Re: [RFC v2 10/14] iomap: fix iomap_dio_zero() for fs bs > system page size

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

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