f2fs: fix deadlock allocating bio_post_read_ctx from mempool

Without any form of coordination, any case where multiple allocations
from the same mempool are needed at a time to make forward progress can
deadlock under memory pressure.

This is the case for struct bio_post_read_ctx, as one can be allocated
to decrypt a Merkle tree page during fsverity_verify_bio(), which itself
is running from a post-read callback for a data bio which has its own
struct bio_post_read_ctx.

Fix this by freeing first bio_post_read_ctx before calling
fsverity_verify_bio().  This works because verity (if enabled) is always
the last post-read step.

This deadlock can be reproduced by trying to read from an encrypted
verity file after reducing NUM_PREALLOC_POST_READ_CTXS to 1 and patching
mempool_alloc() to pretend that pool->alloc() always fails.

Note that since NUM_PREALLOC_POST_READ_CTXS is actually 128, to actually
hit this bug in practice would require reading from lots of encrypted
verity files at the same time.  But it's theoretically possible, as N
available objects doesn't guarantee forward progress when > N/2 threads
each need 2 objects at a time.

Fixes: 95ae251f ("f2fs: add fs-verity support")
Signed-off-by: NEric Biggers <ebiggers@google.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -204,19 +204,32 @@ static void f2fs_verity_work(struct work_struct *work)
 {
 	struct bio_post_read_ctx *ctx =
 		container_of(work, struct bio_post_read_ctx, work);
+	struct bio *bio = ctx->bio;
+#ifdef CONFIG_F2FS_FS_COMPRESSION
+	unsigned int enabled_steps = ctx->enabled_steps;
+#endif
+
+	/*
+	 * fsverity_verify_bio() may call readpages() again, and while verity
+	 * will be disabled for this, decryption may still be needed, resulting
+	 * in another bio_post_read_ctx being allocated.  So to prevent
+	 * deadlocks we need to release the current ctx to the mempool first.
+	 * This assumes that verity is the last post-read step.
+	 */
+	mempool_free(ctx, bio_post_read_ctx_pool);
+	bio->bi_private = NULL;
 
 #ifdef CONFIG_F2FS_FS_COMPRESSION
 	/* previous step is decompression */
-	if (ctx->enabled_steps & (1 << STEP_DECOMPRESS)) {
-
-		f2fs_verify_bio(ctx->bio);
-		f2fs_release_read_bio(ctx->bio);
+	if (enabled_steps & (1 << STEP_DECOMPRESS)) {
+		f2fs_verify_bio(bio);
+		f2fs_release_read_bio(bio);
 		return;
 	}
 #endif
 
-	fsverity_verify_bio(ctx->bio);
-	__f2fs_read_end_io(ctx->bio, false, false);
+	fsverity_verify_bio(bio);
+	__f2fs_read_end_io(bio, false, false);
 }
 
 static void f2fs_post_read_work(struct work_struct *work)