linux-ext4 - Re: [PATCH 16/17] block: use iomap for writes to block devices

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Date:   Wed, 24 May 2023 08:27:13 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Miklos Szeredi <miklos@...redi.hu>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        ceph-devel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/17] block: use iomap for writes to block devices

On Fri, May 19, 2023 at 04:22:01PM +0200, Hannes Reinecke wrote:
> On 4/24/23 07:49, Christoph Hellwig wrote:
> > Use iomap in buffer_head compat mode to write to block devices.
> > 
> > Signed-off-by: Christoph Hellwig <hch@....de>
> > ---
> >   block/Kconfig |  1 +
> >   block/fops.c  | 33 +++++++++++++++++++++++++++++----
> >   2 files changed, 30 insertions(+), 4 deletions(-)
> > 
> > diff --git a/block/Kconfig b/block/Kconfig
> > index 941b2dca70db73..672b08f0096ab4 100644
> > --- a/block/Kconfig
> > +++ b/block/Kconfig
> > @@ -5,6 +5,7 @@
> >   menuconfig BLOCK
> >          bool "Enable the block layer" if EXPERT
> >          default y
> > +       select IOMAP
> >          select SBITMAP
> >          help
> >   	 Provide block layer support for the kernel.
> > diff --git a/block/fops.c b/block/fops.c
> > index 318247832a7bcf..7910636f8df33b 100644
> > --- a/block/fops.c
> > +++ b/block/fops.c
> > @@ -15,6 +15,7 @@
> >   #include <linux/falloc.h>
> >   #include <linux/suspend.h>
> >   #include <linux/fs.h>
> > +#include <linux/iomap.h>
> >   #include <linux/module.h>
> >   #include "blk.h"
> > @@ -386,6 +387,27 @@ static ssize_t blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
> >   	return __blkdev_direct_IO(iocb, iter, bio_max_segs(nr_pages));
> >   }
> > +static int blkdev_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> > +		unsigned int flags, struct iomap *iomap, struct iomap *srcmap)
> > +{
> > +	struct block_device *bdev = I_BDEV(inode);
> > +	loff_t isize = i_size_read(inode);
> > +
> > +	iomap->bdev = bdev;
> > +	iomap->offset = ALIGN_DOWN(offset, bdev_logical_block_size(bdev));
> > +	if (WARN_ON_ONCE(iomap->offset >= isize))
> > +		return -EIO;
> 
> I'm hitting this during booting:
> [    5.016324]  <TASK>
> [    5.030256]  iomap_iter+0x11a/0x350
> [    5.030264]  iomap_readahead+0x1eb/0x2c0
> [    5.030272]  read_pages+0x5d/0x220
> [    5.030279]  page_cache_ra_unbounded+0x131/0x180
> [    5.030284]  filemap_get_pages+0xff/0x5a0

Why is filemap_get_pages() using unbounded readahead? Surely
readahead should be limited to reading within EOF....

> [    5.030292]  filemap_read+0xca/0x320
> [    5.030296]  ? aa_file_perm+0x126/0x500
> [    5.040216]  ? touch_atime+0xc8/0x150
> [    5.040224]  blkdev_read_iter+0xb0/0x150
> [    5.040228]  vfs_read+0x226/0x2d0
> [    5.040234]  ksys_read+0xa5/0xe0
> [    5.040238]  do_syscall_64+0x5b/0x80
> 
> Maybe we should consider this patch:
> 
> diff --git a/block/fops.c b/block/fops.c
> index 524b8a828aad..d202fb663f25 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -386,10 +386,13 @@ static int blkdev_iomap_begin(struct inode *inode,
> loff_t offset, loff_t length,
> 
>         iomap->bdev = bdev;
>         iomap->offset = ALIGN_DOWN(offset, bdev_logical_block_size(bdev));
> -       if (WARN_ON_ONCE(iomap->offset >= isize))
> -               return -EIO;
> -       iomap->type = IOMAP_MAPPED;
> -       iomap->addr = iomap->offset;
> +       if (WARN_ON_ONCE(iomap->offset >= isize)) {
> +               iomap->type = IOMAP_HOLE;
> +               iomap->addr = IOMAP_NULL_ADDR;
> +       } else {
> +               iomap->type = IOMAP_MAPPED;
> +               iomap->addr = iomap->offset;
> +       }

I think Christoph's code is correct. IMO, any attempt to read beyond
the end of the device should throw out a warning and return an
error, not silently return zeros.

If readahead is trying to read beyond the end of the device, then it
really seems to me like the problem here is readahead, not the iomap
code detecting the OOB read request....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com