Re: [PATCH 1/2] mm: multi-gen LRU: retry folios written back while isolated

[Minchan Kim <minchan@kernel.org>]

On Tue, Nov 15, 2022 at 06:38:07PM -0700, Yu Zhao wrote:
> The page reclaim isolates a batch of folios from the tail of one of
> the LRU lists and works on those folios one by one. For a suitable
> swap-backed folio, if the swap device is async, it queues that folio
> for writeback. After the page reclaim finishes an entire batch, it
> puts back the folios it queued for writeback to the head of the
> original LRU list.
> 
> In the meantime, the page writeback flushes the queued folios also by
> batches. Its batching logic is independent from that of the page
> reclaim. For each of the folios it writes back, the page writeback
> calls folio_rotate_reclaimable() which tries to rotate a folio to the
> tail.
> 
> folio_rotate_reclaimable() only works for a folio after the page
> reclaim has put it back. If an async swap device is fast enough, the
> page writeback can finish with that folio while the page reclaim is
> still working on the rest of the batch containing it. In this case,
> that folio will remain at the head and the page reclaim will not retry
> it before reaching there.
> 
> This patch adds a retry to evict_folios(). After evict_folios() has
> finished an entire batch and before it puts back folios it cannot free
> immediately, it retries those that may have missed the rotation.

Can we make something like this?

    shrink_folio_list(struct list_head *folio_list, struct list_head *folio_wb_list, )
        pageout
            goto keep
        ..
        ..

        keep:
            if (folio_test_writeback(folio) &&
                    folio_test_reclaim(folio))
                list_add(&folio->lru, &ret_writeback_folio);
    
    move_folios_to_lru(&folio_list, &folio_wb_list);
        struct folio *wb_folio = lru_to_folio(folio_wb_list);

        /*
         * If writeback is already done, move the page into tail.
         * Otherwise, put the page into head and folio_rotate_reclaimable
         * will move it to the tail when the writeback is done
         */
        if (!folio_test_writeback(wb_folio)) &&
                    folio_test_reclaim(wb_folio))
            lruvec_add_folio_tail(lruvec, folio);