linux-kernel - [PATCH] vfs: reduce stack usage by shrinking struct kiocb

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Date:	Sun, 27 Apr 2008 06:17:36 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	David Chinner <dgc@....com>, Benjamin LaHaise <bcrl@...ck.org>
Cc:	xfs@....sgi.com, Eric Sandeen <sandeen@...deen.net>,
	Adrian Bunk <bunk@...nel.org>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-aio@...ck.org
Subject: [PATCH] vfs: reduce stack usage by shrinking struct kiocb

Hi Al, Benjamin, David,

struct kiocb is placed on stack by, for example, do_sync_write().
Eventually it contributes to xfs writeout path's stack usage, among others.
This is *the* path which causes 4k stack overflows on i386 with xfs.

This patch trivially reorders fields of this structure,
and makes some of them smaller.

Reordering helps 64-bit architectures:
int, void*, int, void* - bad,
int, int, void*, void* - better.

These fields are made smaller:
ki_flags: long -> short: possible values are 0,1,2, so short is enough.
ki_nr_segs: ulong -> uint: nobody uses 4 billion element writev's
                           (and it would not work anyway)
ki_cur_seg: same

For 32bit x86, it makes this struct only 4 bytes smaller.
This isn't much, but it helps not only xfs, but all filesystems.

For 64-bit case savings are a bit more significant,
as ulong -> uint actually makes a difference, and reordering
of 64-bit fields eliminates some padding.

Only compile tested. Observed stack reductions on 32 bits:

-sock_recvmsg [vmlinux]:                        196
-sock_sendmsg [vmlinux]:                        196
+sock_recvmsg [vmlinux]:                        192
+sock_sendmsg [vmlinux]:                        192
-do_sync_write [vmlinux]:                       140
-do_sync_read [vmlinux]:                        140
+do_sync_write [vmlinux]:                       136
+do_sync_read [vmlinux]:                        136
-do_sync_readv_writev [vmlinux]:                132
+do_sync_readv_writev [vmlinux]:                128

Signed-off-by: Denys Vlasenko <vda.linux@...glemail.com>
--
vda

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