linux-kernel - Re: Simplify load_unaligned_zeropad() (was Re: [GIT PULL] Ceph updates for 5.20-rc1)

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Message-ID: <CAHk-=whSGBmH7zKvD-=qJLkWPSGZo1cM7GyLH=8cuide7+ri_Q@mail.gmail.com>
Date:   Sun, 14 Aug 2022 20:43:09 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Mike Rapoport <rppt@...nel.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: Simplify load_unaligned_zeropad() (was Re: [GIT PULL] Ceph
 updates for 5.20-rc1)

On Sun, Aug 14, 2022 at 3:59 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> If TDX has problems with it, then TDX needs to be fixed. And it's
> simple enough - just make sure you have a guard page between any
> kernel RAM mapping and whatever odd crazy page.

.. thinking about this more, I thought we had already done that in the
memory initialization code - ie make sure that we always leave a gap
between any page we mark and any IO memory after it.

But it's possible that I'm confused with the IO window allocation
code, which does the reverse (ie actively try to avoid starting
allocations close to the end-of-RAM because there is often
undocumented stolen memory there)

I'd much rather lose one page from the page allocator at the end of a
RAM region than lose the ability to do string word operations.

Of course, it's also entirely possible that even if my memory about us
already trying to do that is right (which it might not be), we might
also have lost that whole thing over time, since we've had a lot of
updates to the bootmem/memblock setup.

Bringing in Mike Rapoport in case he can point to the code (or lack there-of).

Mike?

               Linus