Re: [PATCH, RFC] x86/mm/pat: Restore large pages after fragmentation

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Apr 17, 2020 at 03:52:28AM +0300, Mika Penttilä wrote:
> Hi!
> 
> On 17.4.2020 0.32, Kirill A. Shutemov wrote:
> > Change of attributes of the pages may lead to fragmentation of direct
> > mapping over time and performance degradation as result.
> > 
> > With current code it's one way road: kernel tries to avoid splitting
> > large pages, but it doesn't restore them back even if page attributes
> > got compatible again.
> > 
> > Any change to the mapping may potentially allow to restore large page.
> > 
> > Hook up into cpa_flush() path to check if there's any pages to be
> > recovered in PUD_SIZE range around pages we've just touched.
> > 
> > CPUs don't like[1] to have to have TLB entries of different size for the
> > same memory, but looks like it's okay as long as these entries have
> > matching attributes[2]. Therefore it's critical to flush TLB before any
> > following changes to the mapping.
> > 
> > Note that we already allow for multiple TLB entries of different sizes
> > for the same memory now in split_large_page() path. It's not a new
> > situation.
> > 
> > set_memory_4k() provides a way to use 4k pages on purpose. Kernel must
> > not remap such pages as large. Re-use one of software PTE bits to
> > indicate such pages.
> > 
> > [1] See Erratum 383 of AMD Family 10h Processors
> > [2] https://lore.kernel.org/linux-mm/1da1b025-cabc-6f04-bde5-e50830d1ecf0@amd.com/
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >   arch/x86/include/asm/pgtable_types.h |   2 +
> >   arch/x86/mm/pat/set_memory.c         | 191 ++++++++++++++++++++++++++-
> >   2 files changed, 188 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> > index b6606fe6cfdf..11ed34804343 100644
> > --- a/arch/x86/include/asm/pgtable_types.h
> > +++ b/arch/x86/include/asm/pgtable_types.h
> > @@ -34,6 +34,7 @@
> >   #define _PAGE_BIT_CPA_TEST	_PAGE_BIT_SOFTW1
> >   #define _PAGE_BIT_UFFD_WP	_PAGE_BIT_SOFTW2 /* userfaultfd wrprotected */
> >   #define _PAGE_BIT_SOFT_DIRTY	_PAGE_BIT_SOFTW3 /* software dirty tracking */
> > +#define _PAGE_BIT_KERNEL_4K	_PAGE_BIT_SOFTW3 /* page must not be converted to large */
> >   #define _PAGE_BIT_DEVMAP	_PAGE_BIT_SOFTW4
> >   /* If _PAGE_BIT_PRESENT is clear, we use these: */
> > @@ -56,6 +57,7 @@
> >   #define _PAGE_PAT_LARGE (_AT(pteval_t, 1) << _PAGE_BIT_PAT_LARGE)
> >   #define _PAGE_SPECIAL	(_AT(pteval_t, 1) << _PAGE_BIT_SPECIAL)
> >   #define _PAGE_CPA_TEST	(_AT(pteval_t, 1) << _PAGE_BIT_CPA_TEST)
> > +#define _PAGE_KERNEL_4K	(_AT(pteval_t, 1) << _PAGE_BIT_KERNEL_4K)
> >   #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
> >   #define _PAGE_PKEY_BIT0	(_AT(pteval_t, 1) << _PAGE_BIT_PKEY_BIT0)
> >   #define _PAGE_PKEY_BIT1	(_AT(pteval_t, 1) << _PAGE_BIT_PKEY_BIT1)
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index 5414fabad1ae..7cb04a436d86 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -344,22 +344,56 @@ static void __cpa_flush_tlb(void *data)
> >   		__flush_tlb_one_kernel(fix_addr(__cpa_addr(cpa, i)));
> >   }
> > +static void restore_large_pages(unsigned long addr, struct list_head *pgtables);
> > +
> > +static void cpa_restore_large_pages(struct cpa_data *cpa,
> > +		struct list_head *pgtables)
> > +{
> > +	unsigned long start, addr, end;
> > +	int i;
> > +
> > +	if (cpa->flags & CPA_PAGES_ARRAY) {
> > +		for (i = 0; i < cpa->numpages; i++)
> > +			restore_large_pages(__cpa_addr(cpa, i), pgtables);
> > +		return;
> > +	}
> > +
> > +	start = __cpa_addr(cpa, 0);
> > +	end = start + PAGE_SIZE * cpa->numpages;
> > +
> > +	for (addr = start; addr >= start && addr < end; addr += PUD_SIZE)
> > +		restore_large_pages(addr, pgtables);
> > +}
> > +
> >   static void cpa_flush(struct cpa_data *data, int cache)
> >   {
> > +	LIST_HEAD(pgtables);
> > +	struct page *page, *tmp;
> >   	struct cpa_data *cpa = data;
> >   	unsigned int i;
> >   	BUG_ON(irqs_disabled() && !early_boot_irqs_disabled);
> > +	cpa_restore_large_pages(data, &pgtables);
> > +
> >   	if (cache && !static_cpu_has(X86_FEATURE_CLFLUSH)) {
> >   		cpa_flush_all(cache);
> > +		list_for_each_entry_safe(page, tmp, &pgtables, lru) {
> > +			list_del(&page->lru);
> > +			__free_page(page);
> > +		}
> >   		return;
> >   	}
> > -	if (cpa->numpages <= tlb_single_page_flush_ceiling)
> > -		on_each_cpu(__cpa_flush_tlb, cpa, 1);
> > -	else
> > +	if (cpa->numpages > tlb_single_page_flush_ceiling || !list_empty(&pgtables))
> >   		flush_tlb_all();
> > +	else
> > +		on_each_cpu(__cpa_flush_tlb, cpa, 1);
> > +
> > +	list_for_each_entry_safe(page, tmp, &pgtables, lru) {
> > +		list_del(&page->lru);
> > +		__free_page(page);
> > +	}
> 
> 
> The pagetable pages are freed here but ren't you leaking the leaf 4K pages
> (except the first which is made large)?

Huh? There's no way to convert one 4k to large page. We convert all pages
in large page region to the large page.

-- 
 Kirill A. Shutemov