linux-kernel - Re: [PATCH 03/21] fs/bdev: Add atomic write support info to statx

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Date:   Mon, 2 Oct 2023 10:51:36 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     Bart Van Assche <bvanassche@....org>,
        Eric Biggers <ebiggers@...nel.org>
Cc:     axboe@...nel.dk, kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org,
        Prasad Singamsetty <prasad.singamsetty@...cle.com>
Subject: Re: [PATCH 03/21] fs/bdev: Add atomic write support info to statx

On 01/10/2023 14:23, Bart Van Assche wrote:
> On 9/29/23 15:49, Eric Biggers wrote:
>> On Fri, Sep 29, 2023 at 10:27:08AM +0000, John Garry wrote:
>>> diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
>>> index 7cab2c65d3d7..c99d7cac2aa6 100644
>>> --- a/include/uapi/linux/stat.h
>>> +++ b/include/uapi/linux/stat.h
>>> @@ -127,7 +127,10 @@ struct statx {
>>>       __u32    stx_dio_mem_align;    /* Memory buffer alignment for 
>>> direct I/O */
>>>       __u32    stx_dio_offset_align;    /* File offset alignment for 
>>> direct I/O */
>>>       /* 0xa0 */
>>> -    __u64    __spare3[12];    /* Spare space for future expansion */
>>> +    __u32    stx_atomic_write_unit_max;
>>> +    __u32    stx_atomic_write_unit_min;
>>
>> Maybe min first and then max?  That seems a bit more natural, and a 
>> lot of the
>> code you've written handle them in that order.

ok, I think it's fine to reorder

>>
>>> +#define STATX_ATTR_WRITE_ATOMIC        0x00400000 /* File supports 
>>> atomic write operations */
>>
>> How would this differ from stx_atomic_write_unit_min != 0?

Yeah, I suppose that we can just not set this for the case of 
stx_atomic_write_unit_min == 0.

> 
> Is it even possible that stx_atomic_write_unit_min == 0? My understanding
> is that all Linux filesystems rely on the assumption that writing a single
> logical block either succeeds or does not happen, even if a power failure
> occurs between writing and reading a logical block.
> 

Maybe they do rely on this, but is it particularly interesting?

BTW, I would not like to provide assurances that every storage media 
produced writes logical blocks atomically.

Thanks,
John