linux-kernel - Re: [PATCH] userfaultfd: don't fail on unrecognized features

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Message-ID: <ZCNrWRKl4nCJX3pg@x1n>
Date:   Tue, 28 Mar 2023 18:34:01 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] userfaultfd: don't fail on unrecognized features

On Tue, Mar 28, 2023 at 02:52:35PM -0700, Axel Rasmussen wrote:
> I don't see being very strict here as useful. Another example might be
> madvise() - for example trying to MADV_PAGEOUT on a kernel that
> doesn't support it. There is no way the kernel can proceed here, since
> it simply doesn't know how to do what you're asking for. In this case
> an error makes sense.

IMHO, PAGEOUT is not a great example.  I wished we can have a way to probe
what madvise() the system supports, and I know many people wanted that too.
I even had a feeling that we'll have it some day.

So now I'm going back to look at this patch assuming I'm reviewing it, I'm
still not convinced the old API needs changing.

Userfaultfd allows probing with features=0 with/without this patch, so I
see this patch as something that doesn't bring a direct functional benefit,
but some kind of api change due to subjective preferences which I cannot
say right or wrong.  Now the patch is already merged.  If we need to change
either this patch or the man page to make them match again, again I'd
prefer we simply revert it to keep everything like before and copy stable.

Thanks,

-- 
Peter Xu