linux-kernel - Re: [RFC v2 10/14] iomap: fix iomap_dio

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Date: Tue, 13 Feb 2024 13:30:40 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	mcgrof@...nel.org, gost.dev@...sung.com, akpm@...ux-foundation.org,
	kbusch@...nel.org, chandan.babu@...cle.com, p.raghav@...sung.com,
	linux-kernel@...r.kernel.org, hare@...e.de, willy@...radead.org,
	linux-mm@...ck.org, david@...morbit.com
Subject: Re: [RFC v2 10/14] iomap: fix iomap_dio_zero() for fs bs > system
 page size

On Tue, Feb 13, 2024 at 10:27:32PM +0100, Pankaj Raghav (Samsung) wrote:
> On Tue, Feb 13, 2024 at 08:30:37AM -0800, Darrick J. Wong wrote:
> > On Tue, Feb 13, 2024 at 10:37:09AM +0100, Pankaj Raghav (Samsung) wrote:
> > > From: Pankaj Raghav <p.raghav@...sung.com>
> > > 
> > > iomap_dio_zero() will pad a fs block with zeroes if the direct IO size
> > > < fs block size. iomap_dio_zero() has an implicit assumption that fs block
> > > size < page_size. This is true for most filesystems at the moment.
> > > 
> > > If the block size > page size, this will send the contents of the page
> > > next to zero page(as len > PAGE_SIZE) to the underlying block device,
> > > causing FS corruption.
> > > 
> > > iomap is a generic infrastructure and it should not make any assumptions
> > > about the fs block size and the page size of the system.
> > > 
> > > Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>
> > > ---
> > >  fs/iomap/direct-io.c | 13 +++++++++++--
> > >  1 file changed, 11 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > > index bcd3f8cf5ea4..04f6c5548136 100644
> > > --- a/fs/iomap/direct-io.c
> > > +++ b/fs/iomap/direct-io.c
> > > @@ -239,14 +239,23 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
> > >  	struct page *page = ZERO_PAGE(0);
> > >  	struct bio *bio;
> > >  
> > > -	bio = iomap_dio_alloc_bio(iter, dio, 1, REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
> > > +	WARN_ON_ONCE(len > (BIO_MAX_VECS * PAGE_SIZE));
> > > +
> > > +	bio = iomap_dio_alloc_bio(iter, dio, BIO_MAX_VECS,
> > > +				  REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
> > >  	fscrypt_set_bio_crypt_ctx(bio, inode, pos >> inode->i_blkbits,
> > >  				  GFP_KERNEL);
> > > +
> > >  	bio->bi_iter.bi_sector = iomap_sector(&iter->iomap, pos);
> > >  	bio->bi_private = dio;
> > >  	bio->bi_end_io = iomap_dio_bio_end_io;
> > >  
> > > -	__bio_add_page(bio, page, len, 0);
> > > +	while (len) {
> > > +		unsigned int io_len = min_t(unsigned int, len, PAGE_SIZE);
> > 
> > What was the result of all that discussion about using the PMD-sized
> > zero-folio the last time this patch was submitted?  Did that prove to be
> > unwieldly, or did it require enough extra surgery to become its own
> > series?
> > 
> 
> It proved a bit unwieldly to me at least as I did not know any straight
> forward way to do it at the time. So I thought I will keep this approach
> as it is, and add support for the PMD-sized zero folio for later
> improvement.
> 
> > (The code here looks good to me.)
> 
> Thanks!

In that case I'll throw it on the testing pile and let's ask brauner to
merge this for 6.9 if nothing blows up.

Reviewed-by: Darrick J. Wong <djwong@...nel.org>

--D

> > 
> > --D
> > 
> > > +
> > > +		__bio_add_page(bio, page, io_len, 0);
> > > +		len -= io_len;
> > > +	}
> > >  	iomap_dio_submit_bio(iter, dio, bio, pos);
> > >  }
> > >  
> > > -- 
> > > 2.43.0
> > > 
> > > 
>