On Sat, Jun 15, 2024 at 06:41:45AM +0200, Mateusz Guzik wrote:
> On Fri, Jun 14, 2024 at 12:34:16PM -0400, Yu Ma wrote:
> > alloc_fd() has a sanity check inside to make sure the FILE object mapping to the
>
> Total nitpick: FILE is the libc thing, I would refer to it as 'struct
> file'. See below for the actual point.
>
> > Combined with patch 1 and 2 in series, pts/blogbench-1.1.0 read improved by
> > 32%, write improved by 15% on Intel ICX 160 cores configuration with v6.8-rc6.
> >
> > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> > ---
> > fs/file.c | 14 ++++++--------
> > 1 file changed, 6 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/file.c b/fs/file.c
> > index a0e94a178c0b..59d62909e2e3 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -548,13 +548,6 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
> > else
> > __clear_close_on_exec(fd, fdt);
> > error = fd;
> > -#if 1
> > - /* Sanity check */
> > - if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
> > - printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
> > - rcu_assign_pointer(fdt->fd[fd], NULL);
> > - }
> > -#endif
> >
>
> I was going to ask when was the last time anyone seen this fire and
> suggest getting rid of it if enough time(tm) passed. Turns out it does
> show up sometimes, latest result I found is 2017 vintage:
> https://groups.google.com/g/syzkaller-bugs/c/jfQ7upCDf9s/m/RQjhDrZ7AQAJ
>
> So you are moving this to another locked area, but one which does not
> execute in the benchmark?
>
> Patch 2/3 states 28% read and 14% write increase, this commit message
> claims it goes up to 32% and 15% respectively -- pretty big. I presume
> this has to do with bouncing a line containing the fd.
>
> I would argue moving this check elsewhere is about as good as removing
> it altogether, but that's for the vfs overlords to decide.
>
> All that aside, looking at disasm of alloc_fd it is pretty clear there
> is time to save, for example:
>
> if (unlikely(nr >= fdt->max_fds)) {
> if (fd >= end) {
> error = -EMFILE;
> goto out;
> }
> error = expand_files(fd, fd);
> if (error < 0)
> goto out;
> if (error)
> goto repeat;
> }
>
Now that I wrote it I noticed the fd < end check has to be performed
regardless of max_fds -- someone could have changed rlimit to a lower
value after using a higher fd. But the main point stands: the call to
expand_files and associated error handling don't have to be there.
> This elides 2 branches and a func call in the common case. Completely
> untested, maybe has some brainfarts, feel free to take without credit
> and further massage the routine.
>
> Moreover my disasm shows that even looking for a bit results in
> a func call(!) to _find_next_zero_bit -- someone(tm) should probably
> massage it into another inline.
>
> After the above massaging is done and if it turns out the check has to
> stay, you can plausibly damage-control it with prefetch -- issue it
> immediately after finding the fd number, before any other work.
>
> All that said, by the above I'm confident there is still *some*
> performance left on the table despite the lock.
>
> > out:
> > spin_unlock(&files->file_lock);
> > @@ -572,7 +565,7 @@ int get_unused_fd_flags(unsigned flags)
> > }
> > EXPORT_SYMBOL(get_unused_fd_flags);
> >
> > -static void __put_unused_fd(struct files_struct *files, unsigned int fd)
> > +static inline void __put_unused_fd(struct files_struct *files, unsigned int fd)
> > {
> > struct fdtable *fdt = files_fdtable(files);
> > __clear_open_fd(fd, fdt);
> > @@ -583,7 +576,12 @@ static void __put_unused_fd(struct files_struct *files, unsigned int fd)
> > void put_unused_fd(unsigned int fd)
> > {
> > struct files_struct *files = current->files;
> > + struct fdtable *fdt = files_fdtable(files);
> > spin_lock(&files->file_lock);
> > + if (unlikely(rcu_access_pointer(fdt->fd[fd]))) {
> > + printk(KERN_WARNING "put_unused_fd: slot %d not NULL!\n", fd);
> > + rcu_assign_pointer(fdt->fd[fd], NULL);
> > + }
> > __put_unused_fd(files, fd);
> > spin_unlock(&files->file_lock);
> > }
