Sender: Tejun Heo <htejun@gmail.com>
Date:   Thu, 2 Feb 2023 11:30:42 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
        stable@vger.kernel.org, cgroups@vger.kernel.org
Subject: Re: [PATCH] fscrypt: Copy the memcg information to the ciphertext
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Hello,

On Tue, Jan 31, 2023 at 10:31:31PM -0800, Eric Biggers wrote:
> > These can usually be handled by explicitly associating the bio's to the
> > desired cgroups using one of bio_associate_blkg*() or
> > bio_clone_blkg_association().
> 
> Here that already happens in wbc_init_bio(), called from io_submit_init_bio() in
> fs/ext4/page-io.c.

Yeah, without bouncing, that's usually how writeback IOs are associated with
their cgroups.

> > It is possible to go through memcg ownership
> > too using set_active_memcg() so that the page is owned by the target cgroup;
> > however, the page ownership doesn't directly map to IO ownership as the
> > relationship depends on the type of the page (e.g. IO ownership for
> > pagecache writeback is determined per-inode, not per-page). If the in-flight
> > pages are limited, it probably is better to set bio association directly.
> 
> ext4 also calls wbc_account_cgroup_owner() for each pagecache page that's
> written out.  It seems this is for a different purpose -- it looks like the
> fs-writeback code is trying to figure out which cgroup "owns" the inode based on
> which cgroup "owns" most of the pagecache pages?

Yeah, there's a difference between how memory and IO track cgroup ownership.
Memory ownership is per-page but IO ownership is per-inode. This is because
splitting writeback IOs of the same inode can perform really badly, so we
try to find the majority dirty page owner cgroup of a given inode and
associate the whole inode to that cgroup.

So, something like md / dm, which gets a bio from filesystem and then
bounces it to another bio, would use either bio_clone_blkg_association() to
copy the association of the original bio (which probably is set through
wbc_init_bio()) or determine the cgroup the bio should belong to somehow and
set it explicitly with bio_associate_blkg(). However, here, as the
filesystem is the one bouncing I guess it can be simpler.

> The bug we're discussing here is that when ext4 writes out a pagecache page in
> an encrypted file, it first encrypts the data into a bounce page, then passes
> the bounce page (which don't have a memcg) to wbc_account_cgroup_owner().  Maybe
> the proper fix is to just pass the pagecache page to wbc_account_cgroup_owner()
> instead?  See below for ext4 (a separate patch would be needed for f2fs):

Yeah, this makes sense to me and is the right thing to do no matter what.
wbc_account_cgroup_owner() should be fed the origin page so that the IO can
be blamed on the owner of that page.

Thanks.

-- 
tejun