Re: [PATCH v2 5/6] ext4: introduce direct IO write path using iomap infrastructure

From: Matthew Bobrowski <mbobrowski@mbobrowski.org>
To: Ritesh Harjani <riteshh@linux.ibm.com>
Cc: tytso@mit.edu, jack@suse.cz, adilger.kernel@dilger.ca,
	linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	david@fromorbit.com, hch@infradead.org, darrick.wong@oracle.com
Subject: Re: [PATCH v2 5/6] ext4: introduce direct IO write path using iomap infrastructure
Date: Wed, 11 Sep 2019 22:39:05 +1000	[thread overview]
Message-ID: <20190911123905.GA26735@bobrowski> (raw)
In-Reply-To: <20190911080853.43B954C04E@d06av22.portsmouth.uk.ibm.com>

On Wed, Sep 11, 2019 at 01:38:52PM +0530, Ritesh Harjani wrote:
> On 9/9/19 2:56 PM, Ritesh Harjani wrote:
> > On 9/9/19 4:49 AM, Matthew Bobrowski wrote:
> > > @@ -217,6 +218,14 @@ static ssize_t ext4_write_checks(struct kiocb
> > > *iocb, struct iov_iter *from)
> > >       if (ret <= 0)
> > >           return ret;
> > > 
> > > +    ret = file_remove_privs(iocb->ki_filp);
> > > +    if (ret)
> > > +        return 0;
> > > +
> > > +    ret = file_update_time(iocb->ki_filp);
> > > +    if (ret)
> > > +        return 0;
> > > +
> > >       if (unlikely(IS_IMMUTABLE(inode)))
> > >           return -EPERM;
> 
> Maybe we can move this up. If file is IMMUTABLE no point in
> calling for above actions (file_remove_privs/file_updatetime).

Yep, sure could do this. In fact, I think we could put this above
generic_write_checks().

> Also why not use file_modified() API which does the same.

Ah, nice. Indeed we can, thanks for simplifying it.

> > > @@ -234,6 +243,34 @@ static ssize_t ext4_write_checks(struct kiocb
> > > *iocb, struct iov_iter *from)
> > >       return iov_iter_count(from);
> > >   }
> > > 
> > > +static ssize_t ext4_buffered_write_iter(struct kiocb *iocb,
> > > +                    struct iov_iter *from)
> > > +{
> > > +    ssize_t ret;
> > > +    struct inode *inode = file_inode(iocb->ki_filp);
> > > +
> > > +    if (iocb->ki_flags & IOCB_NOWAIT)
> > > +        return -EOPNOTSUPP;
> > > +
> > > +    if (!inode_trylock(inode))
> > > +        inode_lock(inode);
> 
> Is it really needed to check for trylock first?
> we can directly call for inode_lock() here.

You're right, no need to do this dance. We can call inode_lock() directly.

> > > +
> > > +    ret = ext4_write_checks(iocb, from);
> > > +    if (ret <= 0)
> > > +        goto out;
> > > +
> > > +    current->backing_dev_info = inode_to_bdi(inode);
> > > +    ret = generic_perform_write(iocb->ki_filp, from, iocb->ki_pos);
> > > +    current->backing_dev_info = NULL;
> > > +out:
> > > +    inode_unlock(inode);
> > > +    if (likely(ret > 0)) {
> > > +        iocb->ki_pos += ret;
> > > +        ret = generic_write_sync(iocb, ret);
> > > +    }
> > > +    return ret;
> > > +}
> > > +
> > > +    if (!ext4_dio_checks(inode)) {
> > > +        inode_unlock(inode);
> > > +        /*
> > > +         * Fallback to buffered IO if the operation on the
> > > +         * inode is not supported by direct IO.
> > > +         */
> > > +        return ext4_buffered_write_iter(iocb, from);
> > > +    }
> > > +
> > > +    ret = ext4_write_checks(iocb, from);
> This can modify the count in iov_iter *from.

Good point. We'll recalculate the iter 'count' again.

Thank you for the review/suggestions, highly appreciated.

--<M>--