linux-kernel - Re: [PATCH RFC v2 2/2] mm: Rework swap handling of zap_pte

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Date:   Tue, 16 Nov 2021 13:06:30 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH RFC v2 2/2] mm: Rework swap handling of zap_pte_range

On Mon, Nov 15, 2021 at 01:57:32PM +0000, Matthew Wilcox wrote:
> On Mon, Nov 15, 2021 at 09:49:51PM +0800, Peter Xu wrote:
> > Clean the code up by merging the device private/exclusive swap entry handling
> > with the rest, then we merge the pte clear operation too.
> > 
> > struct* page is defined in multiple places in the function, move it upward.
> 
> Is that actually a good thing?  There was a time when declaring
> variables more locally helped compilers with liveness analysis and
> register allocation.  Compilers are probably smarter now.
> 

I see, I don't know the history of that, but I did give it a shot with a patch
that recovered all the "struct page*" back to the origin, I found that it'll
generated exactly the same assembly of unmap_page_range (actually, the whole
mm/memory.o) no matter what.

I only tested on an aarch64 system, with below gcc version:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/aarch64-redhat-linux/11/lto-wrapper
Target: aarch64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,fortran,objc,obj-c++,ada,go,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --with-isl=/builddir/build/BUILD/gcc-11.0.1-20210324/obj-aarch64-redhat-linux/isl-install --enable-gnu-indirect-function --build=aarch64-redhat-linux
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.0.1 20210324 (Red Hat 11.0.1-0) (GCC)

Thanks,

-- 
Peter Xu