linux-kernel - Re: [Patch v9 07/10] block: Add fops atomic write support

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Date: Fri, 21 Jun 2024 13:02:34 +0100
From: John Garry <john.g.garry@...cle.com>
To: Hannes Reinecke <hare@...e.de>, axboe@...nel.dk, kbusch@...nel.org,
        hch@....de, sagi@...mberg.me, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, viro@...iv.linux.org.uk,
        brauner@...nel.org, dchinner@...hat.com, jack@...e.cz
Cc: djwong@...nel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-scsi@...r.kernel.org, ojaswin@...ux.ibm.com, linux-aio@...ck.org,
        linux-btrfs@...r.kernel.org, io-uring@...r.kernel.org,
        nilay@...ux.ibm.com, ritesh.list@...il.com, willy@...radead.org,
        agk@...hat.com, snitzer@...nel.org, mpatocka@...hat.com,
        dm-devel@...ts.linux.dev
Subject: Re: [Patch v9 07/10] block: Add fops atomic write support

On 21/06/2024 07:13, Hannes Reinecke wrote:
> On 6/20/24 14:53, John Garry wrote:
>> Support atomic writes by submitting a single BIO with the REQ_ATOMIC set.
>>
>> It must be ensured that the atomic write adheres to its rules, like
>> naturally aligned offset, so call blkdev_dio_invalid() ->
>> blkdev_atomic_write_valid() [with renaming blkdev_dio_unaligned() to
>> blkdev_dio_invalid()] for this purpose. The BIO submission path currently
>> checks for atomic writes which are too large, so no need to check here.
>>
>> In blkdev_direct_IO(), if the nr_pages exceeds BIO_MAX_VECS, then we 
>> cannot
>> produce a single BIO, so error in this case.
>>
>> Finally set FMODE_CAN_ATOMIC_WRITE when the bdev can support atomic 
>> writes
>> and the associated file flag is for O_DIRECT.
>>
>> Reviewed-by: Martin K. Petersen <martin.petersen@...cle.com>
>> Signed-off-by: John Garry <john.g.garry@...cle.com>
>> ---
>>   block/fops.c | 20 +++++++++++++++++---
>>   1 file changed, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/fops.c b/block/fops.c
>> index 376265935714..be36c9fbd500 100644
>> --- a/block/fops.c
>> +++ b/block/fops.c
>> @@ -34,9 +34,12 @@ static blk_opf_t dio_bio_write_op(struct kiocb *iocb)
>>       return opf;
>>   }
>> -static bool blkdev_dio_unaligned(struct block_device *bdev, loff_t pos,
>> -                  struct iov_iter *iter)
>> +static bool blkdev_dio_invalid(struct block_device *bdev, loff_t pos,
>> +                struct iov_iter *iter, bool is_atomic)
>>   {
>> +    if (is_atomic && !generic_atomic_write_valid(iter, pos))
>> +        return true;
>> +
>>       return pos & (bdev_logical_block_size(bdev) - 1) ||
>>           !bdev_iter_is_aligned(bdev, iter);
>>   }
>> @@ -72,6 +75,8 @@ static ssize_t __blkdev_direct_IO_simple(struct 
>> kiocb *iocb,
>>       bio.bi_iter.bi_sector = pos >> SECTOR_SHIFT;
>>       bio.bi_write_hint = file_inode(iocb->ki_filp)->i_write_hint;
>>       bio.bi_ioprio = iocb->ki_ioprio;
>> +    if (iocb->ki_flags & IOCB_ATOMIC)
>> +        bio.bi_opf |= REQ_ATOMIC;
>>       ret = bio_iov_iter_get_pages(&bio, iter);
>>       if (unlikely(ret))
>> @@ -343,6 +348,9 @@ static ssize_t __blkdev_direct_IO_async(struct 
>> kiocb *iocb,
>>           task_io_account_write(bio->bi_iter.bi_size);
>>       }
>> +    if (iocb->ki_flags & IOCB_ATOMIC)
>> +        bio->bi_opf |= REQ_ATOMIC;
>> +
>>       if (iocb->ki_flags & IOCB_NOWAIT)
>>           bio->bi_opf |= REQ_NOWAIT;
>> @@ -359,12 +367,13 @@ static ssize_t __blkdev_direct_IO_async(struct 
>> kiocb *iocb,
>>   static ssize_t blkdev_direct_IO(struct kiocb *iocb, struct iov_iter 
>> *iter)
>>   {
>>       struct block_device *bdev = I_BDEV(iocb->ki_filp->f_mapping->host);
>> +    bool is_atomic = iocb->ki_flags & IOCB_ATOMIC;
>>       unsigned int nr_pages;
>>       if (!iov_iter_count(iter))
>>           return 0;
>> -    if (blkdev_dio_unaligned(bdev, iocb->ki_pos, iter))
>> +    if (blkdev_dio_invalid(bdev, iocb->ki_pos, iter, is_atomic))
> 
> Why not passing in iocb->ki_flags here?
> Or, indeed, the entire iocb?

We could (pass the iocb), but we only need to look up one thing - 
ki_pos. We already have is_atomic local. I am just trying to make things 
as efficient as possible. If you really think it's better (to pass 
iocb), then it can be changed.

Thanks,
John