On Thu, 7 Jun 2007, Eric Dumazet wrote:
> Davide Libenzi a écrit :
> > On Thu, 7 Jun 2007, Eric Dumazet wrote:
> >
> > > Davide, are you sure we want FIFO for non sequential allocations ?
> > >
> > > This tends to use all the fmap slots, and not very cache friendly
> > > if an app does a lot of [open(),...,close()] things. We already got a perf
> > > drop because of RCUification of file freeing (FIFO mode instead of LIFO
> > > given by kmalloc()/kfree())
> > >
> > > If the idea behind this FIFO was security (ie not easy for an app to
> > > predict next glibc file handle), we/glibc might use yet another
> > > FD_SECUREMODE flag, wich ORed with O_NONSEQFD would ask to fdmap_newfd()
> > > to take the tail of fmap->slist, not head.
> >
> > Uli, would it be OK to rely only on base randomization and use a LIFO
> > instead? We have base randomization, plus LIFO does not mean strictly
> > sequential like legacy allocator, just more compatc and cache friendly.
> >
>
> I am afraid randomization wont really work if /sbin/init or /bin/bash for
> example uses one (or more) unseq fd :
> The 'random base' will be propagated at fork()/exec() time ?
As I said to Uli, we can't move the base while fds are in there. We can
re-randomize it when it's empty. This can also be done (it's a trivial and
fast operation - just set fmap->base to a new value) even every time the
fd count on the map touches zero.
- Davide
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