Re: [RFC PATCH 0/3] support large folio for mlock

[Yin, Fengwei]

Hi David,

On 7/10/2023 5:32 PM, David Hildenbrand wrote:
> On 09.07.23 15:25, Yin, Fengwei wrote:
>>
>>
>> On 7/8/2023 12:02 PM, Matthew Wilcox wrote:
>>> I would be tempted to allocate memory & copy to the new mlocked VMA.
>>> The old folio will go on the deferred_list and be split later, or its
>>> valid parts will be written to swap and then it can be freed.
>> If the large folio splitting failure is because of GUP pages, can we
>> do copy here?
>>
>> Let's say, if the GUP page is target of DMA operation and DMA operation
>> is ongoing. We allocated a new page and copy GUP page content to the
>> new page, the data in the new page can be corrupted.
> 
> No, we may only replace anon pages that are flagged as maybe shared (!PageAnonExclusive). We must not replace pages that are exclusive (PageAnonExclusive) unless we first try marking them maybe shared. Clearing will fail if the page maybe pinned.
Thanks a lot for clarification.

So my understanding is that if large folio splitting fails, it's not always
true that we can allocate new folios, copy original large folio content to
new folios, remove original large folio from VMA and map the new folios to
VMA (like it's only true if original large folio is marked as maybe shared).


Regards
Yin, Fengwei

> 
> page_try_share_anon_rmap() implements the clearing logic, taking care of synchronizing against concurrent GUP-fast.
> 
> There are some additional nasty details regarding O_DIRECT. But once it completely switched from using FOLL_GET to properly using FOLL_PIN (a lot of that conversion already happened IIRC), we're fine in that regard.
>