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

[Yin, Fengwei]

On 7/8/2023 1:26 AM, Matthew Wilcox wrote:
> On Sat, Jul 08, 2023 at 12:52:18AM +0800, Yin Fengwei wrote:
>> This series identified the large folio for mlock to two types:
>>   - The large folio is in VM_LOCKED VMA range
>>   - The large folio cross VM_LOCKED VMA boundary
> 
> This is somewhere that I think our fixation on MUST USE PMD ENTRIES
> has led us astray.  Today when the arguments to mlock() cross a folio
> boundary, we split the PMD entry but leave the folio intact.  That means
> that we continue to manage the folio as a single entry on the LRU list.
> But userspace may have no idea that we're doing this.  It may have made
> several calls to mmap() 256kB at once, they've all been coalesced into
> a single VMA and khugepaged has come along behind its back and created
> a 2MB THP.  Now userspace calls mlock() and instead of treating that as
> a hint that oops, maybe we shouldn't've done that, we do our utmost to
> preserve the 2MB folio.
> 
> I think this whole approach needs rethinking.  IMO, anonymous folios
> should not cross VMA boundaries.  Tell me why I'm wrong.

No. You are not wrong. :). That concept to keep anonymous folio not
cross VMA boundary is decent.


I tried to split the large folio when it cross VMA boundary for mlock().
As it's possible that the folio split fails, we always need to deal with
this case. I decided to postpone all large folio splitting to page
reclaim phase. The benefits we could get:
   - If the range is munlocked before page reclaim pick this folio,
     we don't need to split the folio.
   - for the system which don't have swap enabled, we don't need to
     split this kind folio.


Regards
Yin, Fengwei