Re: [patch 12/12] mm: rmap opt

From: Nick Piggin
Date: Tue Nov 22 2005 - 18:05:02 EST

Hugh Dickins wrote:
On Mon, 21 Nov 2005, Nick Piggin wrote:

Optimise rmap functions by minimising atomic operations when
we know there will be no concurrent modifications.

It's not quite right yet. A few minor points first:

Thanks for looking at it.

You ought to convert the page_add_anon_rmap in fs/exec.c to
page_add_new_anon_rmap: that won't give a huge leap in performance,
but it will save someone coming along later and wondering why that
particular one isn't "new_".

Yep, you mentioned that before but I must have lost the hunk.

The mod to page-flags.h at the end: nowhere is __SetPageReferenced
used, just cut the page-flags.h change out of your patch.

Perhaps that was at one time a half-way house to removing the
SetPageReferenced from do_anonymous_page: I support you in that
removal (I've several times argued that if it's needed there, then
it's also needed in several other like places which lack it; and I
think you concluded that it's just not needed); but you ought at least
to confess to that in the change comments, if it's not a separate patch.

You're right. I'll split that and fix the page-flags.h.

I've spent longest staring at page_remove_rmap. Here's how it looks:

void page_remove_rmap(struct page *page)
int fast = (page_mapcount(page) == 1) &
PageAnon(page) & (!PageSwapCache(page));

/* fast page may become SwapCache here, but nothing new will map it. */
if (fast)
else if (atomic_add_negative(-1, &page->_mapcount))
BUG_ON(page_mapcount(page) < 0);
if (page_test_and_clear_dirty(page))
return; /* non zero mapcount */
/* [comment snipped for these purposes] */

Well, C doesn't yet allow indentation to take the place of braces:
I think you'll find your /proc/meminfo Mapped goes up and up, since
only on s390 will page_test_and_clear_dirty ever say yes.

Thanks. It is fairly obscure, and possibly has memory ordering problems.
Also the conditional jumps and icache usage are increased, so it isn't
as clear a win as the add_new_anon_rmap's. I'll drop this part for the


SUSE Labs, Novell Inc.

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