From: Oscar Salvador <osalvador@suse.de>
To: David Hildenbrand <david@redhat.com>
Cc: akpm@linux-foundation.org, mhocko@kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org, vbabka@suse.cz,
pasha.tatashin@soleen.com
Subject: Re: [PATCH 2/5] mm,memory_hotplug: Allocate memmap from the added memory range
Date: Mon, 25 Jan 2021 11:56:02 +0100 [thread overview]
Message-ID: <20210125105557.GA28363@linux> (raw)
In-Reply-To: <20210125103951.GA27851@linux>
On Mon, Jan 25, 2021 at 11:39:55AM +0100, Oscar Salvador wrote:
> > Interresting, so we automatically support differeing sizeof(struct
> > page). I guess it will be problematic in case of sizeof(struct page) !=
> > 64, because then, we might not have multiples of 2MB for the memmap of a
> > memory block.
> >
> > IIRC, it could happen that if we add two consecutive memory blocks, that
> > the second one might reuse parts of the vmemmap residing on the first
> > memory block. If you remove the first one, you might be in trouble.
> >
> > E.g., on x86-64
> > vmemmap_populate()->vmemmap_populate_hugepages()->vmemmap_alloc_block_buf():
> > - Populate a huge page
> >
> > vmemmap_free()->remove_pagetable()...->remove_pmd_table():
> > - memchr_inv() will leave the hugepage populated.
> >
> > Do we want to fence that off, maybe in mhp_supports_memmap_on_memory()?
> > Or do we somehow want to fix that? We should never populate partial huge
> > pages from an altmap ...
> >
> > But maybe I am missing something.
>
> No, you are not missing anything.
>
> I think that remove_pmd_table() should be fixed.
> vmemmap_populate_hugepages() allocates PMD_SIZE chunk and marks the PMD as
> PAGE_KERNEL_LARGE, so when remove_pmd_table() sees that 1) the PMD
> is large and 2) the chunk is not aligned, the memset() should be writing
> PAGE_INUSE for a PMD chunk.
>
> I do not really a see a reason why this should not happen.
> Actually, we will be leaving pagetables behind as we never get to free
> the PMD chunk when sizeof(struct page) is not a multiple of 2MB.
>
> I plan to fix remove_pmd_table(), but for now let us not allow to use this feature
> if the size of a struct page is not 64.
> Let us keep it simply for the time being, shall we?
My first intention was:
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index b5a3fa4033d3..0c9756a2eb24 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1044,32 +1044,14 @@ remove_pmd_table(pmd_t *pmd_start, unsigned long addr, unsigned long end,
continue;
if (pmd_large(*pmd)) {
- if (IS_ALIGNED(addr, PMD_SIZE) &&
- IS_ALIGNED(next, PMD_SIZE)) {
- if (!direct)
- free_hugepage_table(pmd_page(*pmd),
- altmap);
-
- spin_lock(&init_mm.page_table_lock);
- pmd_clear(pmd);
- spin_unlock(&init_mm.page_table_lock);
- pages++;
- } else {
- /* If here, we are freeing vmemmap pages. */
- memset((void *)addr, PAGE_INUSE, next - addr);
-
- page_addr = page_address(pmd_page(*pmd));
- if (!memchr_inv(page_addr, PAGE_INUSE,
- PMD_SIZE)) {
- free_hugepage_table(pmd_page(*pmd),
- altmap);
-
- spin_lock(&init_mm.page_table_lock);
- pmd_clear(pmd);
- spin_unlock(&init_mm.page_table_lock);
- }
- }
+ if (!direct)
+ free_hugepage_table(pmd_page(*pmd),
+ altmap);
+ spin_lock(&init_mm.page_table_lock);
+ pmd_clear(pmd);
+ spin_unlock(&init_mm.page_table_lock);
+ pages++;
continue;
}
I did not try it out yet and this might explode badly, but AFAICS, a PMD size
chunk is always allocated even when the section does not spand 2MB.
E.g: sizeof(struct page) = 56.
PAGES_PER_SECTION * 64 = 2097152
PAGES_PER_SECTION * 56 = 1835008
Even in the latter case, vmemmap_populate_hugepages will allocate a PMD size chunk
to satisfy the unaligned range.
So, unless I am missing something, it should not be a problem to free that 2MB in
remove_pmd_table when 1) the PMD is large and 2) the range is not aligned.
--
Oscar Salvador
SUSE L3
next prev parent reply other threads:[~2021-01-25 10:56 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-17 13:07 [PATCH 0/5] Allocate memmap from hotadded memory (per device) Oscar Salvador
2020-12-17 13:07 ` [PATCH 1/5] mm: Introduce ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE Oscar Salvador
2021-01-11 16:52 ` David Hildenbrand
2021-01-12 7:26 ` Oscar Salvador
2021-01-12 10:12 ` David Hildenbrand
2021-01-12 11:17 ` Oscar Salvador
2021-01-12 11:17 ` David Hildenbrand
2020-12-17 13:07 ` [PATCH 2/5] mm,memory_hotplug: Allocate memmap from the added memory range Oscar Salvador
2021-01-18 10:11 ` Oscar Salvador
2021-01-19 13:58 ` David Hildenbrand
2021-01-25 10:39 ` Oscar Salvador
2021-01-25 10:56 ` Oscar Salvador [this message]
2021-01-25 11:02 ` David Hildenbrand
2021-01-25 13:36 ` Oscar Salvador
2021-01-25 10:57 ` David Hildenbrand
2021-01-25 11:18 ` Oscar Salvador
2021-01-25 11:23 ` David Hildenbrand
2020-12-17 13:07 ` [PATCH 3/5] acpi,memhotplug: Enable MHP_MEMMAP_ON_MEMORY when supported Oscar Salvador
2021-01-11 16:53 ` David Hildenbrand
2020-12-17 13:07 ` [PATCH 4/5] powerpc/memhotplug: " Oscar Salvador
2021-01-11 16:55 ` David Hildenbrand
2021-01-12 13:31 ` Oscar Salvador
2020-12-17 13:07 ` [PATCH 5/5] mm,memory_hotplug: Add kernel boot option to enable memmap_on_memory Oscar Salvador
2021-01-11 9:05 ` [PATCH 0/5] Allocate memmap from hotadded memory (per device) Oscar Salvador
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