linux-kernel - Re: [PATCH RFC 03/10] iomap: Support CoW-based atomic writes

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Message-ID: <6ad6f42a-e3e0-4c13-859c-07f5e7bd5ec8@oracle.com>
Date: Thu, 6 Feb 2025 11:21:13 +0000
From: John Garry <john.g.garry@...cle.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: brauner@...nel.org, cem@...nel.org, dchinner@...hat.com, hch@....de,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, ojaswin@...ux.ibm.com,
        ritesh.list@...il.com, martin.petersen@...cle.com
Subject: Re: [PATCH RFC 03/10] iomap: Support CoW-based atomic writes


>>   			if (iomi.pos >= dio->i_size ||
>> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
>> index 75bf54e76f3b..0a0b6798f517 100644
>> --- a/include/linux/iomap.h
>> +++ b/include/linux/iomap.h
>> @@ -56,6 +56,8 @@ struct vm_fault;
>>    *
>>    * IOMAP_F_BOUNDARY indicates that I/O and I/O completions for this iomap must
>>    * never be merged with the mapping before it.
>> + *
>> + * IOMAP_F_ATOMIC_COW indicates that we require atomic CoW end IO handling.
> 
> It more indicates that the filesystem is using copy on write to handle
> an untorn write, and will provide the ioend support necessary to commit
> the remapping atomically, right?

yes, correct

> 
>>    */
>>   #define IOMAP_F_NEW		(1U << 0)
>>   #define IOMAP_F_DIRTY		(1U << 1)
>> @@ -68,6 +70,7 @@ struct vm_fault;
>>   #endif /* CONFIG_BUFFER_HEAD */
>>   #define IOMAP_F_XATTR		(1U << 5)
>>   #define IOMAP_F_BOUNDARY	(1U << 6)
>> +#define IOMAP_F_ATOMIC_COW	(1U << 7)
>>   
>>   /*
>>    * Flags set by the core iomap code during operations:
>> @@ -183,6 +186,7 @@ struct iomap_folio_ops {
>>   #define IOMAP_DAX		0
>>   #endif /* CONFIG_FS_DAX */
>>   #define IOMAP_ATOMIC		(1 << 9)
>> +#define IOMAP_ATOMIC_COW	(1 << 10)
> 
> What does IOMAP_ATOMIC_COW do?  There's no description for it (or for
> IOMAP_ATOMIC). 

I'll add a description for both.

> Can you have IOMAP_ATOMIC and IOMAP_ATOMIC_COW both set?

Yes

> Or are they mutually exclusive?

I am not thinking that it might be neater to have a distinct flag for 
IOMAP_ATOMIC when we want to try an atomic bio - maybe IOMAP_ATOMIC_HW 
or IOMAP_ATOMIC_BIO? And then also IOMAP_DIO_ATOMIC_BIO (in addition to 
IOMAP_DIO_ATOMIC_COW).

> 
> I'm guessing from the code that ATOMIC_COW requires ATOMIC to be set,
> but I wonder why because there's no documentation update in the header
> files or in Documentation/filesystems/iomap/.

Will do.

Thanks,
John