by Christoph Hellwig

[permalink] [raw]

On Jul 25, 2022, at 12:12 PM, Eric Biggers <[email protected]> wrote:
>
> On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote:
>> On 07/22, Eric Biggers wrote:
>>> From: Eric Biggers <[email protected]>
>>>
>>> Currently, if an f2fs filesystem is mounted with the mode=lfs and
>>> io_bits mount options, DIO reads are allowed but DIO writes are not.
>>> Allowing DIO reads but not DIO writes is an unusual restriction, which
>>> is likely to be surprising to applications, namely any application that
>>> both reads and writes from a file (using O_DIRECT). This behavior is
>>> also incompatible with the proposed STATX_DIOALIGN extension to statx.
>>> Given this, let's drop the support for DIO reads in this configuration.
>>
>> IIRC, we allowed DIO reads since applications complained a lower performance.
>> So, I'm afraid this change will make another confusion to users. Could
>> you please apply the new bahavior only for STATX_DIOALIGN?
>>
>
> Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this
> weird case where DIO reads are allowed but not DIO writes. So the question is
> whether this case actually matters, in which case we should make STATX_DIOALIGN
> distinguish between DIO reads and DIO writes, or whether it's some odd edge case
> that doesn't really matter, in which case we could just fix it or make
> STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some
> insight here. What sort of applications want DIO reads but not DIO writes?
> Is this common at all?

I don't think this is f2fs related, but some backup applications I'm aware
of are using DIO reads to avoid polluting the page cache when reading large
numbers of files. They don't care about DIO writes, since that is usually
slower than async writes due to the sync before returning from the syscall.

Also, IMHO it doesn't make sense to remove useful functionality because the
new STATX_DIOALIGN fields don't handle this. At worst the application will
still get an error when trying a DIO write, but in most cases they will
not use the brand new STATX call in the first place, and if this is documented
then any application that starts to use it should be able to handle it.

Cheers, Andreas

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2022-07-31 03:11:31

by Jaegeuk Kim

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Subject: Re: [PATCH v4 6/9] f2fs: don't allow DIO reads but not DIO writes

On 07/25, Eric Biggers wrote:
> On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote:
> > On 07/22, Eric Biggers wrote:
> > > From: Eric Biggers <[email protected]>
> > >
> > > Currently, if an f2fs filesystem is mounted with the mode=lfs and
> > > io_bits mount options, DIO reads are allowed but DIO writes are not.
> > > Allowing DIO reads but not DIO writes is an unusual restriction, which
> > > is likely to be surprising to applications, namely any application that
> > > both reads and writes from a file (using O_DIRECT). This behavior is
> > > also incompatible with the proposed STATX_DIOALIGN extension to statx.
> > > Given this, let's drop the support for DIO reads in this configuration.
> >
> > IIRC, we allowed DIO reads since applications complained a lower performance.
> > So, I'm afraid this change will make another confusion to users. Could
> > you please apply the new bahavior only for STATX_DIOALIGN?
> >
>
> Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this
> weird case where DIO reads are allowed but not DIO writes. So the question is
> whether this case actually matters, in which case we should make STATX_DIOALIGN
> distinguish between DIO reads and DIO writes, or whether it's some odd edge case
> that doesn't really matter, in which case we could just fix it or make
> STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some
> insight here. What sort of applications want DIO reads but not DIO writes?
> Is this common at all?

I think there's no specific application to use the LFS mode at this
moment, but I'd like to allow DIO read for zoned device which will be
used for Android devices.

>
> - Eric

2022-08-16 04:23:51

by Eric Biggers

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Subject: Re: [PATCH v4 6/9] f2fs: don't allow DIO reads but not DIO writes

On Sat, Jul 30, 2022 at 08:08:26PM -0700, Jaegeuk Kim wrote:
> On 07/25, Eric Biggers wrote:
> > On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote:
> > > On 07/22, Eric Biggers wrote:
> > > > From: Eric Biggers <[email protected]>
> > > >
> > > > Currently, if an f2fs filesystem is mounted with the mode=lfs and
> > > > io_bits mount options, DIO reads are allowed but DIO writes are not.
> > > > Allowing DIO reads but not DIO writes is an unusual restriction, which
> > > > is likely to be surprising to applications, namely any application that
> > > > both reads and writes from a file (using O_DIRECT). This behavior is
> > > > also incompatible with the proposed STATX_DIOALIGN extension to statx.
> > > > Given this, let's drop the support for DIO reads in this configuration.
> > >
> > > IIRC, we allowed DIO reads since applications complained a lower performance.
> > > So, I'm afraid this change will make another confusion to users. Could
> > > you please apply the new bahavior only for STATX_DIOALIGN?
> > >
> >
> > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this
> > weird case where DIO reads are allowed but not DIO writes. So the question is
> > whether this case actually matters, in which case we should make STATX_DIOALIGN
> > distinguish between DIO reads and DIO writes, or whether it's some odd edge case
> > that doesn't really matter, in which case we could just fix it or make
> > STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some
> > insight here. What sort of applications want DIO reads but not DIO writes?
> > Is this common at all?
>
> I think there's no specific application to use the LFS mode at this
> moment, but I'd like to allow DIO read for zoned device which will be
> used for Android devices.
>

So if the zoned device feature becomes widely adopted, then STATX_DIOALIGN will
be useless on all Android devices? That sounds undesirable. Are you sure that
supporting DIO reads but not DIO writes actually works? Does it not cause
problems for existing applications?

What we need to do is make a decision about whether this means we should build
in a stx_dio_direction field (indicating no support / readonly support /
writeonly support / readwrite support) into the API from the beginning. If we
don't do that, then I don't think we could simply add such a field later, as the
statx_dio_*_align fields will have already been assigned their meaning. I think
we'd instead have to "duplicate" the API, with STATX_DIOROALIGN and
statx_dio_ro_*_align fields. That seems uglier than building a directional
indicator into the API from the beginning. On the other hand, requiring all
programs to check stx_dio_direction would add complexity to using the API.

Any thoughts on this?

- Eric

2022-08-16 10:31:26

by Dave Chinner

[permalink] [raw]

Subject: Re: [PATCH v4 6/9] f2fs: don't allow DIO reads but not DIO writes

On Mon, Aug 15, 2022 at 05:55:45PM -0700, Eric Biggers wrote:
> On Sat, Jul 30, 2022 at 08:08:26PM -0700, Jaegeuk Kim wrote:
> > On 07/25, Eric Biggers wrote:
> > > On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote:
> > > > On 07/22, Eric Biggers wrote:
> > > > > From: Eric Biggers <[email protected]>
> > > > >
> > > > > Currently, if an f2fs filesystem is mounted with the mode=lfs and
> > > > > io_bits mount options, DIO reads are allowed but DIO writes are not.
> > > > > Allowing DIO reads but not DIO writes is an unusual restriction, which
> > > > > is likely to be surprising to applications, namely any application that
> > > > > both reads and writes from a file (using O_DIRECT). This behavior is
> > > > > also incompatible with the proposed STATX_DIOALIGN extension to statx.
> > > > > Given this, let's drop the support for DIO reads in this configuration.
> > > >
> > > > IIRC, we allowed DIO reads since applications complained a lower performance.
> > > > So, I'm afraid this change will make another confusion to users. Could
> > > > you please apply the new bahavior only for STATX_DIOALIGN?
> > > >
> > >
> > > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this
> > > weird case where DIO reads are allowed but not DIO writes. So the question is
> > > whether this case actually matters, in which case we should make STATX_DIOALIGN
> > > distinguish between DIO reads and DIO writes, or whether it's some odd edge case
> > > that doesn't really matter, in which case we could just fix it or make
> > > STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some
> > > insight here. What sort of applications want DIO reads but not DIO writes?
> > > Is this common at all?
> >
> > I think there's no specific application to use the LFS mode at this
> > moment, but I'd like to allow DIO read for zoned device which will be
> > used for Android devices.
> >
>
> So if the zoned device feature becomes widely adopted, then STATX_DIOALIGN will
> be useless on all Android devices? That sounds undesirable. Are you sure that
> supporting DIO reads but not DIO writes actually works? Does it not cause
> problems for existing applications?

What purpose does DIO in only one direction actually serve? All it
means is that we're forcibly mixing buffered and direct IO to the
same file and that simply never ends well from a data coherency POV.

Hence I'd suggest that mixing DIO reads and buffered writes like
this ends up exposing uses to the worst of both worlds - all of the
problems with none of the benefits...

> What we need to do is make a decision about whether this means we should build
> in a stx_dio_direction field (indicating no support / readonly support /
> writeonly support / readwrite support) into the API from the beginning. If we
> don't do that, then I don't think we could simply add such a field later, as the
> statx_dio_*_align fields will have already been assigned their meaning. I think
> we'd instead have to "duplicate" the API, with STATX_DIOROALIGN and
> statx_dio_ro_*_align fields. That seems uglier than building a directional
> indicator into the API from the beginning. On the other hand, requiring all
> programs to check stx_dio_direction would add complexity to using the API.
>
> Any thoughts on this?

Decide whether partial, single direction DIO serves a useful purpose
before trying to work out what is needed in the API to indicate that
this sort of crazy will be supported....

Cheers,

Dave.
--
Dave Chinner
[email protected]

2022-08-16 16:58:21

On Aug 19, 2022, at 5:09 PM, Eric Biggers <[email protected]> wrote:
>
> On Tue, Aug 16, 2022 at 10:42:29AM -0600, Andreas Dilger wrote:
>>
>> IMHO, this whole discussion is putting the cart before the horse.
>> Changing existing (and useful) IO behavior to accommodate an API that
>> nobody has ever used, and is unlikely to even be widely used, doesn't
>> make sense to me. Most applications won't check or care about the new
>> DIO size fields, since they've lived this long without statx() returning
>> this info, and will just pick a "large enough" size (4KB, 1MB, whatever)
>> that gives them the performance they need. They *WILL* care if the app
>> is suddenly unable to read data from a file in ways that have worked for
>> a long time.
>>
>> Even if apps are modified to check these new DIO size fields, and then
>> try to DIO write to a file in f2fs that doesn't allow it, then f2fs will
>> return an error, which is what it would have done without the statx()
>> changes, so no harm done AFAICS.
>>
>> Even with a more-complex DIO status return that handles a "direction"
>> field (which IMHO is needlessly complex), there is always the potential
>> for a TOCTOU race where a file changes between checking and access, so
>> the userspace code would need to handle this.
>
> I'm having trouble making sense of your argument here; you seem to be saying
> that STATX_DIOALIGN isn't useful, so it doesn't matter if we design it
> correctly? That line of reasoning is concerning, as it's certainly intended
> to be useful, and if it's not useful there's no point in adding it.
>
> Are there any specific concerns that you have, besides TOCTOU races and the
> lack of support for read-only DIO?

My main concern is disabling useful functionality that exists today to appease
the new DIO size API. Whether STATX_DIOALIGN will become widely used by
applications or not is hard to say at this point.

If there were separate STATX_DIOREAD and STATX_DIOWRITE flags in the returned
data, and the alignment is provided as it is today, that would be enough IMHO
to address the original use case without significant complexity.

> I don't think that TOCTOU races are a real concern here. Generally DIO
> constraints would only change if the application doing DIO intentionally does
> something to the file, or if there are changes that involve the filesystem
> being taken offline, e.g. the filesystem being mounted with significantly
> different options or being moved to a different block device. And, well,
> everything else in stat()/statx() is subject to TOCTOU as well, but is still
> used...

I was thinking of background filesystem operations like compression, LVM
migration to new storage with a different sector size, etc. that may change
the DIO characteristics of the file even while it is open. Not that I think
this will happen frequently, but it is possible, and applications shouldn't
explode if the DIO parameters change and they get an error.

Cheers, Andreas

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2022-08-26 17:32:55

by Jeff Layton

[permalink] [raw]

Subject: Re: [PATCH v4 0/9] make statx() return DIO alignment information

On Fri, 2022-07-22 at 00:12 -0700, Eric Biggers wrote:
> This patchset makes the statx() system call return direct I/O (DIO)
> alignment information. This allows userspace to easily determine
> whether a file supports DIO, and if so with what alignment restrictions.
>
> Patch 1 adds the basic VFS support for STATX_DIOALIGN. Patch 2 wires it
> up for all block device files. The remaining patches wire it up for
> regular files on ext4, f2fs, and xfs. Support for regular files on
> other filesystems can be added later.
>
> I've also written a man-pages patch, which I'm sending separately.
>
> Note, f2fs has one corner case where DIO reads are allowed but not DIO
> writes. The proposed statx fields can't represent this. My proposal
> (patch 6) is to just eliminate this case, as it seems much too weird.
> But I'd appreciate any feedback on that part.
>
> This patchset applies to v5.19-rc7.
>
> Changed v3 => v4:
> - Added xfs support.
>
> - Moved the helper function for block devices into block/bdev.c.
>
> - Adjusted the ext4 patch to not introduce a bug where misaligned DIO
> starts being allowed on encrypted files when it gets combined with
> the patch "iomap: add support for dma aligned direct-io" that is
> queued in the block tree for 5.20.
>
> - Made a simplification in fscrypt_dio_supported().
>
> Changed v2 => v3:
> - Dropped the stx_offset_align_optimal field, since its purpose
> wasn't clearly distinguished from the existing stx_blksize.
>
> - Renamed STATX_IOALIGN to STATX_DIOALIGN, to reflect the new focus
> on DIO only.
>
> - Similarly, renamed stx_{mem,offset}_align_dio to
> stx_dio_{mem,offset}_align, to reflect the new focus on DIO only.
>
> - Wired up STATX_DIOALIGN on block device files.
>
> Changed v1 => v2:
> - No changes.
>
> Eric Biggers (9):
> statx: add direct I/O alignment information
> vfs: support STATX_DIOALIGN on block devices
> fscrypt: change fscrypt_dio_supported() to prepare for STATX_DIOALIGN
> ext4: support STATX_DIOALIGN
> f2fs: move f2fs_force_buffered_io() into file.c
> f2fs: don't allow DIO reads but not DIO writes
> f2fs: simplify f2fs_force_buffered_io()
> f2fs: support STATX_DIOALIGN
> xfs: support STATX_DIOALIGN
>
> block/bdev.c | 25 ++++++++++++++++++++
> fs/crypto/inline_crypt.c | 49 +++++++++++++++++++--------------------
> fs/ext4/ext4.h | 1 +
> fs/ext4/file.c | 37 ++++++++++++++++++++---------
> fs/ext4/inode.c | 36 ++++++++++++++++++++++++++++
> fs/f2fs/f2fs.h | 45 -----------------------------------
> fs/f2fs/file.c | 45 ++++++++++++++++++++++++++++++++++-
> fs/stat.c | 14 +++++++++++
> fs/xfs/xfs_iops.c | 9 +++++++
> include/linux/blkdev.h | 4 ++++
> include/linux/fscrypt.h | 7 ++----
> include/linux/stat.h | 2 ++
> include/uapi/linux/stat.h | 4 +++-
> 13 files changed, 190 insertions(+), 88 deletions(-)
>
> base-commit: ff6992735ade75aae3e35d16b17da1008d753d28

Hi Eric,

Can I ask what your plans are with this set? I didn't see it in
linux-next yet, so I wasn't sure when you were looking to get it merged.
I'm working on patches to add a new statx field for the i_version
counter as well and I want to make sure that our work doesn't collide.

Thanks,
--
Jeff Layton <[email protected]>

2022-08-27 07:13:02

by Eric Biggers

[permalink] [raw]

Subject: Re: [PATCH v4 0/9] make statx() return DIO alignment information

On Fri, Aug 26, 2022 at 01:19:37PM -0400, Jeff Layton wrote:
> On Fri, 2022-07-22 at 00:12 -0700, Eric Biggers wrote:
> > This patchset makes the statx() system call return direct I/O (DIO)
> > alignment information. This allows userspace to easily determine
> > whether a file supports DIO, and if so with what alignment restrictions.
> >
> > Patch 1 adds the basic VFS support for STATX_DIOALIGN. Patch 2 wires it
> > up for all block device files. The remaining patches wire it up for
> > regular files on ext4, f2fs, and xfs. Support for regular files on
> > other filesystems can be added later.
> >
> > I've also written a man-pages patch, which I'm sending separately.
> >
> > Note, f2fs has one corner case where DIO reads are allowed but not DIO
> > writes. The proposed statx fields can't represent this. My proposal
> > (patch 6) is to just eliminate this case, as it seems much too weird.
> > But I'd appreciate any feedback on that part.
> >
> > This patchset applies to v5.19-rc7.
> >
> > Changed v3 => v4:
> > - Added xfs support.
> >
> > - Moved the helper function for block devices into block/bdev.c.
> >
> > - Adjusted the ext4 patch to not introduce a bug where misaligned DIO
> > starts being allowed on encrypted files when it gets combined with
> > the patch "iomap: add support for dma aligned direct-io" that is
> > queued in the block tree for 5.20.
> >
> > - Made a simplification in fscrypt_dio_supported().
> >
> > Changed v2 => v3:
> > - Dropped the stx_offset_align_optimal field, since its purpose
> > wasn't clearly distinguished from the existing stx_blksize.
> >
> > - Renamed STATX_IOALIGN to STATX_DIOALIGN, to reflect the new focus
> > on DIO only.
> >
> > - Similarly, renamed stx_{mem,offset}_align_dio to
> > stx_dio_{mem,offset}_align, to reflect the new focus on DIO only.
> >
> > - Wired up STATX_DIOALIGN on block device files.
> >
> > Changed v1 => v2:
> > - No changes.
> >
> > Eric Biggers (9):
> > statx: add direct I/O alignment information
> > vfs: support STATX_DIOALIGN on block devices
> > fscrypt: change fscrypt_dio_supported() to prepare for STATX_DIOALIGN
> > ext4: support STATX_DIOALIGN
> > f2fs: move f2fs_force_buffered_io() into file.c
> > f2fs: don't allow DIO reads but not DIO writes
> > f2fs: simplify f2fs_force_buffered_io()
> > f2fs: support STATX_DIOALIGN
> > xfs: support STATX_DIOALIGN
> >
> > block/bdev.c | 25 ++++++++++++++++++++
> > fs/crypto/inline_crypt.c | 49 +++++++++++++++++++--------------------
> > fs/ext4/ext4.h | 1 +
> > fs/ext4/file.c | 37 ++++++++++++++++++++---------
> > fs/ext4/inode.c | 36 ++++++++++++++++++++++++++++
> > fs/f2fs/f2fs.h | 45 -----------------------------------
> > fs/f2fs/file.c | 45 ++++++++++++++++++++++++++++++++++-
> > fs/stat.c | 14 +++++++++++
> > fs/xfs/xfs_iops.c | 9 +++++++
> > include/linux/blkdev.h | 4 ++++
> > include/linux/fscrypt.h | 7 ++----
> > include/linux/stat.h | 2 ++
> > include/uapi/linux/stat.h | 4 +++-
> > 13 files changed, 190 insertions(+), 88 deletions(-)
> >
> > base-commit: ff6992735ade75aae3e35d16b17da1008d753d28
>
> Hi Eric,
>
> Can I ask what your plans are with this set? I didn't see it in
> linux-next yet, so I wasn't sure when you were looking to get it merged.
> I'm working on patches to add a new statx field for the i_version
> counter as well and I want to make sure that our work doesn't collide.
>

I've just sent v5. I guess I'll try to get it merged for 6.1. We were a bit
stuck on the read-only DIO issue. All things considered though, including that
Christoph thinks it's possible for f2fs to support DIO writes on zoned block
devices, I'm willing to bet that read-only DIO doesn't really matter enough for
it to be worth it to add a direction field to STATX_DIOALIGN (which would make
it harder to use STATX_DIOALIGN, as the field would always have to be checked).

- Eric