Can fallocate() ops be emulated better using SMB request compounding?

[Paulo Alcantara]

Jeremy Allison <jra at samba.org> writes:

> On Thu, Dec 07, 2023 at 03:58:46PM +0000, David Howells wrote:
>>Hi Steve, Namjae, Jeremy,
>>
>>At the moment certain fallocate() operations aren't very well implemented in
>>the cifs filesystem on Linux, either because the protocol doesn't fully
>>support them or because the ops being used don't also set the EOF marker at
>>the same time and a separate RPC must be made to do that.
>>
>>For instance:
>>
>> - FALLOC_FL_ZERO_RANGE does some zeroing and then sets the EOF as two
>>   distinctly separate operations.  The code prevents you from doing this op
>>   under some circumstances as it doesn't have an oplock and doesn't want to
>>   race with a third party (note that smb3_punch_hole() doesn't have this
>>   check).
>>
>> - FALLOC_FL_COLLAPSE_RANGE uses COPYCHUNK to move the file down and then sets
>>   the EOF as two separate operations as there is no protocol op for this.
>>   However, the copy will likely fail if the ranges overlap and it's
>>   non-atomic with respect to a third party.
>>
>> - FALLOC_FL_INSERT_RANGE has the same issues as FALLOC_FL_COLLAPSE_RANGE.
>>
>>Question: Would it be possible to do all of these better by using compounding
>>with SMB2_FLAGS_RELATED_OPERATIONS?  In particular, if two components of a
>>compound are marked related, does the second get skipped if the first fails?
>
> Yes:
>
> https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-smb2/46dd4182-62d3-4e30-9fe5-e2ec124edca1
>
> "When the current operation requires a FileId and the previous operation
> either contains or generates a FileId, if the previous operation fails
> with an error, the server SHOULD<253> fail the current operation with
> the same error code returned by the previous operation."

David, you could extend smb2_compound_op() like I did for compound
create+{get,set}_reparse+getinfo+close in [1][2][3].

[1] https://lore.kernel.org/r/20231126025510.28147-2-pc@manguebit.com
[2] https://lore.kernel.org/r/20231126025510.28147-3-pc@manguebit.com
[3] https://lore.kernel.org/r/20231126025510.28147-8-pc@manguebit.com