On Thu, Jan 30, 2025 at 12:00:44PM +0200, Kirill A. Shutemov wrote:
> The recently introduced PG_dropbehind allows for freeing folios
> immediately after writeback. Unlike PG_reclaim, it does not need vmscan
> to be involved to get the folio freed.
>
> Instead of using folio_set_reclaim(), use folio_set_dropbehind() in
> pageout().
>
> It is safe to leave PG_dropbehind on the folio if, for some reason
> (bug?), the folio is not in a writeback state after ->writepage().
> In these cases, the kernel had to clear PG_reclaim as it shared a page
> flag bit with PG_readahead.
Is it correct to say that leaving PG_dropbehind on folios which doesn't
have writeback state after ->writepage() (i.e. store to zswap) is fine
because PG_dropbehind is not in PAGE_FLAGS_CHECK_AT_FREE?
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
> mm/vmscan.c | 9 +++------
> 1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bc1826020159..c97adb0fdaa4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -692,19 +692,16 @@ static pageout_t pageout(struct folio *folio, struct address_space *mapping,
> if (shmem_mapping(mapping) && folio_test_large(folio))
> wbc.list = folio_list;
>
> - folio_set_reclaim(folio);
> + folio_set_dropbehind(folio);
> +
> res = mapping->a_ops->writepage(&folio->page, &wbc);
> if (res < 0)
> handle_write_error(mapping, folio, res);
> if (res == AOP_WRITEPAGE_ACTIVATE) {
> - folio_clear_reclaim(folio);
> + folio_clear_dropbehind(folio);
> return PAGE_ACTIVATE;
> }
>
> - if (!folio_test_writeback(folio)) {
> - /* synchronous write or broken a_ops? */
> - folio_clear_reclaim(folio);
> - }
> trace_mm_vmscan_write_folio(folio);
> node_stat_add_folio(folio, NR_VMSCAN_WRITE);
> return PAGE_SUCCESS;
> --
> 2.47.2
>
