[adding Steve, since he worked on THP for 32-bit ARM] On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote: > On Tue, 23 Feb 2016 13:32:21 +0300 > "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > > The theory is that the splitting bit effetely masked bogus pmd_present(): > > we had pmd_trans_splitting() in all code path and that prevented mm from > > touching the pmd. Once pmd_trans_splitting() has gone, mm proceed with the > > pmd where it shouldn't and here's a boom. > > Well, I don't think pmd_present() == true is bogus for a trans_huge pmd under > splitting, after all there is a page behind the the pmd. Also, if it was > bogus, and it would need to be false, why should it be marked !pmd_present() > only at the pmdp_invalidate() step before the pmd_populate()? It clearly > is pmd_present() before that, on all architectures, and if there was any > problem/race with that, setting it to !pmd_present() at this stage would > only (marginally) reduce the race window. > > BTW, PowerPC and Sparc seem to do the same thing in pmdp_invalidate(), > i.e. they do not set pmd_present() == false, only mark it so that it would > not generate a new TLB entry, just like on s390. After all, the function > is called pmdp_invalidate(), and I think the comment in mm/huge_memory.c > before that call is just a little ambiguous in its wording. When it says > "mark the pmd notpresent" it probably means "mark it so that it will not > generate a new TLB entry", which is also what the comment is really about: > prevent huge and small entries in the TLB for the same page at the same > time. > > FWIW, and since the ARM arch-list is already on cc, I think there is > an issue with pmdp_invalidate() on ARM, since it also seems to clear > the trans_huge (and formerly trans_splitting) bit, which actually makes > the pmd !pmd_present(), but it violates the other requirement from the > comment: > "the pmd_trans_huge and pmd_trans_splitting must remain set at all times > on the pmd until the split is complete for this pmd" I've only been testing this for arm64 (where I'm yet to see a problem), but we use the generic pmdp_invalidate implementation from mm/pgtable-generic.c there. On arm64, pmd_trans_huge will return true after pmd_mknotpresent. On arm, it does look to be buggy, since it nukes the entire entry... Steve? Will -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
