* [PATCH] ext4: fix deadlock allocating crypto bounce page from mempool
@ 2019-12-31 18:11 Eric Biggers
2020-01-13 19:44 ` Theodore Y. Ts'o
0 siblings, 1 reply; 2+ messages in thread
From: Eric Biggers @ 2019-12-31 18:11 UTC (permalink / raw)
To: linux-ext4; +Cc: linux-fscrypt
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
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] ext4: fix deadlock allocating crypto bounce page from mempool
2019-12-31 18:11 [PATCH] ext4: fix deadlock allocating crypto bounce page from mempool Eric Biggers
@ 2020-01-13 19:44 ` Theodore Y. Ts'o
0 siblings, 0 replies; 2+ messages in thread
From: Theodore Y. Ts'o @ 2020-01-13 19:44 UTC (permalink / raw)
To: Eric Biggers; +Cc: linux-ext4, linux-fscrypt
On Tue, Dec 31, 2019 at 12:11:49PM -0600, Eric Biggers wrote:
> 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>
Applied, thanks.
- Ted
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