On Mon, 8 Sep 2014, Naoya Horiguchi wrote: > On Mon, Sep 08, 2014 at 12:13:16AM -0700, Hugh Dickins wrote: > > On Fri, 5 Sep 2014, Naoya Horiguchi wrote: > > > On Wed, Sep 03, 2014 at 02:17:41PM -0700, Hugh Dickins wrote: > > > > > > > One subtlety to take care over: it's a long time since I've had to > > > > worry about pmd folding and pud folding (what happens when you only > > > > have 2 or 3 levels of page table instead of the full 4): macros get > > > > defined to each other, and levels get optimized out (perhaps > > > > differently on different architectures). > > > > > > > > So although at first sight the lock to take in follow_huge_pud() > > > > would seem to be mm->page_table_lock, I am not at this point certain > > > > that that's necessarily so - sometimes pud_huge might be pmd_huge, > > > > and the size PMD_SIZE, and pmd_lockptr appropriate at what appears > > > > to be the pud level. Maybe: needs checking through the architectures > > > > and their configs, not obvious to me. > > > > > > I think that every architecture uses mm->page_table_lock for pud-level > > > locking at least for now, but that could be changed in the future, > > > for example when 1GB hugepages or pud-based hugepages become common and > > > someone are interested in splitting lock for pud level. > > > > I'm not convinced by your answer, that you understand the (perhaps > > imaginary!) issue I'm referring to. Try grep for __PAGETABLE_P.D_FOLDED. > > > > Our infrastructure allows for 4 levels of pagetable, pgd pud pmd pte, > > but many architectures/configurations support only 2 or 3 levels. > > What pud functions and pmd functions work out to be in those > > configs is confusing, and varies from architecture to architecture. > > > > In particular, pud and pmd may be different expressions of the same > > thing (with 1 pmd per pud, instead of say 512). In that case PUD_SIZE > > will equal PMD_SIZE: and then at the pud level huge_pte_lockptr() > > will be using split locking instead of mm->page_table_lock. > > Is it a possible problem? It seems to me that in such system no one > can create pud-based hugepages and care about pud level locking.
Maybe it is not a possible problem, I already said I'm not certain. (Maybe I just need to try a couple of x86_32 builds with printks, to find that it is a real problem; but I haven't tried, and x86_32 would not disprove it for the other architectures.) But again, your answer does not convince me that you begin to understand the issue: please read again what I wrote. I am not talking about pud-based hugepages, I'm talking about pmd-based hugepages when the pud level is identical to the pmd level. Hopefully, you're seeing the issue from a different viewpoint than I am, and from your good viewpoint the answer is obvious, whereas from my muddled viewpoint it is not; but you're not making that clear to me. What is certain is that we do not need to worry about this in a patch fixing follow_huge_pmd() alone: it only becomes an issue in a patch extending properly locked FOLL_GET support to follow_huge_pud(), which I think you've decided to set aside for now. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
