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Commit d18de5a2 authored by Eric Dumazet's avatar Eric Dumazet Committed by Linus Torvalds
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[PATCH] don't insert pipe dentries into dentry_hashtable. 



We currently insert pipe dentries into the global dentry hashtable.  This
is suboptimal because there is currently no way these entries can be used
for a lookup().  (/proc/xxx/fd/xxx uses a different mechanism).  Inserting
them in dentry hashtable slows dcache lookups.

To let __dpath() still work correctly (ie not adding a " (deleted)") after
dentry name, we do :

 - Right after d_alloc(), pretend they are hashed by clearing the
   DCACHE_UNHASHED bit.

 - Call d_instantiate() instead of d_add() : dentry is not inserted in
   hash table.

__dpath() & friends work as intended during dentry lifetime.

 - At dismantle time, once dput() must clear the dentry, setting again
   DCACHE_UNHASHED bit inside the custom d_delete() function provided by
   pipe code, so that dput() can just kill_it.

This patch, combined with (avoid RCU for never hashed dentries) reduced
time of { pipe(p); close(p[0]); close(p[1]);} on my UP machine (1.6GHz
Pentium-M) from 3.23 us to 2.86 us (But this patch does not depend on other
patches, only bench results)

Signed-off-by: default avatarEric Dumazet <dada1@cosmosbay.com>
Acked-by: default avatarDavid Miller <davem@davemloft.net>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 1c69d921

fs/pipe.c

+17 −5
Original line number Diff line number Diff line
@@ -830,7 +830,14 @@ void free_pipe_info(struct inode *inode) 
static struct vfsmount *pipe_mnt __read_mostly;

static int pipefs_delete_dentry(struct dentry *dentry)

{

	return 1;

	/*

	 * At creation time, we pretended this dentry was hashed

	 * (by clearing DCACHE_UNHASHED bit in d_flags)

	 * At delete time, we restore the truth : not hashed.

	 * (so that dput() can proceed correctly)

	 */

	dentry->d_flags |= DCACHE_UNHASHED;

	return 0;

}



static struct dentry_operations pipefs_dentry_operations = {
@@ -891,17 +898,22 @@ struct file *create_write_pipe(void) 
	if (!inode)

		goto err_file;



	sprintf(name, "[%lu]", inode->i_ino);

	this.len = sprintf(name, "[%lu]", inode->i_ino);

	this.name = name;

	this.len = strlen(name);

	this.hash = inode->i_ino; /* will go */

	this.hash = 0;

	err = -ENOMEM;

	dentry = d_alloc(pipe_mnt->mnt_sb->s_root, &this);

	if (!dentry)

		goto err_inode;



	dentry->d_op = &pipefs_dentry_operations;

	d_add(dentry, inode);

	/*

	 * We dont want to publish this dentry into global dentry hash table.

	 * We pretend dentry is already hashed, by unsetting DCACHE_UNHASHED

	 * This permits a working /proc/$pid/fd/XXX on pipes

	 */

	dentry->d_flags &= ~DCACHE_UNHASHED;

	d_instantiate(dentry, inode);

	f->f_vfsmnt = mntget(pipe_mnt);

	f->f_dentry = dentry;

	f->f_mapping = inode->i_mapping;