Re: [PATCH 4/9] ext4: Drop pointless IO submission from ext4_bio_write_page()

From: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: Ted Tso <tytso@mit.edu>,
	linux-ext4@vger.kernel.org, Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 4/9] ext4: Drop pointless IO submission from ext4_bio_write_page()
Date: Thu, 1 Dec 2022 19:10:25 +0530	[thread overview]
Message-ID: <20221201134025.qagwnl4ezhes46jo@riteshh-domain> (raw)
In-Reply-To: <20221201103519.n32kes6llulr2mcx@quack3>

On 22/12/01 11:35AM, Jan Kara wrote:
> On Thu 01-12-22 12:36:55, Ritesh Harjani (IBM) wrote:
> > On 22/11/30 05:35PM, Jan Kara wrote:
> > > We submit outstanding IO in ext4_bio_write_page() if we find a buffer we
> > > are not going to write. This is however pointless because we already
> > > handle submission of previous IO in case we detect newly added buffer
> > > head is discontiguous. So just delete the pointless IO submission call.
> >
> > Agreed. io_submit_add_bh() is anyway called at the end for submitting buffers.
> > And io_submit_add_bh() also has the logic to:
> > 1. submit a discontiguous bio
> > 2. Also submit a bio if the bio gets full (submit_and_retry label).
> >
> > Hence calling ext4_io_submit() early is not required.
> >
> > I guess the same will also hold true for at this place.
> > https://elixir.bootlin.com/linux/v6.1-rc7/source/fs/ext4/page-io.c#L524
>
> So there the submission is needed because we are OOM and are going to wait
> for some memory to free. If we have some bio accumulated, it is pinning
> pages in writeback state and memory reclaim can be waiting on them. So if
> we don't submit, it is a deadlock possibility or at least asking for
> trouble.

Aah! right. I didn't see the ret == -ENOMEM compare there.

Thanks!
-ritesh