On Thu, Feb 25, 2016 at 03:49:33PM +0000, Steve Capper wrote: > On 23 February 2016 at 18:47, Will Deacon <will.deacon@xxxxxxx> wrote: > > [adding Steve, since he worked on THP for 32-bit ARM] > > Apologies for my late reply... > > > > > 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? > > pmd_mknotpresent on arm looks inconsistent with the other > architectures and can be changed. > > Having had a look at the usage, I can't see it causing an immediate > problem (that needs to be addressed by an emergency patch). > We don't have a notion of splitting pmds (so there is no splitting > information to lose), and the only usage I could see of > pmd_mknotpresent was: > > pmdp_invalidate(vma, haddr, pmd); > pmd_populate(mm, pmd, pgtable); > > In mm/huge_memory.c, around line 3588. > > So we invalidate the entry (which puts down a faulting entry from > pmd_mknotpresent and invalidates tlb), then immediately put down a > table entry with pmd_populate. > > I have run a 32-bit ARM test kernel and exacerbated THP splits (that's > what took me time), and I didn't notice any problems with 4.5-rc5. If I read code correctly, your pmd_mknotpresent() makes the pmd pmd_none(), right? If yes, it's a problem. It introduces race I've described here: https://marc.info/?l=linux-mm&m=144723658100512&w=4 Basically, if zap_pmd_range() would see pmd_none() between pmdp_mknotpresent() and pmd_populate(), we're screwed. The race window is small, but it's there. -- Kirill A. Shutemov -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>