On Thu, 7 Jun 2007, Eric Dumazet wrote:
> On Thu, 7 Jun 2007 07:59:47 -0400
> Kyle Moffett <[email protected]> wrote:
>
>
> > Likewise there are a massive group of other libraries (especially
> > user-interface and server related ones) that would really like to
> > have support for creating file-descriptors without the top-level
> > application closing them randomly (like several shells seem to).
> >
>
> True, shells are sometimes quite strange.
>
> For example, bash uses file descriptor 255 (FD_CLOEXEC)
>
> When it forks a new process, child gets a file table with 256 slots.
>
> At exec() time, 255 is closed but file table doesnt shrink.
> (shrinking is done at fork() time only)
>
> With fdmap, that means each process started by bash uses at least
> 256 * sizeof(list_head) bytes, ie 4096 bytes on x86_64, even if only three
> file-descriptors are opened (0,1,2)
>
> FD_CLOFORK should help here (BTW : current patch from Davide doesnt take this
> into account and might need a change in fdmap_top_open_fd())
Yes, the CLOFORK flag is there, but it needs to be taken in account in
fdmap_top_open_fd().
> 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.
That was the reason, yes. If we agree that the base randomization is
enough, we can use a LIFO.
- Davide
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