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Subject: Re: [PATCH] kcmp: add separate Kconfig symbol for kcmp syscall
To:     Matthew Wilcox <willy@infradead.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Cyrill Gorcunov <gorcunov@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20200710075632.14661-1-linux@rasmusvillemoes.dk>
 <20200710155719.GN12769@casper.infradead.org>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <20a80bdc-7a5d-c08b-e27b-bea38c378b6c@rasmusvillemoes.dk>
Date:   Tue, 13 Oct 2020 21:45:40 +0200
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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
>> CONFIG_CHECKPOINT_RESTORE.
> 
> 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.

Rasmus