Re: [PATCH] xfs: always free inline data before resetting inode fork during ifree

[Sasha Levin]

On Sun, Mar 25, 2018 at 6:33 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Fri, Mar 23, 2018 at 06:23:02PM +0000, Luis R. Rodriguez wrote:
>> On Fri, Mar 23, 2018 at 10:26:20AM -0700, Darrick J. Wong wrote:
>> > On Fri, Mar 23, 2018 at 05:08:13PM +0000, Luis R. Rodriguez wrote:
>> > > On Thu, Mar 22, 2018 at 08:41:45PM -0700, Darrick J. Wong wrote:
>> > > > On Fri, Mar 23, 2018 at 01:30:37AM +0000, Luis R. Rodriguez wrote:
>> > > > > On Wed, Nov 22, 2017 at 10:01:37PM -0800, Darrick J. Wong wrote:
>> > > > > > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
>> > > > > > index 61d1cb7..8012741 100644
>> > > > > > --- a/fs/xfs/xfs_inode.c
>> > > > > > +++ b/fs/xfs/xfs_inode.c
>> > > > > > @@ -2401,6 +2401,24 @@ xfs_ifree_cluster(
>> > > > > >  }
>> > > > > >
>> > > > > >  /*
>> > > > > > + * Free any local-format buffers sitting around before we reset to
>> > > > > > + * extents format.
>> > > > > > + */
>> > > > > > +static inline void
>> > > > > > +xfs_ifree_local_data(
>> > > > > > +   struct xfs_inode        *ip,
>> > > > > > +   int                     whichfork)
>> > > > > > +{
>> > > > > > +   struct xfs_ifork        *ifp;
>> > > > > > +
>> > > > > > +   if (XFS_IFORK_FORMAT(ip, whichfork) != XFS_DINODE_FMT_LOCAL)
>> > > > > > +           return;
>> > > > >
>> > > > > I'm new to all this so this was a bit hard to follow. I'm confused with how
>> > > > > commit 43518812d2 ("xfs: remove support for inlining data/extents into the
>> > > > > inode fork") exacerbated the leak, isn't that commit about
>> > > > > XFS_DINODE_FMT_EXTENTS?
>> > > >
>> > > > Not specifically _EXTENTS, merely any fork (EXTENTS or LOCAL) whose
>> > > > incore data was small enough to fit in if_inline_ata.
>> > >
>> > > Got it, I thought those were XFS_DINODE_FMT_EXTENTS by definition.
>> > >
>> > > > > Did we have cases where the format was XFS_DINODE_FMT_LOCAL and yet
>> > > > > ifp->if_u1.if_data == ifp->if_u2.if_inline_data ?
>> > > >
>> > > > An empty directory is 6 bytes, which is what you get with a fresh mkdir
>> > > > or after deleting everything in the directory.  Prior to the 43518812d2
>> > > > patch we could get away with not even checking if we had to free if_data
>> > > > when deleting a directory because it fit within if_inline_data.
>> > >
>> > > Ah got it. So your fix *is* also applicable even prior to commit 43518812d2.
>> >
>> > You'd have to modify the patch so that it doesn't try to kmem_free
>> > if_data if if_data == if_inline_data but otherwise (in theory) I think
>> > that the concept applies to pre-4.15 kernels.
>> >
>> > (YMMV, please do run this through QA/kmemleak just in case I'm wrong, etc...)
>>
>> Well... so we need a resolution and better get testing this already given that
>> *I believe* the new auto-selection algorithm used to cherry pick patches onto
>> stable for linux-4.14.y (covered on a paper [0] and when used, stable patches
>> are prefixed with AUTOSEL, a recent discussion covered this in November 2017
>> [1]) recommended to merge your commit 98c4f78dcdd8 ("xfs: always free inline
>> data before resetting inode fork during ifree") as stable commit 1eccdbd4836a41
>> on v4.14.17 *without* merging commit 43518812d2 ("xfs: remove support for
>> inlining data/extents into the inode fork").
>
> Yikes. That sets off all my "how to break filesysetms for fun and
> profit" alarm bells. This is like playing russian roulette with all
> our user's data.  XFS fixes that look like they are simple often
> have subtle dependencies in them that automated backports won't ever
> be able to understand, and if we don't get that right, we break
> stuff.

On the other hand, XFS has a few commits that fix possible
corruptions, that have never ended up in a stable tree. Isn't it just
as bad ("playing roulette") for users?

> Filesystems aren't like drivers or memory management - you can't
> reboot to fix a filesystem corruption or data loss bug. User's tend
> to care a lot more about their data and cat photos than they do
> about how often the dodgy hardware they bought on ebay needs OS
> rebooting to get working again..

Thank you for your input Dave. Let me give you the background for why
I've been doing it this way up to now to explain my reasoning, and
where I was wrong.

After I've built the initial framework for this, I ran it through a
set of kernel versions and ended up with a massively large set of
commits that were detected as bug fixes but didn't end up getting in
stable trees.

My thinking back then was that I can start building branches of
proposed commits, send them to their respective authors/maintainers,
and remove any patches that maintainers objected to their inclusion.
This process ran for a few months, and a few thousand patches (between
all kernel trees) went in this way. The rate of rejection (either when
a maintainer objects to an inclusion, or a regression discovered after
a kernel was released) was on par (and even lower) than the one for
"regular" patches tagged for stable.

What I didn't account for correctly was that people are way more
busier than I have expected, so a lot of these mails were lost or
ignored in mailboxes, so some of these patches never received review
before going in a stable tree.

I'm trying to fix this with a different approach (more below).

>> I do wonder if other XFS folks are *at least* aware that the auto-selection
>> algorithm now currently merging patches onto stable for XFS?
>
> No I wasn't aware that this was happening.  I'm kinda shit scared
> right now hearing about how automated backports of random kernel
> patches are being done with minimal oversight and no visibility to
> the subsystem developers. When did this start happening?

About half a year ago. I'm not sure about the no visibility part -
maintainers and authors would receive at least 3 mails for each patch
that got in this way, and would have at least a week (usually a lot
more) to object to the inclusion. Did you not receive any mails from
me?

> At this point I'd be much more comfortable if XFS was blacklisted
> until there's solid subsystem developer visibility of the iautomated
> backports, not to mention a solid set of automated regression
> testing backing this automated backport proceedure.

I'll be winding down what I'm trying to do now, and will be trying to
address these concerns from maintainers in a few different ways.

I've started working on a framework to automate reviews of sent
patches to lkml by my framework, this will allow me to do the
following:

 - I would send a reply to the original patch sent to LKML within a
few hours for patches that have a high probability for being a bug fix
rather than sending a brand new mail a few months after this patch
made it upstream. This will help reviews as this commit is still fresh
in the author+maintainers head.
 - I will include the results of builds for various build testing (I
got that working now). At this point I suspect this will mostly help
Greg with patches that are already sent with stable tags.
 - This will turn into an opt-in rather than opt-out, but it will be
extremely easy to opt in (something like replying with "ack" to have
that patch included in the proposed stable branches).
 - In the future, I'd also like to create a per-subsystem testing
procedure (so for example, for xfs - run xfstest). I'll try working
with maintainers of each subsystem to create something they're happy
with. Given this discussion, I'll make XFS my first attempt at this :)

The mails will look something like this (an example based on a recent
XFS commit):

> From: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
> To: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
> To: linux-xfs@xxxxxxxxxxxxxxx, "Darrick J . Wong" <darrick.wong@xxxxxxxxxx>
> Cc: Brian Foster <bfoster@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic
> In-Reply-To: <20180306102638.25322-1-vbendel@xxxxxxxxxx>
> References: <20180306102638.25322-1-vbendel@xxxxxxxxxx>
>
> Hi Vratislav Bendel,
>
> [This is an automated email]
>
> This commit has been processed by the -stable helper bot and determined
> to be a high probability candidate for -stable trees. (score: 6.4845)
>
> The bot has tested the following trees: v4.15.12, v4.14.29, v4.9.89, v4.4.123, v4.1.50, v3.18.101.
>
> v4.15.12: OK!
> v4.14.29: OK!
> v4.9.89: OK!
> v4.4.123: OK!
> v4.1.50: OK!
> v3.18.101: OK!
>
> Please reply with "ack" to have this patch included in the appropriate stable trees.
>
> --
> Thanks,
> Sasha

If you look at the recent history for fs/xfs, there were no commits in
the past half a year or so that were submitted to any stable tree in
the "traditional" way. There are no XFS fixes in the 4.14 LTS tree
besides the ones submitted with the autoselection method. This is not
finger pointing at XFS, but rather at the -stable process itself. It's
difficult to keep track on which branches authors need to test their
patches on, what sort of tests they need to do, and how they should
tag their commits. In quite a few cases the effort to properly tag a
commit for stable takes more effort that writing the code for that
commit, which deters people from working with stable.

Thanks again for your input.