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On Mon, 23 May 2005, William Lee Irwin III wrote: > On Mon, May 23, 2005 at 05:14:06PM -0700, Andrew Morton wrote: > > I must say that I continue to find this approach a bit queazifying. > > After some reading of the code I'd agree that yes, it's not possible for us > > to get here with `pte' pointing at the first slot of the pte page, but it's > > not 100% obvious and it's possible that someone will come along later and > > will change things in try_to_unmap_cluster() which cause this unmap to > > suddenly do the wrong thing in rare circumstances. > > IOW: I'd sleep better at night if we took a temporary and actually unmapped > > the thing which we we got back from pte_offset_map().. Am I being silly? There's a similar argument for queasiness in all the other (8 or more) instances of the idiom. I think we originally adopted (and I furthered) this pte_unmap(pte - 1) idiom because in the majority of architecture's configurations pte_unmap does nothing at all, so we resented assigning a pointless variable in some critical loops. > Not at all. I merely attempt to minimize diffsize by default. An > alternative implementation follows (changelog etc. to be taken > from the prior patch) in case it saves the time (however short) needed > to write it yourself. Either of wli's patches is fine with me. There are several levels on which try_to_unmap_cluster is harder to understand than the others, and no good reason to resist the variable assignment. We could rewrite pte_unmap to avoid the issue completely, since its job is to unmap (or pretend to unmap) KM_PTE0's pte if the address is in the fixmap area: but changing it to tolerate an off-by-one address gives a queasy feeling too. Thanks, Hugh - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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