Re: [PATCH 2/2] btrfs: Make btrfs_direct_write atomic with respect to inode_lock

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

On 14:13 15/12, Darrick J. Wong wrote:
> On Tue, Dec 15, 2020 at 12:06:36PM -0600, Goldwyn Rodrigues wrote:
> > From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> > 
> > btrfs_direct_write() fallsback to buffered write in case btrfs is not
> > able to perform or complete a direct I/O. During the fallback
> > inode lock is unlocked and relocked. This does not guarantee the
> > atomicity of the entire write since the lock can be acquired by another
> > write between unlock and relock.
> > 
> > __btrfs_buffered_write() is used to perform the direct fallback write,
> > which performs the write without acquiring the lock or checks.
> 
> Er... can you grab the inode lock before deciding which of the IO
> path(s) you're going to take?  Then you'd always have an atomic write
> even if fallback happens.

No, since this is a fallback option which also works if the I/O is
incomplete.

> 
> (Also vaguely wondering why this needs even more slicing and dicing of
> the iomap directio functions...)

I would most likely go with Dave's method of storing the flag in the
function and calling iomap dio functions without IOCB_DSYNC flag. This
way we don't have to change iomap.

-- 
Goldwyn