linux-kernel - Re: [PATCH] mm: userfaultfd: add UFFDIO_CONTINUE_MODE

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Date:   Wed, 15 Feb 2023 08:37:43 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Shuah Khan <shuah@...nel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Nadav Amit <namit@...are.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org,
        James Houghton <jthoughton@...gle.com>
Subject: Re: [PATCH] mm: userfaultfd: add UFFDIO_CONTINUE_MODE_WP to install
 WP PTEs

On Tue, Feb 14, 2023 at 07:32:29PM -0500, Peter Xu wrote:
> On Tue, Feb 14, 2023 at 02:37:51PM -0800, Axel Rasmussen wrote:
> > Agreed, it would likely be a nice cleanup. Peter, any objections? I
> > wouldn't mind writing a commit to do this sort of refactor, and rebase
> > my change on top of that.
> 
> No objection here.  Personally I actually prefer keeping the parameters
> around if possible because it's straightforward and no thinking of any
> possible indirect accesses all over the place. But maybe growing as long as
> 8 is still a moot point..  It's just that I don't really know whether it'll
> look that good if we put everything into a struct*.
> 
> Things like src_start/dst_start/.. do not look good to be there: each layer
> could loop over its own range of start/end/... so even if not in the
> function parameter we'll need a variable to hold them anyway.
> 
> But I do see a few low hanging fruits:
> 
>   - I don't see why we need to pass over mmap_changing over all of the
>     __mcopy_atomic() callers.  One chance is we simply pass in the ctx* to
>     replace "dst_mm + mmap_changing".

Now ctx* is completely private to fs/userfaultfd.c and I think it'd be
better to keep it this way.
 
>   - Merge mcopy_atomic_mode and mode, having last 2 bits for the existing
>     three modes, then bit 3 for WP, good enough to set it for the new case.

Agree, having flags instead of an enum and bools sounds better to me.
 
>   - Optionally, we can avoid passing over dst_mm/src_mm all around, when
>     dst_vma/src_vma is there?

+1
 
> How about we start from simple?
> 
> -- 
> Peter Xu
> 

-- 
Sincerely yours,
Mike.