linux-kernel - Re: [PATCH v2 1/5] mm/shmem: Unconditionally set pte dirty in mfill_atomic_install

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Message-ID: <2f1bfe82-9bb7-957c-2b32-2ccf8a48e70a@redhat.com>
Date:   Fri, 3 Sep 2021 09:42:34 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: [PATCH v2 1/5] mm/shmem: Unconditionally set pte dirty in
 mfill_atomic_install_pte

On 02.09.21 22:17, Peter Xu wrote:
> It was conditionally done previously, as there's one shmem special case that we
> use SetPageDirty() instead.  However that's not necessary and it should be
> easier and cleaner to do it unconditionally in mfill_atomic_install_pte().
> 
> The most recent discussion about this is here, where Hugh explained the history
> of SetPageDirty() and why it's possible that it's not required at all:
> 
> https://lore.kernel.org/lkml/alpine.LSU.2.11.2104121657050.1097@eggly.anvils/
> 
> Currently mfill_atomic_install_pte() has three callers:
> 
>          1. shmem_mfill_atomic_pte
>          2. mcopy_atomic_pte
>          3. mcontinue_atomic_pte
> 
> After the change: case (1) should have its SetPageDirty replaced by the dirty
> bit on pte (so we unify them together, finally), case (2) should have no
> functional change at all as it has page_in_cache==false, case (3) may add a
> dirty bit to the pte.  However since case (3) is UFFDIO_CONTINUE for shmem,
> it's merely 100% sure the page is dirty after all, so should not make a real
> difference either.

Would it be worth adding VM_BUG_ON() to make sure that "100%" is really 
the case?

> 
> This should make it much easier to follow on which case will set dirty for
> uffd, as we'll simply set it all now for all uffd related ioctls.  Meanwhile,
> no special handling of SetPageDirty() if there's no need.

To me this all sounds sane, but I'm certainly not an expert on that 
code, so ...

-- 
Thanks,

David / dhildenb