linux-kernel - Re: [PATCH v7 0/8] add support for direct I/O with fscrypt using blk-crypto

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Date:   Tue, 17 Nov 2020 09:20:56 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Satya Tangirala <satyat@...gle.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...nel.org>, Chao Yu <chao@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        linux-fscrypt@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-block@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v7 0/8] add support for direct I/O with fscrypt using
 blk-crypto

On Tue, Nov 17, 2020 at 12:15:26PM -0500, Theodore Y. Ts'o wrote:
> What is the expected use case for Direct I/O using fscrypt?  This
> isn't a problem which is unique to fscrypt, but one of the really
> unfortunate aspects of the DIO interface is the silent fallback to
> buffered I/O.  We've lived with this because DIO goes back decades,
> and the original use case was to keep enterprise databases happy, and
> the rules around what is necessary for DIO to work was relatively well
> understood.
> 
> But with fscrypt, there's going to be some additional requirements
> (e.g., using inline crypto) required or else DIO silently fall back to
> buffered I/O for encrypted files.  Depending on the intended use case
> of DIO with fscrypt, this caveat might or might not be unfortunately
> surprising for applications.
> 
> I wonder if we should have some kind of interface so we can more
> explicitly allow applications to query exactly what the requirements
> might be for a particular file vis-a-vis Direct I/O.  What are the
> memory alignment requirements, what are the file offset alignment
> requirements, what are the write size requirements, for a particular
> file.

In Ye Olde days there was XFS_IOC_DIOINFO to communicate all that (xfs
hardcodes 512b file offset alignment), but in this modern era perhaps
it's time to shovel that into statx...

--D

> 
> 						- Ted