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From: David Hildenbrand <david@redhat.com>
To: Ryan Roberts <ryan.roberts@arm.com>,
	"Yin, Fengwei" <fengwei.yin@intel.com>, Zi Yan <ziy@nvidia.com>,
	Matthew Wilcox <willy@infradead.org>, Yu Zhao <yuzhao@google.com>
Cc: Linux-MM <linux-mm@kvack.org>
Subject: Re: Prerequisites for Large Anon Folios
Date: Mon, 24 Jul 2023 13:42:11 +0200	[thread overview]
Message-ID: <8d68377e-7dab-e117-d47d-06f018a99067@redhat.com> (raw)
In-Reply-To: <3b08e3d9-29c3-6451-91a3-826f9a28f34d@arm.com>

On 24.07.23 11:46, Ryan Roberts wrote:
> On 24/07/2023 10:33, Yin, Fengwei wrote:
>>
>>
>> On 7/24/2023 5:04 PM, Ryan Roberts wrote:
>>> On 23/07/2023 13:33, Yin, Fengwei wrote:
>>>>
>>>>
>>>> On 7/20/2023 5:41 PM, Ryan Roberts wrote:
>>>>> Hi All,
>>>>>
>>>>> As discussed at Matthew's call yesterday evening, I've put together a list of
>>>>> items that need to be done as prerequisites for merging large anonymous folios
>>>>> support.
>>>>>
>>>>> It would be great to get some review and confirmation as to whether anything is
>>>>> missing or incorrect. Most items have an assignee - in that case it would be
>>>>> good to check that my understanding that you are working on the item is correct.
>>>>>
>>>>> I think most things are independent, with the exception of "shared vs exclusive
>>>>> mappings", which I think becomes a dependency for a couple of things (marked in
>>>>> depender description); again would be good to confirm.
>>>>>
>>>>> Finally, although I'm concentrating on the prerequisites to clear the path for
>>>>> merging an MVP Large Anon Folios implementation, I've included one "enhancement"
>>>>> item ("large folios in swap cache"), solely because we explicitly discussed it
>>>>> last night. My view is that enhancements can come after the initial large anon
>>>>> folios merge. Over time, I plan to add other enhancements (e.g. retain large
>>>>> folios over COW, etc).
>>>>>
>>>>> I'm posting the table as yaml as that seemed easiest for email. You can convert
>>>>> to csv with something like this in Python:
>>>>>
>>>>>    import yaml
>>>>>    import pandas as pd
>>>>>    pd.DataFrame(yaml.safe_load(open('work-items.yml'))).to_csv('work-items.csv')
>>>>>
>>>>> Thanks,
>>>>> Ryan
>>>> Should we add the mremap case to the list? Like how to handle the case that mremap
>>>> happens in the middle of large anonymous folio and fails to split it.
>>>
>>> What's the issue that you see here? My opinion is that if we do nothing special
>>> for mremap(), it neither breaks correctness nor performance when we enable large
>>> anon folios. So on that basis, its not a prerequisite and I'd rather leave it
>>> off the list. We might want to do something later as an enhancement though?
>> The issue is related with anonymous folio->index.
>>
>> If mremap happens in the middle of the large folio, current code doesn't split it.
>> So the large folio will be split to two parts: one is in original place and another
>> is in the new place. These two parts which are in different VMA have same folio->index.
>> Can rmap_walk_anon() work with this situation? vma_address() combined with head page.
>> Can it work for the pages not in same vma as head page?
>>
>> I could miss something here. Will try to build test against it.
> 
> Ahh, I see. So the rmap is broken for large anon folios that have pages mapped
> non-contiguously in VA? In that case, I agree that this is a big issue for
> correctness and therefore a prerequisite!

I think existing rmap code should be able to handled that, otherwise 
that would be severely broken. A simple partial mremap() on an ordinary 
PMD-mapped THP would already trigger that.

In any case, we have to make PTE-mapped THPs a first-class citizen.

-- 
Cheers,

David / dhildenb



  parent reply	other threads:[~2023-07-24 11:42 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-07-20  9:41 Prerequisites for Large Anon Folios Ryan Roberts
2023-07-23 12:33 ` Yin, Fengwei
2023-07-24  9:04   ` Ryan Roberts
2023-07-24  9:33     ` Yin, Fengwei
2023-07-24  9:46       ` Ryan Roberts
2023-07-24  9:54         ` Yin, Fengwei
2023-07-24 11:42         ` David Hildenbrand [this message]
2023-08-30 10:08       ` Ryan Roberts
2023-08-31  0:01         ` Yin, Fengwei
2023-08-31  7:16           ` Ryan Roberts
2023-08-30 10:44 ` Ryan Roberts
2023-08-30 16:20   ` David Hildenbrand
2023-08-31  7:26     ` Ryan Roberts
2023-08-31  7:59       ` David Hildenbrand
2023-08-31  9:04         ` Ryan Roberts
2023-09-01 14:44           ` David Hildenbrand
2023-09-04 10:06             ` Ryan Roberts
2023-09-05 20:54               ` David Rientjes
2023-08-31  0:08   ` Yin, Fengwei
2023-08-31  7:18     ` Ryan Roberts
2023-08-31  7:38       ` Yin, Fengwei

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