On Wed, Jun 15, 2016 at 10:42:05AM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@xxxxxx>
>
> 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@xxxxxx>
> ---
> 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.
Ping, any thoughts on this?
> 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
>
--
Omar
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