[PATCH mm-new 2/2] mm/shmem: writeout free swap if swap_writeout() reactivates

[Hugh Dickins <hughd@google.com>]

If swap_writeout() returns AOP_WRITEPAGE_ACTIVATE (for example, because
zswap cannot compress and memcg disables writeback), there is no virtue
in keeping that folio in swap cache and holding the swap allocation:
shmem_writeout() switch it back to shmem page cache before returning.

Folio lock is held, and folio->memcg_data remains set throughout, so
there is no need to get into any memcg or memsw charge complications:
swap_free_nr() and delete_from_swap_cache() do as much as is needed (but
beware the race with shmem_free_swap() when inode truncated or evicted).

Doing the same for an anonymous folio is harder, since it will usually
have been unmapped, with references to the swap left in the page tables.
Adding a function to remap the folio would be fun, but not worthwhile
unless it has other uses, or an urgent bug with anon is demonstrated.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/shmem.c | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 33675361031b..5a7ce4c8bad6 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1655,6 +1655,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
 
 	if (!folio_alloc_swap(folio, __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN)) {
 		bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages);
+		int error;
 
 		/*
 		 * Add inode to shmem_unuse()'s list of swapped-out inodes,
@@ -1675,7 +1676,37 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
 		shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap));
 
 		BUG_ON(folio_mapped(folio));
-		return swap_writeout(folio, plug);
+		error = swap_writeout(folio, plug);
+		if (error != AOP_WRITEPAGE_ACTIVATE) {
+			/* folio has been unlocked */
+			return error;
+		}
+
+		/*
+		 * The intention here is to avoid holding on to the swap when
+		 * zswap was unable to compress and unable to writeback; but
+		 * it will be appropriate if other reactivate cases are added.
+		 */
+		error = shmem_add_to_page_cache(folio, mapping, index,
+				swp_to_radix_entry(folio->swap),
+				__GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
+		/* Swap entry might be erased by racing shmem_free_swap() */
+		if (!error) {
+			spin_lock(&info->lock);
+			info->swapped -= nr_pages;
+			spin_unlock(&info->lock);
+			swap_free_nr(folio->swap, nr_pages);
+		}
+
+		/*
+		 * The delete_from_swap_cache() below could be left for
+		 * shrink_folio_list()'s folio_free_swap() to dispose of;
+		 * but I'm a little nervous about letting this folio out of
+		 * shmem_writeout() in a hybrid half-tmpfs-half-swap state
+		 * e.g. folio_mapping(folio) might give an unexpected answer.
+		 */
+		delete_from_swap_cache(folio);
+		goto redirty;
 	}
 	if (nr_pages > 1)
 		goto try_split;
-- 
2.43.0