linux-kernel - Re: [PATCH v2 1/3] userfaultfd: UFFDIO

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Date:   Tue, 3 Oct 2023 10:56:01 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, brauner@...nel.org, shuah@...nel.org,
        aarcange@...hat.com, lokeshgidra@...gle.com, hughd@...gle.com,
        mhocko@...e.com, axelrasmussen@...gle.com, rppt@...nel.org,
        willy@...radead.org, Liam.Howlett@...cle.com, jannh@...gle.com,
        zhangpeng362@...wei.com, bgeffon@...gle.com,
        kaleshsingh@...gle.com, ngeoffray@...gle.com, jdduke@...gle.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation

On Mon, Oct 2, 2023 at 10:30 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 02.10.23 17:23, Peter Xu wrote:
> > On Mon, Oct 02, 2023 at 04:42:50PM +0200, David Hildenbrand wrote:
> >> On 23.09.23 03:31, Suren Baghdasaryan wrote:
> >>> From: Andrea Arcangeli <aarcange@...hat.com>
> >>>
> >>> As far as the rmap code is concerned, UFFDIO_REMAP only alters the
> >>> page->mapping and page->index. It does it while holding the page
> >>> lock. However folio_referenced() is doing rmap walks without taking the
> >>> folio lock first, so folio_lock_anon_vma_read() must be updated to
> >>> re-check that the folio->mapping didn't change after we obtained the
> >>> anon_vma read lock.
> >>
> >> I'm curious: why don't we need this for existing users of
> >> page_move_anon_rmap()? What's special about UFFDIO_REMAP?
> >
> > Totally no expert on anon vma so I'm prone to errors, but IIUC the
> > difference here is root anon vma cannot change in page_move_anon_rmap(),
> > while UFFDIO_REMAP can.
>
> That does sound reasonable, thanks.
>
> Probably we can do better with the patch description (once [1] is used
> to move the folio to the other anon_vma).

I'll develop the next version with your patches in the baseline.
Hopefully by the time of my posting your patches will be in the
mm-unstable.

>
> "mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()
>
> For now, folio_move_anon_rmap() was only used to move a folio to a
> different anon_vma after fork(), whereby the root anon_vma stayed
> unchanged. For that, it was sufficient to hold the page lock when
> calling folio_move_anon_rmap().
>
> However, we want to make use of folio_move_anon_rmap() to move folios
> between VMAs that have a different root anon_vma. As folio_referenced()
> performs an RMAP walk without holding the page lock but only holding the
> anon_vma in read mode, holding the page lock is insufficient.
>
> When moving to an anon_vma with a different root anon_vma, we'll have to
> hold both, the page lock and the anon_vma lock in write mode.
> Consequently, whenever we succeeded in folio_lock_anon_vma_read() to
> read-lock the anon_vma, we have to re-check if the mapping was changed
> in the meantime. If that was the case, we have to retry.
>
> Note that folio_move_anon_rmap() must only be called if the anon page is
> exclusive to a process, and must not be called on KSM folios.
>
> This is a preparation for UFFDIO_REMAP, which will hold the page lock,
> the anon_vma lock in write mode, and the mmap_lock in read mode.
> "

Thanks for taking time to write this up! Looks really clear to me.
I'll reuse it.

>
> In addition, we should document these locking details for
> folio_move_anon_rmap() and probably not mention UFFDIO_REMAP in the
> comment in folio_lock_anon_vma_read(), but instead say
> "folio_move_anon_rmap() might have changed the anon_vma as we might not
> hold the page lock here."

Sounds good. Will add.

>
>
> [1] https://lkml.kernel.org/r/20231002142949.235104-3-david@redhat.com
>
> --
> Cheers,
>
> David / dhildenb
>