linux-ext4 - [PATCH] fscrypt: Copy the memcg information to the ciphertext page

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Date:   Sun, 29 Jan 2023 12:18:51 +0000
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     "Theodore Y . Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...nel.org>,
        linux-fscrypt@...r.kernel.org
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, stable@...r.kernel.org
Subject: [PATCH] fscrypt: Copy the memcg information to the ciphertext page

Both f2fs and ext4 end up passing the ciphertext page to
wbc_account_cgroup_owner().  At the moment, the ciphertext page appears
to belong to no cgroup, so it is accounted to the root_mem_cgroup instead
of whatever cgroup the original page was in.

It's hard to say how far back this is a bug.  The crypto code shared
between ext4 & f2fs was created in May 2015 with commit 0b81d0779072,
but neither filesystem did anything with memcg_data before then.  memcg
writeback accounting was added to ext4 in July 2015 in commit 001e4a8775f6
and it wasn't added to f2fs until January 2018 (commit 578c647879f7).

I'm going with the ext4 commit since this is the first commit where
there was a difference in behaviour between encrypted and unencrypted
filesystems.

Fixes: 001e4a8775f6 ("ext4: implement cgroup writeback support")
Cc: stable@...r.kernel.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 fs/crypto/crypto.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index e78be66bbf01..a4e76f96f291 100644
--- a/fs/crypto/crypto.c
+++ b/fs/crypto/crypto.c
@@ -205,6 +205,9 @@ struct page *fscrypt_encrypt_pagecache_blocks(struct page *page,
 	}
 	SetPagePrivate(ciphertext_page);
 	set_page_private(ciphertext_page, (unsigned long)page);
+#ifdef CONFIG_MEMCG
+	ciphertext_page->memcg_data = page->memcg_data;
+#endif
 	return ciphertext_page;
 }
 EXPORT_SYMBOL(fscrypt_encrypt_pagecache_blocks);
-- 
2.35.1