Re: [RFC v2 0/2] ext4: Add multi-fsblock atomic write support using bigalloc

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, May 08, 2025 at 08:05:27PM +0530, Ritesh Harjani wrote:
> John Garry <john.g.garry@oracle.com> writes:
> 
> > On 30/04/2025 06:20, Ritesh Harjani (IBM) wrote:
> >> This is still an early preview (RFC v2) of multi-fsblock atomic write. Since the
> >> core design of the feature looks ready, wanted to post this for some early
> >> feedback. We will break this into more smaller and meaningful patches in later
> >> revision. However to simplify the review of the core design changes, this
> >> version is limited to just two patches. Individual patches might have more
> >> details in the commit msg.
> >> 
> >> Note: This overall needs more careful review (other than the core design) which
> >> I will be doing in parallel. However it would be helpful if one can provide any
> >> feedback on the core design changes. Specially around ext4_iomap_alloc()
> >> changes, ->end_io() changes and a new get block flag
> >> EXT4_GET_BLOCKS_QUERY_LEAF_BLOCKS.
> >
> > I gave this a try and it looks ok, specifically atomic writing mixed 
> > mappings.
> >
> 
> Thanks John for taking a look.
> 
> > I'll try to look closer that the implementation details.
> 
> We are in the process of sending v3 (hopefully by tonight) which is an
> improved version w.r.t error handling, journal credits and few other
> changes. Although nothing has changed w.r.t the design aspect.
> 
> > But I do note 
> > that you use blkdev_issue_zeroout() to pre-zero any unwritten range 
> > which is being atomically written.
> 
> Yes, that is how internally ext4_map_blocks() with
> EXT4_GET_BLOCKS_CREATE_ZERO will return us the allocated blocks. During
> block allocation, on mixed mapping range, we ensure that the entire range
> becomes a contiguous mapped extent before starting any data writes.
> That means calling ext4_map_blocks() in a loop with
> EXT4_GET_BLOCKS_CREATE_ZERO, so that it can zero out any unwritten
> extents in the requested region.
> I assume writing over a mixed mapping region is not a performance
> critical path. 
> 
> Do you forsee any problems with the approach (since you said "But I do note...")?

It's a little dumb to write zeroes just so you can atomicwrite a block.
However, ext4 lacks an out of place write handler, so I don't think
there's much else that can be done easily.

--D

> -ritesh