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Date: Mon, 13 Jan 2020 14:44:52 -0500 From: "Theodore Y. Ts'o" <tytso@....edu> To: Eric Biggers <ebiggers@...nel.org> Cc: linux-ext4@...r.kernel.org, linux-fscrypt@...r.kernel.org Subject: Re: [PATCH] ext4: fix deadlock allocating crypto bounce page from mempool On Tue, Dec 31, 2019 at 12:11:49PM -0600, Eric Biggers wrote: > From: Eric Biggers <ebiggers@...gle.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@...r.kernel.org > Signed-off-by: Eric Biggers <ebiggers@...gle.com> Applied, thanks. - Ted
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