On Tue, Dec 31, 2019 at 12:11:49PM -0600, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Applied, thanks.
- Ted
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