On Fri, Feb 08, 2013 at 04:58:09PM -0700, Ian Lepore wrote: > I discovered today that fcntl(fd, F_READAHEAD, 0) doesn't work as > advertised. It's supposed to disable readahead, but instead it restores > the default readahead behavior (if it had previously been changed), and > there is no way to disable readahead.[1] I think the attached patch > fixes it, but it's not immediately clear from the patch why; here's the > deal... > > The amount of readahead is calculated by sequential_heuristic() in > vfs_vnops.c. If the FRDAHEAD flag is set on the file it uses the value > stored in the file's f_seqcount, otherwise it calculates a value (and > updates f_seqcount, which doesn't ever happen when FRDAHEAD is set). > > So the patch causes the FRDAHEAD flag to be set even in the case of the > readahead amount being zero. Because it seems like a useful concept, it > still allows the readahead to be restored to default behavior, now by > passing a negative value. > > Does this look right to those of you who understand this part of the > system better than I do? It looks correct.
I do wonder if it better to keep the actual behaviour as is and set
the readahead amount to default on the arg == 0, disabling readahead
for arg < 0. It is too exteme, most likely, please proceed.
>
> -- Ian
>
> [1] No way using F_READAHEAD; I know about POSIX_FADV_RANDOM.
> Index: sys/kern/kern_descrip.c
> ===================================================================
> --- sys/kern/kern_descrip.c (revision 246337)
> +++ sys/kern/kern_descrip.c (working copy)
> @@ -776,7 +776,7 @@
> }
> fhold(fp);
> FILEDESC_SUNLOCK(fdp);
> - if (arg != 0) {
> + if (arg >= 0) {
> vp = fp->f_vnode;
> error = vn_lock(vp, LK_SHARED);
> if (error != 0) {
> Index: lib/libc/sys/fcntl.2
> ===================================================================
> --- lib/libc/sys/fcntl.2 (revision 246337)
> +++ lib/libc/sys/fcntl.2 (working copy)
> @@ -28,7 +28,7 @@
> .\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94
> .\" $FreeBSD$
> .\"
> -.Dd July 27, 2012
> +.Dd February 8, 2013
> .Dt FCNTL 2
> .Os
> .Sh NAME
> @@ -171,7 +171,7 @@
> which is rounded up to the nearest block size.
> A zero value in
> .Fa arg
> -turns off read ahead.
> +turns off read ahead, a negative value restores the system default.
> .It Dv F_RDAHEAD
> Equivalent to Darwin counterpart which sets read ahead amount of 128KB
> when the third argument,
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