linux-ext4 - Re: generic_quota

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Date:   Mon, 6 Jun 2022 09:37:38 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        jfs-discussion@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org,
        "Darrick J . Wong" <djwong@...nel.org>, linux-ext4@...r.kernel.org,
        Jan Kara <jack@...e.com>
Subject: Re: generic_quota_read

On Fri 03-06-22 15:40:13, Matthew Wilcox wrote:
> On Thu, May 26, 2022 at 10:43:51PM -0700, Christoph Hellwig wrote:
> > >  static ssize_t jfs_quota_read(struct super_block *sb, int type, char *data,
> > > +			      size_t len, loff_t pos)
> >
> > And this whole helper is generic now.  It might be worth to move it
> > into fs/quota/dquot.c as generic_quota_read.
> 
> I've been working on that this week.  Unfortunately, you have to convert
> both quota_read and quota_write at the same time, it turns out.  At
> least ext4_quota_write() uses the bdev's inode's page cache to back
> the buffer_heads, so quota_read() and quota_write() are incoherent
> with each other:
> 
> 00017 gqr: mapping:00000000ee19acfb index:0x1 pos:0x1470 len:0x30
> 00017 4qw: mapping:000000007f9a811e index:0x18405 pos:0x1440 len:0x30

Yes, reads and writes have to use the same cache. Otherwise bad things
happen...

> I don't know if there's a way around this.  Can't really use
> read_mapping_folio() on the bdev's inode in generic_quota_read() -- the
> blocks for a given page might be fragmented on disk.  I don't know
> if there's a way to tell ext4_bread() to use the inode's page cache
> instead of the bdev's.

There's no way for ext4_bread() to read from inode's page cache. And that
is deliberate - ext4_bread() is used for filesystem metadata (and quota is
treated as filesystem metadata) and we use bdev page cache for all the
metadata.

> And if we did that, would it even work as being part of a transaction?

In principle it could work because we would then treat quota as journalled
data and jbd2 supports that. But honestly, special-casing quota as
journalled data IMHO brings more hassle on the write side than it can save
by sharing some code on the read side.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR