From: Theodore Ts'o Subject: Re: [RFC PATCH] coredump: avoid ext4 auto_da_alloc for core file Date: Sun, 3 Jul 2016 22:24:55 -0400 Message-ID: <20160704022455.GA28883@thunk.org> References: <5cdda475417b2719dced162cce89a283153cb818.1466012020.git.osandov@fb.com> <20160629183444.GA3030@vader> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Al Viro , linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, kernel-team@fb.com To: Omar Sandoval Return-path: Content-Disposition: inline In-Reply-To: <20160629183444.GA3030@vader> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Wed, Jun 29, 2016 at 11:34:44AM -0700, Omar Sandoval wrote: > > 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 > > --- > > 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 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 Signed-off-by: Theodore Ts'o --- 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