Messages in this thread | | | Date | Thu, 15 Oct 2009 23:41:19 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [PATCH 2/9] swap_info: change to array of pointers |
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On Thu, 15 Oct 2009, KAMEZAWA Hiroyuki wrote: > On Thu, 15 Oct 2009 01:48:01 +0100 (BST) > Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote: > > --- si1/mm/swapfile.c 2009-10-14 21:25:58.000000000 +0100 > > +++ si2/mm/swapfile.c 2009-10-14 21:26:09.000000000 +0100 > > @@ -49,7 +49,7 @@ static const char Unused_offset[] = "Unu > > > > static struct swap_list_t swap_list = {-1, -1}; > > > > -static struct swap_info_struct swap_info[MAX_SWAPFILES]; > > +static struct swap_info_struct *swap_info[MAX_SWAPFILES]; > > > > Could you add some comment like this ? > == > nr_swapfile is never decreased. > swap_info[type] pointer will never be invalid if it turns to be valid once. > > > for (i = 0; i < nr_swapfiles; i++) { > smp_rmp(); > sis = swap_info[type]; > .... > } > Then, we can execute above without checking sis is valid or not. > smp_rmb() is required when we do above loop without swap_lock().
I do describe this (too briefly?) in the comment on smp_wmb() where swap_info[type] is set and nr_swapfiles raised, in swapon (see below). And make a quick same-line comment on the corresponding smp_rmb()s.
Those seem more useful to me than such a comment on the static struct swap_info_struct *swap_info[MAX_SWAPFILES];
I was about to add (now, in writing this mail) that /proc/swaps is the only thing that reads them without swap_lock; but that's not true, of course, swap_duplicate and swap_free (or their helpers) make preliminary checks without swap_lock - but the difference there is that (unless the pagetable has become corrupted) they're dealing with a swap entry which was previously valid, so can by this time rely upon swap_info[type] and nr_swapfiles to be safe.
> swapon_mutex() will be no help. > > Whether sis is used or not can be detelcted by sis->flags. > > > @@ -1675,11 +1674,13 @@ static void *swap_start(struct seq_file > > if (!l) > > return SEQ_START_TOKEN; > > > > - for (i = 0; i < nr_swapfiles; i++, ptr++) { > > - if (!(ptr->flags & SWP_USED) || !ptr->swap_map) > > + for (type = 0; type < nr_swapfiles; type++) { > > + smp_rmb(); /* read nr_swapfiles before swap_info[type] */ > > + si = swap_info[type]; > > if (!si) ? > > > + if (!(si->flags & SWP_USED) || !si->swap_map) > > continue; > > if (!--l) > > - return ptr; > > + return si; > > } ... > > static void *swap_next(struct seq_file *swap, void *v, loff_t *pos) > > { > > - struct swap_info_struct *ptr; > > - struct swap_info_struct *endptr = swap_info + nr_swapfiles; > > + struct swap_info_struct *si = v; > > + int type; > > > > if (v == SEQ_START_TOKEN) > > - ptr = swap_info; > > - else { > > - ptr = v; > > - ptr++; > > - } > > + type = 0; > > + else > > + type = si->type + 1; > > > > - for (; ptr < endptr; ptr++) { > > - if (!(ptr->flags & SWP_USED) || !ptr->swap_map) > > + for (; type < nr_swapfiles; type++) { > > + smp_rmb(); /* read nr_swapfiles before swap_info[type] */ > > + si = swap_info[type]; > > + if (!(si->flags & SWP_USED) || !si->swap_map) ... > > @@ -1799,23 +1800,45 @@ SYSCALL_DEFINE2(swapon, const char __use ... > > - if (type >= nr_swapfiles) > > - nr_swapfiles = type+1; > > - memset(p, 0, sizeof(*p)); > > INIT_LIST_HEAD(&p->extent_list); > > + if (type >= nr_swapfiles) { > > + p->type = type; > > + swap_info[type] = p; > > + /* > > + * Write swap_info[type] before nr_swapfiles, in case a > > + * racing procfs swap_start() or swap_next() is reading them. > > + * (We never shrink nr_swapfiles, we never free this entry.) > > + */ > > + smp_wmb(); > > + nr_swapfiles++; > > + } else { > > + kfree(p); > > + p = swap_info[type]; > > + /* > > + * Do not memset this entry: a racing procfs swap_next() > > + * would be relying on p->type to remain valid. > > + */ > > + } ...
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