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Date: Thu, 19 Jun 2008 17:01:25 +0100 From: "Jan Beulich" <jbeulich@...ell.com> To: "Jeremy Fitzhardinge" <jeremy@...p.org> Cc: <mingo@...e.hu>, <tglx@...utronix.de>, <linux-kernel@...r.kernel.org>, <hpa@...or.com> Subject: Re: [PATCH] i386: fix vmalloc_sync_all() for Xen >>>> + if (!test_bit(sync_index(address), insync)) { >>>> >>>> >>> It's probably worth reversing this test and removing a layer of indentation. >>> >> >> How? There's a second if() following this one, so we can't just 'continue;' >> here. >> > >That second if() block seems completely redundant: > > if (address == start && test_bit(pgd_index(address), insync)) > start = address + PGDIR_SIZE; > >All it does it update "start", but start isn't used anywhere else in the >loop. Since start is a static variable, it must be updated this way. The intention here is to shorten the loop in later runs - since kernel page table entries never go away, this is possible. Possibly just using the insync array would be sufficient, but when I first coded this I wanted to avoid as much overhead as was possible. >>>> spin_lock_irqsave(&pgd_lock, flags); >>>> + if (unlikely(list_empty(&pgd_list))) { >>>> + spin_unlock_irqrestore(&pgd_lock, flags); >>>> + return; >>>> + } >>>> >>>> >>> This seems a bit warty. If the list is empty, then won't the >>> list_for_each_entry() just fall through? Presumably this only applies >>> to boot, since pgd_list won't be empty on a running system with usermode >>> processes. Is there a correctness issue here, or is it just a >>> micro-optimisation? >>> >> >> No, it isn't. Note the setting to NULL of page, which after the loop gets >> tested for. list_for_each_entry() would never yield a NULL page, even >> if the list is empty. > >Does that matter? If pgd_list is empty, then it's in sync by >definition. Why does it need special-casing? Yes, certainly. But it would result in all insync bits set, which would be wrong - only non-empty page directory entries can be in sync. >>>> list_for_each_entry(page, &pgd_list, lru) { >>>> if (!vmalloc_sync_one(page_address(page), >>>> - address)) >>>> + address)) { >>>> + BUG_ON(list_first_entry(&pgd_list, >>>> + struct page, >>>> + lru) != page); >>>> >>>> >>> What condition is this testing for? >>> >> >> This is a replacement of the BUG_ON() that an earlier patch from you >> removed: Failure of vmalloc_sync_one() must happen on the first >> entry or never, and this is what is being checked for here. >> > >Could you add a comment? Sure, though there was none originally, and the intention seemed quite clear to me. Jan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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