From: Omar Sandoval Subject: [RFC PATCH] coredump: avoid ext4 auto_da_alloc for core file Date: Wed, 15 Jun 2016 10:42:05 -0700 Message-ID: <5cdda475417b2719dced162cce89a283153cb818.1466012020.git.osandov@fb.com> Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, kernel-team@fb.com To: Al Viro , Theodore Ts'o Return-path: Received: from mail-pf0-f178.google.com ([209.85.192.178]:35582 "EHLO mail-pf0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932114AbcFORmR (ORCPT ); Wed, 15 Jun 2016 13:42:17 -0400 Received: by mail-pf0-f178.google.com with SMTP id c2so10882369pfa.2 for ; Wed, 15 Jun 2016 10:42:17 -0700 (PDT) Sender: linux-ext4-owner@vger.kernel.org List-ID: From: Omar Sandoval 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"). Thanks. fs/coredump.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/fs/coredump.c b/fs/coredump.c index 281b768000e6..9da7357773f0 100644 --- a/fs/coredump.c +++ b/fs/coredump.c @@ -741,8 +741,10 @@ void do_coredump(const siginfo_t *siginfo) goto close_fail; if (!(cprm.file->f_mode & FMODE_CAN_WRITE)) goto close_fail; - if (do_truncate(cprm.file->f_path.dentry, 0, 0, cprm.file)) - goto close_fail; + if (i_size_read(file_inode(cprm.file)) != 0) { + if (do_truncate(cprm.file->f_path.dentry, 0, 0, cprm.file)) + goto close_fail; + } } /* get us an unshared descriptor table; almost always a no-op */ -- 2.8.3