Re: [PATCH v2 06/11] xfs: Reflink CoW-based atomic write support

John Garry
2025-02-25T18:02:34Z

On 25/02/2025 17:37, Darrick J. Wong wrote:
> On Tue, Feb 25, 2025 at 10:58:56AM +0000, John Garry wrote:
>> On 24/02/2025 20:32, Darrick J. Wong wrote:
>>> On Thu, Feb 13, 2025 at 01:56:14PM +0000, John Garry wrote:
>>>> For CoW-based atomic write support, always allocate a cow hole in
>>>> xfs_reflink_allocate_cow() to write the new data.
>>>>
>>>> The semantics is that if @atomic is set, we will be passed a CoW fork
>>>> extent mapping for no error returned.
>>>>
>>>> Signed-off-by: John Garry <john.g.garry@oracle.com>
>>>> ---
>>>> fs/xfs/xfs_iomap.c | 2 +-
>>>> fs/xfs/xfs_reflink.c | 12 +++++++-----
>>>> fs/xfs/xfs_reflink.h | 2 +-
>>>> 3 files changed, 9 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
>>>> index d61460309a78..ab79f0080288 100644
>>>> --- a/fs/xfs/xfs_iomap.c
>>>> +++ b/fs/xfs/xfs_iomap.c
>>>> @@ -865,7 +865,7 @@ xfs_direct_write_iomap_begin(
>>>> /* may drop and re-acquire the ilock */
>>>> error = xfs_reflink_allocate_cow(ip, &imap, &cmap, &shared,
>>>> &lockmode,
>>>> - (flags & IOMAP_DIRECT) || IS_DAX(inode));
>>>> + (flags & IOMAP_DIRECT) || IS_DAX(inode), false);
>>>
>>> Now I'm /really/ think it's time for some reflink allocation flags,
>>> because the function signature now involves two booleans...
>>
>> ok, but the @convert_now arg is passed to other functions from
>> xfs_reflink_allocate_cow() - so would you prefer to create a bool
>> @convert_now inside xfs_reflink_allocate_cow() and pass that bool as before?
>> Or pass the flags all the way down to end users of @convert_now?
>>
>>>
>>>> if (error)
>>>> goto out_unlock;
>>>> if (shared)
>>>> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
>>>> index 8428f7b26ee6..3dab3ba900a3 100644
>>>> --- a/fs/xfs/xfs_reflink.c
>>>> +++ b/fs/xfs/xfs_reflink.c
>>>> @@ -435,7 +435,8 @@ xfs_reflink_fill_cow_hole(
>>>> struct xfs_bmbt_irec *cmap,
>>>> bool *shared,
>>>> uint *lockmode,
>>>> - bool convert_now)
>>>> + bool convert_now,
>>>> + bool atomic)
>>>
>>> ...but this can come later.
>>
>> Do you mean that this would just be a new flag to set?
>
> Sorry, I meant that the double booleans -> flags conversion could be a
> cleanup patch at the end of the series. But first we'd have to figure
> out where we want the flags boundaries to be -- do we just pass the
> IOMAP_{DIRECT,DAX,ATOMIC_*} flags directly to the reflink code and let
> it figure out what to do?

We have the odd case of @convert_now being set from IS_DAX(inode) in
xfs_direct_write_iomap_begin() -> xfs_reflink_allocate_cow(), so that
thwarts the idea of passing the IOMAP flags directly. BTW, it may be
possible to clear up that IS_DAX() usage - I'm not sure, so I'll check
again.

> Or do we make the xfs_iomap.c code translate
> that into XFS_REFLINK_ALLOC_* flags?

That is what I was thinking of doing. But, as mentioned, it needs to be
decided if we pass XFS_REFLINK_ALLOC_* to callees of
xfs_reflink_allocate_cow(). I'm thinking 'no', as it will only create churn.

>
> Either way, that is not something that needs to be done in this patch.

Sure

Thanks,
John