB flushing rules when changing a valid
>>> virtual address mapping in case of potential issues in future.
>>
>> Right. One point i need to clarify. I do not object this change but
>> want you to clarify this (not an issue in practice) in commit log
>> to let others know they do not need to bp this.
>>
>>>
>>> [1] https://lore.kernel.org/all/YmT%2F%2FhuUbFX+KHcy@FVFYT0MHHV2J.usts.net/
>>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>> ---
>>>   mm/rmap.c | 40 ++++++++++++++++++++++------------------
>>>   1 file changed, 22 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 61e63db..4f0d115 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1535,15 +1535,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>   			 * do this outside rmap routines.
>>>   			 */
>>>   			VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
>>> +			/*
>>> +			 * huge_pmd_unshare may unmap an entire PMD page.
>>> +			 * There is no way of knowing exactly which PMDs may
>>> +			 * be cached for this mm, so we must flush them all.
>>> +			 * start/end were already adjusted above to cover this
>>> +			 * range.
>>> +			 */
>>> +			flush_cache_range(vma, range.start, range.end);
>>> +
>>
>> flush_cache_range() is always called even if we do not need to flush.
>> How about introducing a new helper like hugetlb_pmd_shared() which
>> returns true for shared PMD? Then:
>>
>> 	if (hugetlb_pmd_shared(mm, vma, pvmw.pte)) {
>> 		flush_cache_range(vma, range.start, range.end);
>> 		huge_pmd_unshare(mm, vma, &address, pvmw.pte);
>> 		flush_tlb_range(vma, range.start, range.end);
>> 	}
>>
>> The code could be a little simpler. Right?
>>
>> Thanks.
>>
> 
> I thought about adding a 'hugetlb_pmd_shared()' interface for another use.
> I believe it could even be used earlier in this call sequence.  Since we
> hold i_mmap_rwsem, we would even test for shared BEFORE calling
> adjust_range_if_pmd_sharing_possible.  We can not make an authoritative test
> in adjust range... because not all callers will be holding i_mmap_rwsem.
> 
> I think we COULD optimize to minimize the flush range.  However, I think
> that would complicate this code even more, and it is difficult enough to
> follow.
> 
> My preference would be to over flush as is done here for correctness and
> simplification.  We can optimize later if desired.

OK. Agree.

> 
> With Muchun's comment that this is not an issue in practice today,
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

Thanks.