linux-kernel - Re: [PATCH 01/13] mm: Update ptep_get

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Message-ID: <CAHk-=wj2MFUfh0juVEeBkZ6hBjp=X_UC3jR5edmZ08mV5bztyg@mail.gmail.com>
Date:   Sat, 29 Oct 2022 13:30:26 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>, X86 ML <x86@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        jroedel@...e.de, ubizjak@...il.com,
        Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

On Sat, Oct 29, 2022 at 1:15 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I can think of three options:
>
>  (a) filesystems just deal with it
>
>  (b) we could move the "page_remove_rmap()" into the "flush-and-free" path too
>
>  (c) we could actually add a spinlock (hashed on the page?) for this
>
> I think (a) is basically our current expectation.

Side note: anybody doing gup + set_page_dirty() won't be fixed by b/c
anyway, so I think (a) is basically the only thing.

And that's true even if you do a page pinning gup, since the source of
the gup may be actively unmapped after the gup.

So a filesystem that thinks that only write, or a rmap-accessible mmap
can turn the page dirty really seems to be fundamentally broken.

And I think that has always been the case, it's just that filesystem
writers may not have been happy with it, and may not have had
test-cases for it.

It's not surprising that the filesystem people then try to blame users.

          Linus