[PATCH] ext4: fix deadlock allocating crypto bounce page from mempool

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

ext4_writepages() on an encrypted file has to encrypt the data, but it
can't modify the pagecache pages in-place, so it encrypts the data into
bounce pages and writes those instead.  All bounce pages are allocated
from a mempool using GFP_NOFS.

This is not correct use of a mempool, and it can deadlock.  This is
because GFP_NOFS includes __GFP_DIRECT_RECLAIM, which enables the "never
fail" mode for mempool_alloc() where a failed allocation will fall back
to waiting for one of the preallocated elements in the pool.

But since this mode is used for all a bio's pages and not just the
first, it can deadlock waiting for pages already in the bio to be freed.

This deadlock can be reproduced by patching mempool_alloc() to pretend
that pool->alloc() always fails (so that it always falls back to the
preallocations), and then creating an encrypted file of size > 128 KiB.

Fix it by only using GFP_NOFS for the first page in the bio.  For
subsequent pages just use GFP_NOWAIT, and if any of those fail, just
submit the bio and start a new one.

This will need to be fixed in f2fs too, but that's less straightforward.

Fixes: c9af28fdd449 ("ext4 crypto: don't let data integrity writebacks fail with ENOMEM")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/ext4/page-io.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index 24aeedb8fc75..68b39e75446a 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -512,17 +512,26 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
 		gfp_t gfp_flags = GFP_NOFS;
 		unsigned int enc_bytes = round_up(len, i_blocksize(inode));
 
+		/*
+		 * Since bounce page allocation uses a mempool, we can only use
+		 * a waiting mask (i.e. request guaranteed allocation) on the
+		 * first page of the bio.  Otherwise it can deadlock.
+		 */
+		if (io->io_bio)
+			gfp_flags = GFP_NOWAIT | __GFP_NOWARN;
 	retry_encrypt:
 		bounce_page = fscrypt_encrypt_pagecache_blocks(page, enc_bytes,
 							       0, gfp_flags);
 		if (IS_ERR(bounce_page)) {
 			ret = PTR_ERR(bounce_page);
-			if (ret == -ENOMEM && wbc->sync_mode == WB_SYNC_ALL) {
-				if (io->io_bio) {
+			if (ret == -ENOMEM &&
+			    (io->io_bio || wbc->sync_mode == WB_SYNC_ALL)) {
+				gfp_flags = GFP_NOFS;
+				if (io->io_bio)
 					ext4_io_submit(io);
-					congestion_wait(BLK_RW_ASYNC, HZ/50);
-				}
-				gfp_flags |= __GFP_NOFAIL;
+				else
+					gfp_flags |= __GFP_NOFAIL;
+				congestion_wait(BLK_RW_ASYNC, HZ/50);
 				goto retry_encrypt;
 			}
 
-- 
2.24.1