Date:   Thu, 4 Aug 2022 13:47:39 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     bugzilla-daemon@...nel.org
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [Bug 216322] Freezing of tasks failed after 60.004 seconds (1
 tasks refusing to freeze... task:fstrim  ext4_trim_fs - Dell XPS 13 9310

On Thu, Aug 04, 2022 at 12:44:45AM +0000, bugzilla-daemon@...nel.org wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=216322
> 
> Theodore Tso (tytso@....edu) changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |tytso@....edu
> 
> --- Comment #2 from Theodore Tso (tytso@....edu) ---
> So the problem is that the FITRIM ioctl does not check if a signal is pending,
> and so if the fstrim program requests that the entire SSD (len=ULLONG_MAX),
> like the broomstick set off by Mickey Mouse in Fantasia's "Sorcerer's
> Apprentive", it will mindlessly send discard requests for any blocks not in use
> by the file system until it is done.   Or to put it another way, "Neither rain,
> nor snow, or a request to freeze the OS, shall stop the FITRIM ioctl from its
> appointed task."  :-)
> 
> The question is how to fix things.   The problem is that the FITRIM ioctl
> interface is pretty horrible.   The fstrim_range.len variable is an IN/OUT
> field where on the input it is the number of bytes that should be trimmed (from
> start to start+len) and when the ioctl returns fstrm_range.len is the number of
> bytes that were actually trimmed.   So this is not really amenable for
> -ERESTARTSYS.
> 
> Worse, the fstrim program in util-linux doesn't handle an EAGAIN error return
> code, so if it gets the EAGAIN after try_to_freeze_tasks send the fake signal
> to the process, fstrim will print to stderr "fstrim: FITRIM ioctl failed" and
> the rest of the file system trim operation will be aborted.
> 
> It might be that the only way we can fix this is to have FITRIM return EAGAIN,
> which will stop the fstrim in its tracks.  This is... not great, but typically
> fstrim is run out of crontab or a systemd timer once a month, so if the user
> tries to suspend right as the fstrim is running, hopefully we'll get lucky next
> month.    We can then try teach fstrim to do the right thing, and so this
> lossage mode would only happen in the combination of a new kernel and an older
> version of util-linux.
> 
> I'm not happy with that solution, but the alternative of creating a new FITRIM2
> ioctl that has a sane interface means that you need an new kernel and a new
> util-linux package, and if you don't, the user will have to deal with a hot
> laptop bag and a drained battery.   And not changing FITRIM's behaviour will
> have the same potential end result, if the user gets unlucky and tries to
> suspend the laptop when there is more than 60 seconds left before FITRIM to
> complete.   :-/
> 
> The other thing I'll note is that every file system has its own FITRIM
> implementation, and I suspect they all have this issue, because the FITRIM
> interface is fundamentally flawed.

I agree that the FITRIM interface is flawed in this way. But
ext4_try_to_trim_range() actually does have fatal_signal_pending() and
will return -ERESTARTSYS if that's true. Or did you have something else in
mind?

Also in that case, I see no reason why we would not be able to adjust
the fstrim_range to make it easier to re-start where we left off if
we're going to return -ERESTARTSYS. I am missing something?

I have not had time to look deeply into the traces, but are you actually
sure that we're not stuck in blkdev_issue_discard() instead?

-Lukas

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