On Fri, Apr 15, 2011 at 05:06:06PM +0200, Andrea Arcangeli wrote: > On Fri, Apr 15, 2011 at 04:39:16PM +0200, Andrea Arcangeli wrote: > > On Fri, Apr 15, 2011 at 11:12:48AM +0100, Mel Gorman wrote: > > > diff --git a/mm/memory.c b/mm/memory.c > > > index 5823698..1659574 100644 > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -3322,7 +3322,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma, > > > * run pte_offset_map on the pmd, if an huge pmd could > > > * materialize from under us from a different thread. > > > */ > > > - if (unlikely(__pte_alloc(mm, vma, pmd, address))) > > > + if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address)) > > I started hacking on this and I noticed it'd be better to extend the > unlikely through the end. At first review I didn't notice the > parenthesis closure stops after pte_none and __pte_alloc is now > uncovered. I'd prefer this: > > if (unlikely(pmd_none(*pmd) && __pte_alloc(mm, vma, pmd, address))) > I had this at one point and then decided to match what we do for pte_alloc_map(). My reasoning was that the most important part of this check is pmd_none(). It's relatively unlikely we even call __pte_alloc which is why I didn't think it belonged in the unlikely block. I also preferred being consistent with pte_alloc_map. > I mean the real unlikely thing is that we return VM_FAULT_OOM, if we > end up calling __pte_alloc or not, depends on the app. Generally it > sounds more frequent that the pte is not none, so it's not wrong, but > it's even less likely that __pte_alloc fails so that can be taken into > account too, and __pte_alloc runs still quite frequently. So either > above or: > > if (unlikely(pmd_none(*pmd)) && unlikely(__pte_alloc(mm, vma, pmd, address))) > I'd prefer this than putting everything inside the same unlikely block. But if this makes a noticeable, why do we not do it for pte_alloc_map, pmd_alloc and other similar functions? > I generally prefer unlikely only when it's 100% sure thing it's less > likely (like the VM_FAULT_OOM), so the first version I guess it's > enough (I'm afraid unlikely for pte_none too, may make gcc generate a > far away jump possibly going out of l1 icache for a case that is only > 512 times less likely at best). My point is that it's certainly hugely > more unlikely that __pte_alloc fails than the pte is none. > For the bug fix, it's best to match what pte_alloc_map, pmd_alloc, pud_alloc and others do in terms of how it uses unlikely. If what we are currently doing is sub-optimal, a single patch should change all the helpers. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx 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>