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: > > > On Thu, 28 Aug 2014, Naoya Horiguchi wrote: > > > > > > > > Reported-by: Hugh Dickins <hughd@xxxxxxxxxx> > > > > Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> > > > > Cc: <stable@xxxxxxxxxxxxxxx> # [3.12+] > > > > > > No ack to this one yet, I'm afraid. > > > > OK, I defer Reported-by until all the problems in this patch are solved. > > I added this Reported-by because Andrew asked how In found this problem, > > and advised me to show the reporter. > > And I didn't intend by this Reported-by that you acked the patch. > > In this case, should I have used some unofficial tag like > > "Not-yet-Reported-by:" to avoid being rude? > > Sorry, misunderstanding, I chose that position to write "No ack to this > one yet" because that is where I would insert my "Acked-by" to the patch > when ready. I just meant that I cannot yet give you my "Acked-by". I see my understanding, thanks. > You were not being rude to me at all, quite the reverse. > > I have no objection to your writing "Reported-by: Hugh...": you are > being polite to acknowledge me, and I was not objecting to that. Great. > Although usually, we save "Reported-by"s for users who have > reported a problem they saw in practice, rather than for fellow > developers who have looked at the code and seen a potential bug - > so I won't mind at all if you end up taking it out. > > > > > > 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. > Many of the hugetlb architectures have a pud_huge() which just returns > 0, and we need not worry about those, nor the follow_huge_addr() powerpc. > But arm64, mips, tile, x86 look more interesting. > > Frankly, I find myself too dumb to be sure of the right answer for all: > and think that when we put the proper locking into follow_huge_pud(), > we shall have to include a PUD_SIZE == PMD_SIZE test, to let the > compiler decide for us which is the appropriate locking to match > huge_pte_lockptr(). Yes, both should be done at the same time. > > Unless Kirill can illuminate: I may be afraid of complications > where actually there are none. Yes. What we need now is to fix follow_huge_pmd(), and combining non-urgent things with it is not easy for me. Thanks, Naoya Horiguchi -- 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>