[RFC PATCH] coredump: avoid ext4 auto_da_alloc for core file

[Omar Sandoval <osandov@osandov.com>]

From: Omar Sandoval <osandov@fb.com>

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 <osandov@fb.com>
---
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