On Tue, 23 Feb 2016 19:19:07 +0100 Gerald Schaefer <gerald.schaefer@xxxxxxxxxx> wrote: > On Tue, 23 Feb 2016 13:32:21 +0300 > "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > > > On Fri, Feb 12, 2016 at 06:16:40PM +0100, Gerald Schaefer wrote: > > > On Fri, 12 Feb 2016 16:57:27 +0100 > > > Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote: > > > > > > > > I'm also confused by pmd_none() is equal to !pmd_present() on s390. Hm? > > > > > > > > Don't know, Gerald or Martin? > > > > > > The implementation frequently changes depending on how many new bits Martin > > > needs to squeeze out :-) > > > We don't have a _PAGE_PRESENT bit for pmds, so pmd_present() just checks if the > > > entry is not empty. pmd_none() of course does the opposite, it checks if it is > > > empty. > > > > I still worry about pmd_present(). It looks wrong to me. I wounder if > > patch below makes a difference. > > > > 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. If I am not mistaken this is true for x86 as well. The generic implementation for pmdp_invalidate sets a new pmd that has been modified with pmd_mknotpresent. For x86 this function removes the _PAGE_PRESENT and _PAGE_PROTNONE bits from the entry. The _PAGE_PSE bit stays set and that makes pmd_present return true. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- 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>