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

Sasha Levin
2018-12-03T09:22:50Z

On Mon, Dec 03, 2018 at 10:23:03AM +1100, Dave Chinner wrote:
>On Sat, Dec 01, 2018 at 02:49:09AM -0500, Sasha Levin wrote:
>> 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.

I think that part of our disagreement is the whole "user reported"
criteria. Looking at myself as an example, unless I experience an
obvious corruption I can reproduce, I am most likely to just ignore it
and recreate the filesystem.

This is even more true for "enterprisy" workloads where data may be
replicated across multiple filesystems, and if one of these fails then
its just silently discarded and replaced.

User reports are hard to come by, not just for XFS but pretty much
anywhere else in the kernel. Our debugging/reporting story is almost as
bad as our QA ;)

A few times above you used the word "unlikely" to indicate that a bug
will never really be hit by real users. I strongly disagree with using
this guess to decide if we're going to backport anything or not. Every
time I meet with the FB folks I keep hearing how they end up hitting
"once in a lifetime" bugs over and over on their infrastructure.

Do we agree that the ideal solution would be backporting every fix, and
having a solid QA system to validate it? Obviously it's not going to
happen in the next year or two, but if we agree on the end goal then
there's no point in this continued arguing about the steps in between :)

>> 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.
>
>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).

One of the concerns I have about stable trees which we both share here
is that no one really uses Linus's tree: it's used as an integration
tree, but very few people actually test their workloads on it. Most
testing ends up hapenning, sadly enough, on stable trees. I see it as an
issue with our process for which I don't have an idea how to solve.

>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.

This is a case where theory collides with the real world. Yes, our QA is
lacking, but we don't have the option of not doing the current process.
If we stop backporting until a future data where our QA problem is
solved we'll end up with what we had before: users stuck on ancient
kernels without a way to upgrade.

With the current model we're aware that bugs sneak through, but we try
to deal with it by both improving our QA, and encouraging users to do
their own extensive QA. If we encourage users to update frequently we
can keep improving our process and the quality of kernels will keep
getting better.

We simply can't go back to the "enterprise distro" days.

--
Thanks,
Sasha