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Commit 5b65c467 authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Ingo Molnar
Browse files

mm, x86/mm: Fix performance regression in get_user_pages_fast() 

The 0-day test bot found a performance regression that was tracked down to
switching x86 to the generic get_user_pages_fast() implementation:

  http://lkml.kernel.org/r/20170710024020.GA26389@yexl-desktop



The regression was caused by the fact that we now use local_irq_save() +
local_irq_restore() in get_user_pages_fast() to disable interrupts.
In x86 implementation local_irq_disable() + local_irq_enable() was used.

The fix is to make get_user_pages_fast() use local_irq_disable(),
leaving local_irq_save() for __get_user_pages_fast() that can be called
with interrupts disabled.

Numbers for pinning a gigabyte of memory, one page a time, 20 repeats:

  Before:  Average: 14.91 ms, stddev: 0.45 ms
  After:   Average: 10.76 ms, stddev: 0.18 ms

Signed-off-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Cc: linux-mm@kvack.org
Fixes: e585513b ("x86/mm/gup: Switch GUP to the generic get_user_page_fast() implementation")
Link: http://lkml.kernel.org/r/20170908215603.9189-3-kirill.shutemov@linux.intel.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 9e52fc2b

mm/gup.c

+58 −39
Original line number Diff line number Diff line
@@ -1618,6 +1618,47 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end, 
	return 1;

}



static void gup_pgd_range(unsigned long addr, unsigned long end,

		int write, struct page **pages, int *nr)

{

	unsigned long next;

	pgd_t *pgdp;



	pgdp = pgd_offset(current->mm, addr);

	do {

		pgd_t pgd = READ_ONCE(*pgdp);



		next = pgd_addr_end(addr, end);

		if (pgd_none(pgd))

			return;

		if (unlikely(pgd_huge(pgd))) {

			if (!gup_huge_pgd(pgd, pgdp, addr, next, write,

					  pages, nr))

				return;

		} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {

			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,

					 PGDIR_SHIFT, next, write, pages, nr))

				return;

		} else if (!gup_p4d_range(pgd, addr, next, write, pages, nr))

			return;

	} while (pgdp++, addr = next, addr != end);

}



#ifndef gup_fast_permitted

/*

 * Check if it's allowed to use __get_user_pages_fast() for the range, or

 * we need to fall back to the slow version:

 */

bool gup_fast_permitted(unsigned long start, int nr_pages, int write)

{

	unsigned long len, end;



	len = (unsigned long) nr_pages << PAGE_SHIFT;

	end = start + len;

	return end >= start;

}

#endif



/*

 * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to

 * the regular GUP. It will only return non-negative values.
@@ -1625,10 +1666,8 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end, 
int __get_user_pages_fast(unsigned long start, int nr_pages, int write,

			  struct page **pages)

{

	struct mm_struct *mm = current->mm;

	unsigned long addr, len, end;

	unsigned long next, flags;

	pgd_t *pgdp;

	unsigned long flags;

	int nr = 0;



	start &= PAGE_MASK;
@@ -1652,44 +1691,14 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write, 
	 * block IPIs that come from THPs splitting.

	 */



	if (gup_fast_permitted(start, nr_pages, write)) {

		local_irq_save(flags);

	pgdp = pgd_offset(mm, addr);

	do {

		pgd_t pgd = READ_ONCE(*pgdp);



		next = pgd_addr_end(addr, end);

		if (pgd_none(pgd))

			break;

		if (unlikely(pgd_huge(pgd))) {

			if (!gup_huge_pgd(pgd, pgdp, addr, next, write,

					  pages, &nr))

				break;

		} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {

			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,

					 PGDIR_SHIFT, next, write, pages, &nr))

				break;

		} else if (!gup_p4d_range(pgd, addr, next, write, pages, &nr))

			break;

	} while (pgdp++, addr = next, addr != end);

		gup_pgd_range(addr, end, write, pages, &nr);

		local_irq_restore(flags);



	return nr;

	}



#ifndef gup_fast_permitted

/*

 * Check if it's allowed to use __get_user_pages_fast() for the range, or

 * we need to fall back to the slow version:

 */

bool gup_fast_permitted(unsigned long start, int nr_pages, int write)

{

	unsigned long len, end;



	len = (unsigned long) nr_pages << PAGE_SHIFT;

	end = start + len;

	return end >= start;

	return nr;

}

#endif



/**

 * get_user_pages_fast() - pin user pages in memory
@@ -1710,12 +1719,22 @@ bool gup_fast_permitted(unsigned long start, int nr_pages, int write) 
int get_user_pages_fast(unsigned long start, int nr_pages, int write,

			struct page **pages)

{

	unsigned long addr, len, end;

	int nr = 0, ret = 0;



	start &= PAGE_MASK;

	addr = start;

	len = (unsigned long) nr_pages << PAGE_SHIFT;

	end = start + len;



	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,

					(void __user *)start, len)))

		return 0;



	if (gup_fast_permitted(start, nr_pages, write)) {

		nr = __get_user_pages_fast(start, nr_pages, write, pages);

		local_irq_disable();

		gup_pgd_range(addr, end, write, pages, &nr);

		local_irq_enable();

		ret = nr;

	}