Messages in this thread | | | Date | Sat, 9 Mar 2013 20:16:43 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/3] coredump: introduce dump_interrupted() |
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On 03/08, Andrew Morton wrote: > > On Fri, 8 Mar 2013 18:59:15 +0100 Oleg Nesterov <oleg@redhat.com> wrote: > > > Change dump_write(), dump_seek() and do_coredump() to check > > signal_pending() and abort if it is true. > > hm, why.
Firstly. we need these changes to ensure that the coredump won't delay suspend, and to ensure it reacts to SIGKILL "quickly enough". A core dump can take a lot of time.
> I think we're missing some context here - this is to support freezing, > yes?
No. This is to document that
- currently we do not support freezing
- why we do not support, and what should we do to support (the comments in dump_interrupted/wait_for_dump_helpers)
If do_coredump() "races" with suspend/etc we simply abort, hopefully this is fine in practice. And even if we decide to change this later, I hope this series can be counted as a preparation.
> An example of why this is needed: the dump_interrupted() check which > was added to dump_seek() is just weird. An lseek is instantaneous, ^^^^^^^^^^^^^ Oh, I simply do not know, this can depend on the filesystem?
> And if the file doesn't support lseek (do such files exist? should we > be returning 0 instead of -ENOMEM?),
(can't comment, I do not know)
> we just sit there in a loop > extending the file with write(). This can take *ages*, but this part > of dump_seek() *didn't* get the signal check!
The loop does dump_write() which checks dump_interrupted() at the start.
> > Ideally it should do try_to_freeze() but then we need the unpleasant > > changes in dump_write() and wait_for_dump_helpers(). So far we simply > > accept the fact that the freezer can truncate a core-dump but at least > > you can reliably suspend. > > OK, so there is some connection between this and suspending. Details, > please...
It is not trivial to change dump_write() to restart if f_op->write() fails because of freezing(). We need to handle the short writes, we need to clear TIF_SIGPENDING (and we can't rely on recalc_sigpending() unless we change it to check PF_DUMPCORE), and somehow we need to avoid the races with freeze_task + __thaw_task.
Everything looks possible but imho doesn't worth a trouble, a coredump truncated by freezer is tolerable. I hope. And again, even if we decide to "fix" this problem we can do this on top of these changes.
Oleg.
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