linux-kernel - Re: [PATCH v8 4/6] block: loop: prepare for supporing direct IO

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Date:	Thu, 30 Jul 2015 11:21:44 -0400
From:	Ming Lei <ming.lei@...onical.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Zach Brown <zab@...bo.net>,
	Maxim Patlasov <mpatlasov@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Tejun Heo <tj@...nel.org>, Dave Chinner <david@...morbit.com>,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v8 4/6] block: loop: prepare for supporing direct IO

On Thu, Jul 30, 2015 at 11:09 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Thu, Jul 30, 2015 at 07:36:22AM -0400, Ming Lei wrote:
>> This patches provides one interface for enabling direct IO
>> from user space:
>>
>>       - userspace(such as losetup) can pass 'file' which is
>>       opened/fcntl as O_DIRECT
>>
>> Also __loop_update_dio() is introduced to check if direct I/O
>> can be used on current loop setting.
>>
>> The last big change is to introduce LO_FLAGS_DIRECT_IO flag
>> for userspace to know if direct IO is used to access backing
>> file.
>
> Maybe i'm missing something because I was too busy to follow the
> current discussion, but this still doesn't check that the loop
> device sector size is aligned to the sector size of the underlying
> filesystem.
>
> E.g. with this you could create a loop device with a 512 byte
> sector size on a filesystem with 4k sector size, and it would attempt
> to use direct I/O and fail.

The I/O size & offset has to be checked in I/O path, see patch 6 please,
could you comment on that patch?

>
>> +     unsigned dio_align = inode->i_sb->s_bdev ?
>> +             (bdev_io_min(inode->i_sb->s_bdev) - 1) : 0;
>> +
>> +     /*
>> +      * We support direct I/O only if lo_offset is aligned
>> +      * with the min I/O size of backing device.
>> +      *
>> +      * Request's offset and size will be checked in I/O path.
>> +      */
>> +     if (dio) {
>> +             if (!dio_align || (lo->lo_offset & dio_align))
>> +                     use_dio = false;
>
> Also this means you'll never use direct I/O on network filesystems,
> which really would benefit from it.

OK, that can be done by removing !dio_align in above check.

Thanks,
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