From: Mike Rapoport <rppt@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Baoquan He <bhe@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
Mike Rapoport <rppt@linux.ibm.com>, Qian Cai <cai@lca.pw>,
Thomas Gleixner <tglx@linutronix.de>,
Vlastimil Babka <vbabka@suse.cz>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org
Subject: Re: [PATCH v3 1/2] x86/setup: don't remove E820_TYPE_RAM for pfn 0
Date: Wed, 13 Jan 2021 17:35:09 +0200 [thread overview]
Message-ID: <20210113153509.GH1106298@kernel.org> (raw)
In-Reply-To: <6ba6bde3-1520-5cd0-f987-32d543f0b79f@redhat.com>
On Wed, Jan 13, 2021 at 01:56:45PM +0100, David Hildenbrand wrote:
> On 11.01.21 20:40, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@linux.ibm.com>
> >
> > The first 4Kb of memory is a BIOS owned area and to avoid its allocation
> > for the kernel it was not listed in e820 tables as memory. As the result,
> > pfn 0 was never recognised by the generic memory management and it is not a
> > part of neither node 0 nor ZONE_DMA.
> >
> > If set_pfnblock_flags_mask() would be ever called for the pageblock
> > corresponding to the first 2Mbytes of memory, having pfn 0 outside of
> > ZONE_DMA would trigger
> >
> > VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
> >
> > Along with reserving the first 4Kb in e820 tables, several first pages are
> > reserved with memblock in several places during setup_arch(). These
> > reservations are enough to ensure the kernel does not touch the BIOS area
> > and it is not necessary to remove E820_TYPE_RAM for pfn 0.
> >
> > Remove the update of e820 table that changes the type of pfn 0 and move the
> > comment describing why it was done to trim_low_memory_range() that reserves
> > the beginning of the memory.
> >
> > Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> > ---
> > arch/x86/kernel/setup.c | 20 +++++++++-----------
> > 1 file changed, 9 insertions(+), 11 deletions(-)
> >
> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > index 740f3bdb3f61..3412c4595efd 100644
> > --- a/arch/x86/kernel/setup.c
> > +++ b/arch/x86/kernel/setup.c
> > @@ -660,17 +660,6 @@ static void __init trim_platform_memory_ranges(void)
> >
> > static void __init trim_bios_range(void)
> > {
> > - /*
> > - * A special case is the first 4Kb of memory;
> > - * This is a BIOS owned area, not kernel ram, but generally
> > - * not listed as such in the E820 table.
> > - *
> > - * This typically reserves additional memory (64KiB by default)
> > - * since some BIOSes are known to corrupt low memory. See the
> > - * Kconfig help text for X86_RESERVE_LOW.
> > - */
> > - e820__range_update(0, PAGE_SIZE, E820_TYPE_RAM, E820_TYPE_RESERVED);
> > -
> > /*
> > * special case: Some BIOSes report the PC BIOS
> > * area (640Kb -> 1Mb) as RAM even though it is not.
> > @@ -728,6 +717,15 @@ early_param("reservelow", parse_reservelow);
> >
> > static void __init trim_low_memory_range(void)
> > {
> > + /*
> > + * A special case is the first 4Kb of memory;
> > + * This is a BIOS owned area, not kernel ram, but generally
> > + * not listed as such in the E820 table.
> > + *
> > + * This typically reserves additional memory (64KiB by default)
> > + * since some BIOSes are known to corrupt low memory. See the
> > + * Kconfig help text for X86_RESERVE_LOW.
> > + */
> > memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
> > }
> >
> >
>
> The only somewhat-confusing thing is that in-between
> e820__memblock_setup() and trim_low_memory_range(), we already have
> memblock allocations. So [0..4095] might look like ordinary memory until
> we reserve it later on.
>
> E.g., reserve_real_mode() does a
>
> mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
> ...
> memblock_reserve(mem, size);
> set_real_mode_mem(mem);
>
> which looks kind of suspicious to me. Most probably I am missing
> something, just wanted to point that out. We might want to do such
> trimming/adjustments before any kind of allocations.
You are right and it looks suspicious, but the first page is reserved at
the very beginning of x86::setup_arch() and, moreover, memblock never
allocates it (look at memblock::memblock_find_in_range_node()).
As for the range 0x1000 <-> reserve_low, we are unlikely to allocate it in
the default top-down mode. The bottom-up mode was only allocating memory
above the kernel so this would also prevent allocation of the lowest
memory, at least until the recent changes for CMA allocation:
https://lore.kernel.org/lkml/20201217201214.3414100-1-guro@fb.com
That said, we'd better consolidate all the trim_some_memory() and move it
closer to the beginning of setup_arch().
I'm going to take a look at it in the next few days.
> --
> Thanks,
>
> David / dhildenb
>
--
Sincerely yours,
Mike.
next prev parent reply other threads:[~2021-01-13 15:35 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-11 19:40 [PATCH v3 0/2] mm: fix initialization of struct page for holes in memory layout Mike Rapoport
2021-01-11 19:40 ` [PATCH v3 1/2] x86/setup: don't remove E820_TYPE_RAM for pfn 0 Mike Rapoport
2021-01-13 8:56 ` Oscar Salvador
2021-01-13 11:23 ` Mike Rapoport
2021-01-13 12:56 ` David Hildenbrand
2021-01-13 15:35 ` Mike Rapoport [this message]
2021-01-21 13:25 ` Borislav Petkov
2021-01-11 19:40 ` [PATCH v3 2/2] mm: fix initialization of struct page for holes in memory layout Mike Rapoport
2021-02-01 9:14 ` David Hildenbrand
2021-02-01 9:39 ` Baoquan He
2021-02-01 14:12 ` Mike Rapoport
2021-01-12 0:58 ` [PATCH v3 0/2] " Andrew Morton
2021-01-12 5:25 ` Mike Rapoport
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