On Wed, Jun 29, 2016 at 11:34:44AM -0700, Omar Sandoval wrote:

quoted

Someone at Facebook reported that their coredumps were much faster when
using a pipe helper than when dumping directly to a file, which doesn't
make much sense. It turns out that this difference is because in
do_coredump(), we truncate the core file and thus trigger the ext4
auto_da_alloc heuristic. We can't use O_TRUNC because we might bail out
of do_coredump() in certain conditions, so instead, avoid truncating
when the file is already empty. In cases where we're actually
overwriting a core file, this won't help, but the common case will be
much better.

Signed-off-by: Omar Sandoval <redacted>
---
Hi, Al and Ted,

This is probably the wrong solution to the problem I described in the
commit message. Do you guys have any better ideas? Something like
0eab928221ba ("ext4: Don't treat a truncation of a zero-length file as
replace-via-truncate") would also work, but that apparently wasn't
right, as it was reverted in 5534fb5bb35a ("ext4: Fix the alloc on close
after a truncate hueristic").

Does this fix things for you?

						- Ted

From bf21c027d84ded545d2c08fa01fd184d29641458 Mon Sep 17 00:00:00 2001

From: Theodore Ts'o <tytso@mit.edu>
Date: Sun, 3 Jul 2016 22:20:49 -0400
Subject: [PATCH] ext4: in ext4_setattr(), only call ext4_truncate() if there is no data to drop

If there are no blocks associated with the inode (and no inline data),
there's no point calling ext4_truncate().  This avoids setting the
replace-via-truncate hueristic if there is an attempt to truncate a
file which is already zero-length --- which is something that happens
in the core dumping code, in case there is an already existing core
file.  In the comon case, there is not a previous core file, so by not
enabling the replace-via-truncate hueristic, we can speed up core
dumps.

Reported-by: Omar Sandoval <redacted>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
 fs/ext4/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 44ee5d9..cd757f8 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c

@@ -5171,7 +5171,7 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
 		 * in data=journal mode to make pages freeable.
 		 */
 		truncate_pagecache(inode, inode->i_size);
-		if (shrink)
+		if (shrink && (inode->i_blocks || ext4_has_inline_data(inode)))
 			ext4_truncate(inode);
 		up_write(&EXT4_I(inode)->i_mmap_sem);
 	}

-- 
2.5.0