linux-kernel - Bloat caused by unnecessary calls to compound

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Date:	Sat, 26 Mar 2016 13:50:49 -0500
From:	Eric Biggers <ebiggers3@...il.com>
To:	linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com
Subject: Bloat caused by unnecessary calls to compound_head()?

Hi,

I noticed that after the recent "page-flags" patchset, there are an excessive
number of calls to compound_head() in certain places.

For example, the frequently executed mark_page_accessed() function already
starts out by calling compound_head(), but then each time it tests a page flag
afterwards, there is an extra, seemingly unnecessary, call to compound_head().
This causes a series of instructions like the following to appear no fewer than
10 times throughout the function:

ffffffff81119db4:       48 8b 53 20             mov    0x20(%rbx),%rdx
ffffffff81119db8:       48 8d 42 ff             lea    -0x1(%rdx),%rax
ffffffff81119dbc:       83 e2 01                and    $0x1,%edx
ffffffff81119dbf:       48 0f 44 c3             cmove  %rbx,%rax
ffffffff81119dc3:       48 8b 00                mov    (%rax),%rax

Part of the problem, I suppose, is that the compiler doesn't know that the pages
can't be linked more than one level deep.

Is this a known tradeoff, and have any possible solutions been considered?

Eric