Commit d7c0e68d authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

mm/ksm: convert break_ksm() to use walk_page_range_vma() 

FOLL_MIGRATION exists only for the purpose of break_ksm(), and actually,
there is not even the need to wait for the migration to finish, we only
want to know if we're dealing with a KSM page.

Using follow_page() just to identify a KSM page overcomplicates GUP code. 
Let's use walk_page_range_vma() instead, because we don't actually care
about the page itself, we only need to know a single property -- no need
to even grab a reference.

So, get rid of follow_page() usage such that we can get rid of
FOLL_MIGRATION now and eventually be able to get rid of follow_page() in
the future.

In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge
performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in
a performance degradation of ~2% (old: ~5010 MiB/s, new: ~4900 MiB/s).  I
don't think we particularly care for now.

Interestingly, the benchmark reduction is due to the single callback. 
Adding a second callback (e.g., pud_entry()) reduces the benchmark by
another 100-200 MiB/s.

Link: https://lkml.kernel.org/r/20221021101141.84170-9-david@redhat.com


Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent e07cda5f

mm/ksm.c

+39 −10
Original line number Original line Diff line number Diff line
@@ -39,6 +39,7 @@ 
#include <linux/freezer.h>

#include <linux/freezer.h>

#include <linux/oom.h>

#include <linux/oom.h>

#include <linux/numa.h>

#include <linux/numa.h>

#include <linux/pagewalk.h>





#include <asm/tlbflush.h>

#include <asm/tlbflush.h>

#include "internal.h"

#include "internal.h"
@@ -419,6 +420,39 @@ static inline bool ksm_test_exit(struct mm_struct *mm) 
	return atomic_read(&mm->mm_users) == 0;

	return atomic_read(&mm->mm_users) == 0;

}

}





static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,

			struct mm_walk *walk)

{

	struct page *page = NULL;

	spinlock_t *ptl;

	pte_t *pte;

	int ret;



	if (pmd_leaf(*pmd) || !pmd_present(*pmd))

		return 0;



	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);

	if (pte_present(*pte)) {

		page = vm_normal_page(walk->vma, addr, *pte);

	} else if (!pte_none(*pte)) {

		swp_entry_t entry = pte_to_swp_entry(*pte);



		/*

		 * As KSM pages remain KSM pages until freed, no need to wait

		 * here for migration to end.

		 */

		if (is_migration_entry(entry))

			page = pfn_swap_entry_to_page(entry);

	}

	ret = page && PageKsm(page);

	pte_unmap_unlock(pte, ptl);

	return ret;

}



static const struct mm_walk_ops break_ksm_ops = {

	.pmd_entry = break_ksm_pmd_entry,

};



/*

/*

 * We use break_ksm to break COW on a ksm page by triggering unsharing,

 * We use break_ksm to break COW on a ksm page by triggering unsharing,

 * such that the ksm page will get replaced by an exclusive anonymous page.

 * such that the ksm page will get replaced by an exclusive anonymous page.
@@ -434,21 +468,16 @@ static inline bool ksm_test_exit(struct mm_struct *mm) 
 */

 */

static int break_ksm(struct vm_area_struct *vma, unsigned long addr)

static int break_ksm(struct vm_area_struct *vma, unsigned long addr)

{

{

	struct page *page;

	vm_fault_t ret = 0;

	vm_fault_t ret = 0;





	do {

	do {

		bool ksm_page = false;

		int ksm_page;





		cond_resched();

		cond_resched();

		page = follow_page(vma, addr,

		ksm_page = walk_page_range_vma(vma, addr, addr + 1,

				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);

					       &break_ksm_ops, NULL);

		if (IS_ERR_OR_NULL(page))

		if (WARN_ON_ONCE(ksm_page < 0))

			break;

			return ksm_page;

		if (PageKsm(page))

			ksm_page = true;

		put_page(page);



		if (!ksm_page)

		if (!ksm_page)

			return 0;

			return 0;

		ret = handle_mm_fault(vma, addr,

		ret = handle_mm_fault(vma, addr,