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From: Rasmus Villemoes <>
To: Matthew Wilcox <>,
	Rasmus Villemoes <>
Cc: Cyrill Gorcunov <>,
	Andrew Morton <>,,
Subject: Re: [PATCH] kcmp: add separate Kconfig symbol for kcmp syscall
Date: Tue, 13 Oct 2020 21:45:40 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 10/07/2020 17.57, Matthew Wilcox wrote:
> On Fri, Jul 10, 2020 at 09:56:31AM +0200, Rasmus Villemoes wrote:
>> The ability to check open file descriptions for equality (without
>> resorting to unreliable fstat() and fcntl(F_GETFL) comparisons) can be
>> useful outside of the checkpoint/restore use case - for example,
>> systemd uses kcmp() to deduplicate the per-service file descriptor
>> store.
>> Make it possible to have the kcmp() syscall without the full
> If systemd is using it, is it even worth making it conditional any more?
> Maybe for CONFIG_EXPERT builds, it could be de-selectable.

[hm, I dropped the ball, sorry for the necromancy]

Well, first, I don't want to change any defaults here, if that is to be
done, it should be a separate patch.

Second, yes, systemd uses it for the de-duplication, and for that reason
recommends CONFIG_CHECKPOINT_RESTORE (at least, according to their
README) - but I'm not aware of any daemons that actually make use of
systemd's file descriptor store, so it's not really something essential
to every systemd-based system out there. It would be nice if systemd
could change its recommendation to just CONFIG_KCMP_SYSCALL.

But it's also useful for others, e.g. I have some code that wants to
temporarily replace stdin/stdout/stderr with some other file
descriptors, but needs to preserve the '0 is a dup of 1, or not' state
(i.e., is the same struct file) - that cannot reliably be determined
from fstat()/lseek(SEEK_CUR)/F_GETFL or whatever else one could throw at
an fd.


      reply	other threads:[~2020-10-13 19:45 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-10  7:56 Rasmus Villemoes
2020-07-10  8:30 ` Cyrill Gorcunov
2020-07-10  9:05   ` Rasmus Villemoes
2020-07-10  9:37     ` Cyrill Gorcunov
2020-07-10 15:51 ` Randy Dunlap
2020-07-10 15:57 ` Matthew Wilcox
2020-10-13 19:45   ` Rasmus Villemoes [this message]

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