linux-ext4 - [PATCH 0/2] ext4, mm: improve partial inode eof zeroing

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Message-ID: <20240919160741.208162-1-bfoster@redhat.com>
Date: Thu, 19 Sep 2024 12:07:39 -0400
From: Brian Foster <bfoster@...hat.com>
To: linux-ext4@...r.kernel.org,
	linux-mm@...ck.org
Cc: linux-fsdevel@...r.kernel.org,
	willy@...radead.org
Subject: [PATCH 0/2] ext4, mm: improve partial inode eof zeroing

Hi all,

I've been poking around at testing zeroing behavior after a couple
recent enhancements to iomap_zero_range() and fsx[1]. Running [1] on
ext4 has uncovered a couple issues that I think share responsibility
between the fs and pagecache.

The details are in the commit logs, but patch 1 updates ext4 to do
partial eof block zeroing in more cases and patch 2 tweaks
pagecache_isize_extended() to do eof folio zeroing similar to as is done
during writeback (i.e., ext4_bio_write_folio(),
iomap_writepage_handle_eof(), etc.). These kind of overlap, but the fs
changes handle the case of a block straddling eof (so we're writing to
disk in that case) and the pagecache changes handle the case of a folio
straddling eof that might be at least partially hole backed (i.e.
sub-page block sizes, so we're just clearing pagecache).

Aside from general review, my biggest questions WRT patch 1 are 1.
whether the journalling bits are handled correctly and 2. whether the
verity case is handled correctly. I recall seeing verity checks around
the code and I don't know enough about the feature to quite understand
why. FWIW, I have run fstests against this using various combinations of
block size and journalling modes without any regression so far. That
includes enabling generic/363 [1] for ext4, which afaict is now possible
with these two proposed changes.

WRT patch 2, I originally tested with unconditional zeroing and added
the dirty check after. This still survives testing, but I'm having
second thoughts on whether that is correct or introduces a small race
window between writeback and an i_size update. I guess there's also a
question of whether the fs or pagecache should be responsible for this,
but given writeback and truncate_setsize() behavior this seemed fairly
consistent to me.

Thoughts, reviews, flames appreciated.

Brian

[1] https://lore.kernel.org/fstests/20240828181534.41054-1-bfoster@redhat.com/

Brian Foster (2):
  ext4: partial zero eof block on unaligned inode size extension
  mm: zero range of eof folio exposed by inode size extension

 fs/ext4/extents.c |  7 ++++++-
 fs/ext4/inode.c   | 51 +++++++++++++++++++++++++++++++++--------------
 mm/truncate.c     | 15 ++++++++++++++
 3 files changed, 57 insertions(+), 16 deletions(-)

-- 
2.45.0