[PATCH] ext4: fix cgroup writeback accounting with fs-layer encryption

[PATCH] ext4: fix cgroup writeback accounting with fs-layer encryption

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

When writing a page from an encrypted file that is using
filesystem-layer encryption (not inline encryption), ext4 encrypts the
pagecache page into a bounce page, then writes the bounce page.

It also passes the bounce page to wbc_account_cgroup_owner().  That's
incorrect, because the bounce page is a newly allocated temporary page
that doesn't have the memory cgroup of the original pagecache page.
This makes wbc_account_cgroup_owner() not account the I/O to the owner
of the pagecache page as it should.

Fix this by always passing the pagecache page to
wbc_account_cgroup_owner().

Fixes: 001e4a8775f6 ("ext4: implement cgroup writeback support")
Cc: stable@vger.kernel.org
Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/ext4/page-io.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index beaec6d81074a..1e4db96a04e63 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -409,7 +409,8 @@ static void io_submit_init_bio(struct ext4_io_submit *io,
 
 static void io_submit_add_bh(struct ext4_io_submit *io,
 			     struct inode *inode,
-			     struct page *page,
+			     struct page *pagecache_page,
+			     struct page *bounce_page,
 			     struct buffer_head *bh)
 {
 	int ret;
@@ -421,10 +422,11 @@ static void io_submit_add_bh(struct ext4_io_submit *io,
 	}
 	if (io->io_bio == NULL)
 		io_submit_init_bio(io, bh);
-	ret = bio_add_page(io->io_bio, page, bh->b_size, bh_offset(bh));
+	ret = bio_add_page(io->io_bio, bounce_page ?: pagecache_page,
+			   bh->b_size, bh_offset(bh));
 	if (ret != bh->b_size)
 		goto submit_and_retry;
-	wbc_account_cgroup_owner(io->io_wbc, page, bh->b_size);
+	wbc_account_cgroup_owner(io->io_wbc, pagecache_page, bh->b_size);
 	io->io_next_block++;
 }
 
@@ -561,8 +563,7 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
 	do {
 		if (!buffer_async_write(bh))
 			continue;
-		io_submit_add_bh(io, inode,
-				 bounce_page ? bounce_page : page, bh);
+		io_submit_add_bh(io, inode, page, bounce_page, bh);
 	} while ((bh = bh->b_this_page) != head);
 unlock:
 	unlock_page(page);

base-commit: 6d796c50f84ca79f1722bb131799e5a5710c4700
-- 
2.39.1

Re: [PATCH] ext4: fix cgroup writeback accounting with fs-layer encryption

[Tejun Heo]

On Thu, Feb 02, 2023 at 04:55:03PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> When writing a page from an encrypted file that is using
> filesystem-layer encryption (not inline encryption), ext4 encrypts the
> pagecache page into a bounce page, then writes the bounce page.
> 
> It also passes the bounce page to wbc_account_cgroup_owner().  That's
> incorrect, because the bounce page is a newly allocated temporary page
> that doesn't have the memory cgroup of the original pagecache page.
> This makes wbc_account_cgroup_owner() not account the I/O to the owner
> of the pagecache page as it should.
> 
> Fix this by always passing the pagecache page to
> wbc_account_cgroup_owner().
> 
> Fixes: 001e4a8775f6 ("ext4: implement cgroup writeback support")
> Cc: stable@vger.kernel.org
> Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun

Re: [PATCH] ext4: fix cgroup writeback accounting with fs-layer encryption

[Eric Biggers]

On Fri, Feb 03, 2023 at 08:19:35AM -1000, Tejun Heo wrote:
> On Thu, Feb 02, 2023 at 04:55:03PM -0800, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> > 
> > When writing a page from an encrypted file that is using
> > filesystem-layer encryption (not inline encryption), ext4 encrypts the
> > pagecache page into a bounce page, then writes the bounce page.
> > 
> > It also passes the bounce page to wbc_account_cgroup_owner().  That's
> > incorrect, because the bounce page is a newly allocated temporary page
> > that doesn't have the memory cgroup of the original pagecache page.
> > This makes wbc_account_cgroup_owner() not account the I/O to the owner
> > of the pagecache page as it should.
> > 
> > Fix this by always passing the pagecache page to
> > wbc_account_cgroup_owner().
> > 
> > Fixes: 001e4a8775f6 ("ext4: implement cgroup writeback support")
> > Cc: stable@vger.kernel.org
> > Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> 
> Thanks.

This patch hasn't been applied yet.  Ted, I was hoping you would take it through
the ext4 tree.  Can you do so when you have a chance?

- Eric

Re: [PATCH] ext4: fix cgroup writeback accounting with fs-layer encryption

[Theodore Ts'o]

On Thu, 2 Feb 2023 16:55:03 -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> When writing a page from an encrypted file that is using
> filesystem-layer encryption (not inline encryption), ext4 encrypts the
> pagecache page into a bounce page, then writes the bounce page.
> 
> It also passes the bounce page to wbc_account_cgroup_owner().  That's
> incorrect, because the bounce page is a newly allocated temporary page
> that doesn't have the memory cgroup of the original pagecache page.
> This makes wbc_account_cgroup_owner() not account the I/O to the owner
> of the pagecache page as it should.
> 
> [...]

Applied, thanks!

[1/1] ext4: fix cgroup writeback accounting with fs-layer encryption
      commit: ffec85d53d0f39ee4680a2cf0795255e000e1feb

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>