Re: [PATCH AUTOSEL 4.14 25/35] iomap: sub-block dio needs to zeroout beyond EOF

Amir Goldstein
2018-12-03T07:11:58Z

On Mon, Dec 3, 2018 at 1:23 AM Dave Chinner <david@fromorbit.com> wrote:
>
> On Sat, Dec 01, 2018 at 02:49:09AM -0500, Sasha Levin wrote:
> > On Sat, Dec 01, 2018 at 08:50:05AM +1100, Dave Chinner wrote:
> > >On Fri, Nov 30, 2018 at 05:14:41AM -0500, Sasha Levin wrote:
> > >>On Fri, Nov 30, 2018 at 09:22:03AM +0100, Greg KH wrote:
> > >>>On Fri, Nov 30, 2018 at 09:40:19AM +1100, Dave Chinner wrote:
> > >>>>I stopped my tests at 5 billion ops yesterday (i.e. 20 billion ops
> > >>>>aggregate) to focus on testing the copy_file_range() changes, but
> > >>>>Darrick's tests are still ongoing and have passed 40 billion ops in
> > >>>>aggregate over the past few days.
> > >>>>
> > >>>>The reason we are running these so long is that we've seen fsx data
> > >>>>corruption failures after 12+ hours of runtime and hundreds of
> > >>>>millions of ops. Hence the testing for backported fixes will need to
> > >>>>replicate these test runs across multiple configurations for
> > >>>>multiple days before we have any confidence that we've actually
> > >>>>fixed the data corruptions and not introduced any new ones.
> > >>>>
> > >>>>If you pull only a small subset of the fixes, the fsx will still
> > >>>>fail and we have no real way of actually verifying that there have
> > >>>>been no regression introduced by the backport. IOWs, there's a
> > >>>>/massive/ amount of QA needed for ensuring that these backports work
> > >>>>correctly.
> > >>>>
> > >>>>Right now the XFS developers don't have the time or resources
> > >>>>available to validate stable backports are correct and regression
> > >>>>fre because we are focussed on ensuring the upstream fixes we've
> > >>>>already made (and are still writing) are solid and reliable.
> > >>>
> > >>>Ok, that's fine, so users of XFS should wait until the 4.20 release
> > >>>before relying on it? :)
> > >>
> > >>It's getting to the point that with the amount of known issues with XFS
> > >>on LTS kernels it makes sense to mark it as CONFIG_BROKEN.
> > >
> > >Really? Where are the bug reports?
> >
> > In 'git log'! You report these every time you fix something in upstream
> > xfs but don't backport it to stable trees:
>
> That is so wrong on so many levels I don't really know where to
> begin. I guess doing a *basic risk analysis* demonstrating that none
> of those fixes are backport candidates is a good start:
>
> > $ git log --oneline v4.18-rc1..v4.18 fs/xfs
> > d4a34e165557 xfs: properly handle free inodes in extent hint validators
>
> Found by QA with generic/229 on a non-standard config. Not user
> reported, unlikely to ever be seen by users.
>
> > 9991274fddb9 xfs: Initialize variables in xfs_alloc_get_rec before using them
>
> Cleaning up coverity reported issues to do with corruption log
> messages. No visible symptoms, Not user reported.
>
> > d8cb5e423789 xfs: fix fdblocks accounting w/ RMAPBT per-AG reservation
>
> Minor free space accounting issue, not user reported, doesn't affect
> normal operation.
>
> > e53c4b598372 xfs: ensure post-EOF zeroing happens after zeroing part of a file
>
> Found with fsx via generic/127. Not user reported, doesn't affect
> userspace operation at all.
>
> > a3a374bf1889 xfs: fix off-by-one error in xfs_rtalloc_query_range
>
> Regression fix for code introduced in 4.18-rc1. Not user reported
> because the code has never been released.
>
> > 232d0a24b0fc xfs: fix uninitialized field in rtbitmap fsmap backend
>
> Coverity warning fix, not user reported, not user impact.
>
> > 5bd88d153998 xfs: recheck reflink state after grabbing ILOCK_SHARED for a write
>
> Fixes warning from generic/166, not user reported. Could affect
> users mixing direct IO with reflink, but we expect people using
> new functionality like reflink to be tracking TOT fairly closely
> anyway.
>
> > f62cb48e4319 xfs: don't allow insert-range to shift extents past the maximum offset
>
> Found by QA w/ generic/465. Not user reported, only affects files in
> the exabyte range so not a real world problem....
>
> > aafe12cee0b1 xfs: don't trip over negative free space in xfs_reserve_blocks
>
> Found during ENOSPC stress tests that depeleted the reserve pool.
> Not user reported, unlikely to ever be hit by users.
>
> > 10ee25268e1f xfs: allow empty transactions while frozen
>
> Removes a spurious warning when running GETFSMAP ioctl on a frozen
> filesystem. Not user reported, highly unlikely any user will ever
> hit this as nothing but XFs utilities use GETFSMAP at the moment.
>
> > e53946dbd31a xfs: xfs_iflush_abort() can be called twice on cluster writeback failure
>
> Bug in corrupted filesystem handling, been there for ~15 years IIRC.
> Not user reported - found by one of our shutdown stress tests
> on a debug kernel (generic/388, IIRC). Highly unlikely to show up in
> the real world given how long the bug has been there.
>
> > 23fcb3340d03 xfs: More robust inode extent count validation
>
> Found by filesystem image fuzzing (i.e. intentional filesystem
> corruption). Not user reported, and the filesystem corruption that
> triggered this problem is so artificial there is really no chance of
> it ever occurring in the real world.
>
> > e2ac836307e3 xfs: simplify xfs_bmap_punch_delalloc_range
>
> Cleanup and simplification. Not a bug fix, not user reported, not a
> backport candidate.
>
> IOWs, there isn't a single commit in this list that is user
> reported, nor anything that I'd consider a stable kernel backport
> candidate because none of them affect normal user workloads. i.e.
> they've all be found by tools designed to break filesystems and
> exercise rarely travelled error paths.
>
> > Since I'm assuming that at least some of them are based on actual issues
> > users hit, and some of those apply to stable kernels, why would users
> > want to use an XFS version which is knowingly buggy?
>
> Your assumption is not only incorrect, it is fundamentally flawed.
> A list of commits containing bug fixes is not a list of bug reports
> from users.
>

Up to here, we are in complete agreement.
Thank you for the effort you've put into showing how much effort it takes
to properly review candidate patches.

Further down, although I can tell that your harsh response is due to Sasha's
provoking suggestion to set CONFIG_BROKEN, IMO, your response can be
perceived as sliding a bit into the territory of telling someone else how to do
their job.

It is one thing to advocate that well tested distro kernels are a better
choice for end users. Greg has always advocated the same.
It is another thing to suggest that the kernel.org stable trees have no value
because they are not being maintained with the same standards as the distro
stable kernels.

The time and place for XFS maintainers to make a judgement about stable
trees is whether or not they are willing to look at bug reports reproduced on
kernel.org stable tree kernels.

Whether or not kernel.org stable trees are useful is a risk/benefit
analysis that
each and every downstream user should be doing themselves. And it is the
responsibility of the the stable tree maintainer to make the choices that affect
their downstream users.

In my personal opinion, as a downstream kernel.org stable tree user,
there will be great value in the community maintained stable trees, if and
when filesystem test suites will be run regularly on stable tree candidates.

Whether or not those stable trees include "minor" bug fixes, as the ones
that you listed above, should not be the concern of XFS maintainer, it should
be the concern of downstream users making their own risk/benefit analysis.

I am very much aware of the paradigm that less changes == less risk, which
is the corner stone of maintaining a stable/maint branch.
But at the same time, you seem to be ignoring the fact that people often make
mistakes when cherry-picking over selectively, because some patches in
the series that look like meaningless re-factoring or ones that fix "minor" bugs
may actually be required for a later bug fix and it is not always evident from
reading the commit messages. So there is more to the risk/benefit analysis
then what you present.

There is no replacement for good test coverage. The XFS subsystem excels
in that department, which makes the validation of stable XFS tree candidates
with xfstests very valuable.

There is no replacement for human review of stable tree patch candidates.
*HOWEVER*! the purpose of this review should be to point out backporting
bugs - it should not be to tell the stable tree maintainer which bugs are
stable tree eligible and which bugs are not.

Please join me in an early New Year's resolution: We shall all strive to make
4.19.y LTS kernel more reliable than previous LTS kernels w.r.t filesystems
in general and XFS in particular.

Cheers to that,
Amir.

> IOWs, backporting them only increases the risk of regressions for
> users, it doesn't reduce the risk of users hitting problems or fix
> any problems that users are at risk of actually hitting. IOWs, all
> of these changes fall on the wrong side of the risk-benefit analysis
> equation.
>
> Risk/benefit analysis is fundamental to software engineering
> processes. Running "git log" is not a risk analysis - it's just
> provides a list of things that you need to perform an analysis on.
> Risk analsysis takes time and effort, and to imply that it is not
> necessary and we should just backport everything makes the incorrect
> assumption that backporting carries no risk at all.
>
> It seems to me that the stable kernel process measures itself on how
> many commits an dhow fast they are backported from mainline kernels,
> and the entire focus of improvement is on backporting /more/ commits
> /faster/. i.e. it's all about the speed and quantity of code being
> moved back to the "stable" kernels. What it should be doing is
> identifying and addressing bugs or flaws that put users are risk or
> that users are reporting.
>
> Further, the speed at which backports occur (i.e. within a day or 3
> of upstream commit) means that the code being backported hasn't had
> time to reach a wide testing audience and have regressions shaken
> out of it. The whole purpose of having progressively stricter -rcX
> upstream kernel releases is to allow the new code to stabilise and
> shake out unforseen regressions before it gets to users. The stable
> process is actually releasing upstream code to users before they can
> even get it in a released upstream kernel (i.e. a .0 kernel, not a
> -rcX).
>
> IOWs, pulling code back to stable kernels before it's had a chance
> to stabilise and be more widely tested in the upstream kernel is
> entirely the wrong thing to be doing. Speed here does not improve
> stability, it just increases the risk of regressions and unforseen
> bugs being introduced into the stable tree. And that's made worse by
> the fact that the -rcX process and widespread upstream testing that
> goes along with it* to catch those bugs and regressions. And that's
> made even worse by the fact that subsystems don't have control over
> what is backported anymore, so they may not even be aware that a fix
> for a fix needs to be sent back to stable kernels.
>
> This is the issue here - the "stable kernel" criteria is not about
> stability - it's being optimised to shovel as much change as
> possible with /as little effort as possible/ back into older code
> bases. That's not a recipe for stability, especially considering the
> relative lack of QA the stable kernels get.
>
> IMO, the whole set of linux kernel processes are being optimised
> around the wrong metrics - we count new features, the number of
> commits per release and the quantity of code that gets changed. We
> then optimise our processes to increase these metrics. IOWs, we're
> optimising for speed and rapid change, not quality, reliability and
> stability.
>
> We are not measuring code quality improvements, how effective our
> code review is, we do not do post-mortem analysis of major failures
> and we most certainly don't change processes to avoid those problems
> in future, etc. And worst of all is that people who want better
> processes to improve code quality, testing, etc get shouted at
> because it may slow down the rate at which we change code. i.e. only
> "speed and quantity" seems to matter to the core upstream kernel
> developement community.
>
> As Darrick said, what we are seeing here is a result of "[...] the
> kernel community's systemic inability to QA new fs features
> properly." I'm taking that one step further - what we are seeing
> here is the kernel community's systemic inability to address
> fundamental engineering process deficiencies because "speed and
> quantity" are considered more important than the quality of the
> product being produced.
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com