mm/shmem: writeout free swap if swap_writeout() reactivates

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.

[hughd@google.com: use shmem_recalc_inode() rather than open coding, per Baolin]
Link: https://lkml.kernel.org/r/101a7d89-290c-545d-8a6d-b1174ed8b1e5@google.com
Link: https://lkml.kernel.org/r/5c911f7a-af7a-5029-1dd4-2e00b66d565c@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: David Rientjes <rientjes@google.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index cdee7273ea8cb97e0dd3e523f67b874f1a140fdf..7570a24e0ae4e7db631dc0cfed98c830f282e2c8 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1631,6 +1631,7 @@ try_split:
 
        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,
@@ -1651,7 +1652,35 @@ try_split:
                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) {
+                       shmem_recalc_inode(inode, 0, -nr_pages);
+                       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;