LSF/MM/BPF TOPIC] Deferred folio splitting: extend or remove/replace?

LSF/MM/BPF TOPIC] Deferred folio splitting: extend or remove/replace?

[David Hildenbrand (Arm)]

Hi,

the "deferred" folio splitting mechanism was originally introduced to 
reclaim memory from partially-mapped anonymous THPs under memory 
pressure. Partially-mapped anonymous folios (often) guarantee that at 
least some memory can get reclaimed easily.

Ever since, we extended it to also handle smaller large folios (mTHPs),
but also to scan for mostly-zero-filled mTHPs that can be reclaimed by 
replacing them with the shared zeropage.

Nowadays, each and every (m)THP we allocate gets added to the deferred 
split list as default, as "split_underused_thp = true", which is rather 
suboptimal.

Historically, handling deferred splits has been rather complicated and 
error-prone.

So I've been wondering whether that complexity still warranted. Could we 
instead just let LRU scanning deal with that and get rid of the shrinker?

Or could a shrinker instead use existing LRU lists?


We could flag folios for being
* Partially mapped (which we already do)
* Possibly over-allocated (unscanned for zero since added)

To prioritize processing them.

One of my motivations for looking into this topic is that with 
CONFIG_NO_PAGE_MAPCOUNT, we can sometimes not reliably detect "partially 
mapped" folios and only know that a folio "might be partially mapped". 
To figure out whether it is actually partially-mapped, we have to walk 
the RMAP.

While I could extend the deferred splitting mechanism to handle such 
partially-mapped folios as well, it would imply taking the deferred 
split locks more frequently when unmapping pages ... not having to mess 
with another list might simplify things.

-- 
Cheers,

David

Re: LSF/MM/BPF TOPIC] Deferred folio splitting: extend or remove/replace?

[David Hildenbrand (Arm)]

On 2/23/26 12:26, David Hildenbrand (Arm) wrote:
> Hi,
> 
> the "deferred" folio splitting mechanism was originally introduced to reclaim
> memory from partially-mapped anonymous THPs under memory pressure. Partially-
> mapped anonymous folios (often) guarantee that at least some memory can get
> reclaimed easily.
> 
> Ever since, we extended it to also handle smaller large folios (mTHPs),
> but also to scan for mostly-zero-filled mTHPs that can be reclaimed by replacing
> them with the shared zeropage.
> 
> Nowadays, each and every (m)THP we allocate gets added to the deferred split
> list as default, as "split_underused_thp = true", which is rather suboptimal.
> 
> Historically, handling deferred splits has been rather complicated and error-prone.
> 
> So I've been wondering whether that complexity still warranted. Could we instead
> just let LRU scanning deal with that and get rid of the shrinker?
> 
> Or could a shrinker instead use existing LRU lists?
> 
> 
> We could flag folios for being
> * Partially mapped (which we already do)
> * Possibly over-allocated (unscanned for zero since added)
> 
> To prioritize processing them.
> 
> One of my motivations for looking into this topic is that with
> CONFIG_NO_PAGE_MAPCOUNT, we can sometimes not reliably detect "partially mapped"
> folios and only know that a folio "might be partially mapped". To figure out
> whether it is actually partially-mapped, we have to walk the RMAP.
> 
> While I could extend the deferred splitting mechanism to handle such partially-
> mapped folios as well, it would imply taking the deferred split locks more
> frequently when unmapping pages ... not having to mess with another list might
> simplify things.
> 

Slides at:
https://share.daveh.de/s/bQsDpD7xigJ6wD8/download?path=&files=LSF_MM%2726_%20Deferred_folio_splitting.pdf

-- 
Cheers,

David