ext4: fix deadlock allocating crypto bounce page from mempool

ext4: fix deadlock allocating crypto bounce page from mempool

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>
Link: https://lore.kernel.org/r/20191231181149.47619-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>