On Fri, 27 Apr 2012 17:42:24 +0900 Minchan Kim <minchan@xxxxxxxxxx> wrote: > Now there are several places to use __vmalloc with GFP_ATOMIC, > GFP_NOIO, GFP_NOFS but unfortunately __vmalloc calls map_vm_area > which calls alloc_pages with GFP_KERNEL to allocate page tables. > It means it's possible to happen deadlock. > I don't know why it doesn't have reported until now. > > Firstly, I tried passing gfp_t to lower functions to support __vmalloc > with such flags but other mm guys don't want and decided that > all of caller should be fixed. > > http://marc.info/?l=linux-kernel&m=133517143616544&w=2 > > To begin with, let's listen other's opinion whether they can fix it > by other approach without calling __vmalloc with such flags. > > So this patch adds warning to detect and to be fixed hopely. > I Cced related maintainers. > If I miss someone, please Cced them. > > side-note: > I added WARN_ON instead of WARN_ONCE to detect all of callers > and each WARN_ON for each flag to detect to use any flag easily. > After we fix all of caller or reduce such caller, we can merge > a warning with WARN_ONCE. Just WARN_ONCE, please. If that exposes some sort of calamity then we can reconsider. > > ... > > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -1700,6 +1700,15 @@ static void *__vmalloc_node(unsigned long size, unsigned long align, > gfp_t gfp_mask, pgprot_t prot, > int node, void *caller) > { > + /* > + * This function calls map_vm_area so that it allocates > + * page table with GFP_KERNEL so caller should avoid using > + * GFP_NOIO, GFP_NOFS and !__GFP_WAIT. > + */ > + WARN_ON(!(gfp_mask & __GFP_WAIT)); > + WARN_ON(!(gfp_mask & __GFP_IO)); > + WARN_ON(!(gfp_mask & __GFP_FS)); > + > return __vmalloc_node_range(size, align, VMALLOC_START, VMALLOC_END, > gfp_mask, prot, node, caller); > } This seems strange. There are many entry points to this code and the patch appears to go into a randomly-chosen middle point in the various call chains and sticks a check in there. Why was __vmalloc_node() chosen? Does this provide full coverage or all entry points? Also, the patch won't warn in the most problematic cases such as vmalloc() being called from a __GFP_NOFS context. Presumably there are might_sleep() warnings somewhere on the allocation path which will catch vmalloc() being called from atomic contexts. I'm not sure what to do about that - we don't have machinery in place to be able to detect when a GFP_KERNEL allocation is deadlockable. Perhaps a lot of hacking on lockdep might get us this - we'd need to teach lockdep about which locks prohibit FS entry, which locks prevent IO entry, etc. And there are secret locks such as ext3/4 journal_start(), and bitlocks and lock_page(). eek. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>