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

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Date:   Tue, 14 Feb 2023 19:32:29 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     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>,
        Mike Rapoport <rppt@...nel.org>, 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 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".

  - 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.

  - Optionally, we can avoid passing over dst_mm/src_mm all around, when
    dst_vma/src_vma is there?

How about we start from simple?

-- 
Peter Xu