mm/mglru: remove redundant swap constrained check upon isolation

Remove the swap-constrained early reject check upon isolation. This
check is a micro optimization when swap IO is not allowed, so folios are
rejected early. But it is redundant and overly broad since
shrink_folio_list() already handles all these cases with proper
granularity.

Notably, this check wrongly rejected lazyfree folios, and it doesn't
cover all rejection cases. shrink_folio_list() uses may_enter_fs(),
which distinguishes non-SWP_FS_OPS devices from filesystem-backed swap
and does all the checks after folio is locked, so flags like swap cache
are stable.

This check also covers dirty file folios, which are not a problem now
since sort_folio() already bumps dirty file folios to the next
generation, but causes trouble for unifying dirty folio writeback
handling.

And there should be no performance impact from removing it. We may have
lost a micro optimization, but unblocked lazyfree reclaim for NOIO
contexts, which is not a common case in the first place.

Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>

Author: ryncsn

mm/vmscan.c

@@ -4509,12 +4509,6 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
 {
 	bool success;
 
-	/* swap constrained */
-	if (!(sc->gfp_mask & __GFP_IO) &&
-	    (folio_test_dirty(folio) ||
-	     (folio_test_anon(folio) && !folio_test_swapcache(folio))))
-		return false;
-
 	/* raced with release_pages() */
 	if (!folio_try_get(folio))
 		return false;