mm/shmem: hold off eviction in shmem_writeout()

Once shmem_writeout() has moved the folio from the page cache to the
swap cache, the folio lock no longer keeps the inode alive: truncation
and eviction find a swap entry, free it and do not wait for anybody.
That did not matter as long as swap_writeout() was the last thing the
function did.  But since it puts the folio back when swap_writeout()
returns AOP_WRITEPAGE_ACTIVATE, it uses the mapping and the inode
afterwards:

  reclaim                               last close of an unlinked file
  shmem_writeout()
    shmem_delete_from_page_cache()
    swap_writeout()
      zswap_store() fails               shmem_evict_inode()
      writeback is disabled               shmem_truncate_range()
      return AOP_WRITEPAGE_ACTIVATE         shmem_free_swap()
                                        destroy_inode()
                                        ... RCU grace period, freed
    shmem_add_to_page_cache(mapping)
      xas_lock_irq()            <- use after free

swap_writeout() behaves that way if zswap is enabled, cannot take the
folio and the memcg or one of its ancestors has memory.zswap.writeback
set to 0.

shmem_unuse() already added infrastructure to deal with that.
Reuse info->stop_eviction to protect the inode during writeout. Increment
it before removing the folio from the page cache, and decrement it after
swap_writeout() and any attempted reinsertion and inode accounting.
In shmem_evict_inode(), wait for the counter to reach zero even if
swapoff has already removed the inode from the swap list. Add some helpers
to make the code easier to digest.

There is a known remaining issue with two swapoffs racing, and the empty
list check not being able to detect that race. That will be addressed
separately later.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: 6344a6d9ce13 ("mm/shmem: writeout free swap if swap_writeout() reactivates")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Acked-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
[ david: code cleanups; improve description ]
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Link: https://patch.msgid.link/20261029-fixes_shmem-v4-6-392fc715d8f6@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

0 files changed, 0 insertions, 0 deletions