Re: [RFC] vfs: don't bother clearing close_on_exec bit for unused fds

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Tue, Nov 03 2015, Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Tue, Nov 3, 2015 at 1:41 AM, Rasmus Villemoes
> <linux@rasmusvillemoes.dk> wrote:
>>
>> I'm sure I've missed something, hence the RFC. But if not, there's
>> probably also a few memsets which become redundant. And the
>> __set_close_on_exec part should probably be its own patch...
>
> The patch looks fine to me. I'm not sure the __set_close_on_exec part
> even makes sense, because if you set that bit, it usually really *is*
> clear before, so testing it beforehand is just pointless.  And if
> somebody really keeps setting the bit, they are doing something stupid
> anyway..

So that's true for the lifetime of a single fd where no-one of course
does fcntl(fd, FD_CLOEXEC) more than once. But the scenario I was
thinking of was when fds get recycled. open(, O_CLOEXEC) => 5, close(5),
open(, O_CLOEXEC) => 5; in that case, letting the close_on_exec bit keep
its value avoids dirtying the cache line on all subsequent allocations
of fd 5 (for example, had Eric's app been using *_CLOEXEC for all its
open's, socket's etc. there wouldn't have been any gain by adding the
conditional to __clear_close_on_exec, but I'd expect to see a similar
gain by doing the symmetric thing). Again, this is assuming that almost
all fd allocations either do or do not apply CLOEXEC - after a while,
->close_on_exec would reach a steady-state where no bits get flipped
anymore.

The "usually really *is* clear" only holds when we do "bother clearing
close_on_exec bit for unused fds", which is what I suggest we don't :-)

I don't think either state of the bit in close_on_exec is more or less
'up-to-date' when its buddy in open_fds is not set.

Rasmus