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 10:41 PM chenridong <chenridong@huawei.com> wrote:
>
>
>
> On 2024/11/18 12:14, Barry Song wrote:
> > 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
>
> shrink_folio_list
>   add_to_swap
>     folio_alloc_swap
>       get_swap_pages
>         scan_swap_map_slots
>         /*
>         * Swapfile is not block device or not using clusters so unable
>         * to allocate large entries.
>         */
>         if (!(si->flags & SWP_BLKDEV) || !si->cluster_info)
>           return 0;
>
> In my test, I use a file as swap, which is not 'SWP_BLKDEV'. So it
> failed to get get_swap_pages.

Alright, a proper non-rotating swap block device would be much
better. In your case, though, cluster allocation isn’t supported.

>
> I think this is a race issue between 'shrink_folio_list' executing and
> writing back asynchronously. In my test, 512 folios(THP split) were
> added to swap, only about 60 folios had not been written back when
> 'move_folios_to_lru' was invoked after 'shrink_folio_list'. What if
> writing back faster? Maybe this will happen even 32 folios(without THP)
> are in the 'folio_list' of shrink_folio_list's inputs.

On a real non-rotate swap device, the race condition would occur only when
contiguous 2MB swap slots are unavailable.

Hi Chris,
I recall you mentioned unifying the code for swap devices and swap files, or
for non-rotating and rotating devices. I assume a swap file (not a block device)
would also be a practical user case?

>
> Best regards,
> Ridong

Thanks
Barry