Re: [RFC PATCH v2 1/1] mm/vmscan: move the written-back folios to the tail of LRU after shrinking

[Matthew Wilcox]

On Mon, Nov 18, 2024 at 05:14:14PM +1300, Barry Song wrote:
> On Mon, Nov 18, 2024 at 5:03 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Sat, Nov 16, 2024 at 09:16:58AM +0000, Chen Ridong wrote:
> > > 2. In shrink_page_list function, if folioN is THP(2M), it may be splited
> > >    and added to swap cache folio by folio. After adding to swap cache,
> > >    it will submit io to writeback folio to swap, which is asynchronous.
> > >    When shrink_page_list is finished, the isolated folios list will be
> > >    moved back to the head of inactive lru. The inactive lru may just look
> > >    like this, with 512 filioes have been move to the head of inactive lru.
> >
> > I was hoping that we'd be able to stop splitting the folio when adding
> > to the swap cache.  Ideally. we'd add the whole 2MB and write it back
> > as a single unit.
> 
> This is already the case: adding to the swapcache doesn’t require splitting
> THPs, but failing to allocate 2MB of contiguous swap slots will.

Agreed we need to understand why this is happening.  As I've said a few
times now, we need to stop requiring contiguity.  Real filesystems don't
need the contiguity (they become less efficient, but they can scatter a
single 2MB folio to multiple places).

Maybe Chris has a solution to this in the works?