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Commit 1db491f7 authored by Fenghua Yu's avatar Fenghua Yu Committed by Ingo Molnar
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x86/mm: Reduce PAE-mode per task pgd allocation overhead from 4K to 32 bytes 



With more embedded systems emerging using Quark, among other
things, 32-bit kernel matters again. 32-bit machine and kernel
uses PAE paging, which currently wastes at least 4K of memory
per process on Linux where we have to reserve an entire page to
support a single 32-byte PGD structure. It would be a very good
thing if we could eliminate that wastage.

PAE paging is used to access more than 4GB memory on x86-32. And
it is required for NX.

In this patch, we still allocate one page for pgd for a Xen
domain and 64-bit kernel because one page pgd is assumed in
these cases. But we can save memory space by only allocating
32-byte pgd for 32-bit PAE kernel when it is not running as a
Xen domain.

Signed-off-by: default avatarFenghua Yu <fenghua.yu@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Glenn Williamson <glenn.p.williamson@intel.com>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1421382601-46912-1-git-send-email-fenghua.yu@intel.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent e07e0d4c

arch/x86/mm/pgtable.c

+78 −3
Original line number Diff line number Diff line
@@ -271,12 +271,87 @@ static void pgd_prepopulate_pmd(struct mm_struct *mm, pgd_t *pgd, pmd_t *pmds[]) 
	}

}



/*

 * Xen paravirt assumes pgd table should be in one page. 64 bit kernel also

 * assumes that pgd should be in one page.

 *

 * But kernel with PAE paging that is not running as a Xen domain

 * only needs to allocate 32 bytes for pgd instead of one page.

 */

#ifdef CONFIG_X86_PAE



#include <linux/slab.h>



#define PGD_SIZE	(PTRS_PER_PGD * sizeof(pgd_t))

#define PGD_ALIGN	32



static struct kmem_cache *pgd_cache;



static int __init pgd_cache_init(void)

{

	/*

	 * When PAE kernel is running as a Xen domain, it does not use

	 * shared kernel pmd. And this requires a whole page for pgd.

	 */

	if (!SHARED_KERNEL_PMD)

		return 0;



	/*

	 * when PAE kernel is not running as a Xen domain, it uses

	 * shared kernel pmd. Shared kernel pmd does not require a whole

	 * page for pgd. We are able to just allocate a 32-byte for pgd.

	 * During boot time, we create a 32-byte slab for pgd table allocation.

	 */

	pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, PGD_ALIGN,

				      SLAB_PANIC, NULL);

	if (!pgd_cache)

		return -ENOMEM;



	return 0;

}

core_initcall(pgd_cache_init);



static inline pgd_t *_pgd_alloc(void)

{

	/*

	 * If no SHARED_KERNEL_PMD, PAE kernel is running as a Xen domain.

	 * We allocate one page for pgd.

	 */

	if (!SHARED_KERNEL_PMD)

		return (pgd_t *)__get_free_page(PGALLOC_GFP);



	/*

	 * Now PAE kernel is not running as a Xen domain. We can allocate

	 * a 32-byte slab for pgd to save memory space.

	 */

	return kmem_cache_alloc(pgd_cache, PGALLOC_GFP);

}



static inline void _pgd_free(pgd_t *pgd)

{

	if (!SHARED_KERNEL_PMD)

		free_page((unsigned long)pgd);

	else

		kmem_cache_free(pgd_cache, pgd);

}

#else

static inline pgd_t *_pgd_alloc(void)

{

	return (pgd_t *)__get_free_page(PGALLOC_GFP);

}



static inline void _pgd_free(pgd_t *pgd)

{

	free_page((unsigned long)pgd);

}

#endif /* CONFIG_X86_PAE */



pgd_t *pgd_alloc(struct mm_struct *mm)

{

	pgd_t *pgd;

	pmd_t *pmds[PREALLOCATED_PMDS];



	pgd = (pgd_t *)__get_free_page(PGALLOC_GFP);

	pgd = _pgd_alloc();



	if (pgd == NULL)

		goto out;
@@ -306,7 +381,7 @@ pgd_t *pgd_alloc(struct mm_struct *mm) 
out_free_pmds:

	free_pmds(pmds);

out_free_pgd:

	free_page((unsigned long)pgd);

	_pgd_free(pgd);

out:

	return NULL;

}
@@ -316,7 +391,7 @@ void pgd_free(struct mm_struct *mm, pgd_t *pgd) 
	pgd_mop_up_pmds(mm, pgd);

	pgd_dtor(pgd);

	paravirt_pgd_free(mm, pgd);

	free_page((unsigned long)pgd);

	_pgd_free(pgd);

}



/*