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

[Barry Song <21cnbao@gmail.com>]

On Mon, Nov 18, 2024 at 5:03 PM Matthew Wilcox <willy@infradead.org> 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.

>
> This is going to become much more important with memdescs.  We'd have to
> allocate 512 struct folios to do this, which would be about 10 4kB pages,
> and if we're trying to swap out memory, we're probably low on memory.
>
> So I don't like this solution you have at all because it doesn't help us
> get to the solution we're going to need in about a year's time.
>

Ridong might need to clarify why this splitting is occurring. If it’s due to the
failure to allocate swap slots, we still need a solution to address it.

Thanks
Barry