On 03.04.2014 [21:49:46 +0530], Aneesh Kumar K.V wrote: > Nishanth Aravamudan <nacc@xxxxxxxxxxxxxxxxxx> writes: > > > On 24.03.2014 [16:02:56 -0700], Nishanth Aravamudan wrote: > >> In KVM guests on Power, if the guest is not backed by hugepages, we see > >> the following in the guest: > >> > >> AnonHugePages: 0 kB > >> HugePages_Total: 0 > >> HugePages_Free: 0 > >> HugePages_Rsvd: 0 > >> HugePages_Surp: 0 > >> Hugepagesize: 64 kB > >> > >> This seems like a configuration issue -- why is a hstate of 64k being > >> registered? > >> > >> I did some debugging and found that the following does trigger, > >> mm/hugetlb.c::hugetlb_init(): > >> > >> /* Some platform decide whether they support huge pages at boot > >> * time. On these, such as powerpc, HPAGE_SHIFT is set to 0 when > >> * there is no such support > >> */ > >> if (HPAGE_SHIFT == 0) > >> return 0; > >> > >> That check is only during init-time. So we don't support hugepages, but > >> none of the hugetlb APIs actually check this condition (HPAGE_SHIFT == > >> 0), so /proc/meminfo above falsely indicates there is a valid hstate (at > >> least one). But note that there is no /sys/kernel/mm/hugepages meaning > >> no hstate was actually registered. > >> > >> Further, it turns out that huge_page_order(default_hstate) is 0, so > >> hugetlb_report_meminfo is doing: > >> > >> 1UL << (huge_page_order(h) + PAGE_SHIFT - 10) > >> > >> which ends up just doing 1 << (PAGE_SHIFT - 10) and since the base page > >> size is 64k, we report a hugepage size of 64k... And allow the user to > >> allocate hugepages via the sysctl, etc. > >> > >> What's the right thing to do here? > >> > >> 1) Should we add checks for HPAGE_SHIFT == 0 to all the hugetlb APIs? It > >> seems like HPAGE_SHIFT == 0 should be the equivalent, functionally, of > >> the config options being off. This seems like a lot of overhead, though, > >> to put everywhere, so maybe I can do it in an arch-specific macro, that > >> in asm-generic defaults to 0 (and so will hopefully be compiled out?). > >> > >> 2) What should hugetlbfs do when HPAGE_SHIFT == 0? Should it be > >> mountable? Obviously if it's mountable, we can't great files there > >> (since the fs will report insufficient space). [1] > > > > Here is my solution to this. Comments appreciated! > > > > In KVM guests on Power, in a guest not backed by hugepages, we see the > > following: > > > > AnonHugePages: 0 kB > > HugePages_Total: 0 > > HugePages_Free: 0 > > HugePages_Rsvd: 0 > > HugePages_Surp: 0 > > Hugepagesize: 64 kB > > > > HPAGE_SHIFT == 0 in this configuration, which indicates that hugepages > > are not supported at boot-time, but this is only checked in > > hugetlb_init(). Extract the check to a helper function, and use it in a > > few relevant places. > > > > This does make hugetlbfs not supported in this environment. I believe > > this is fine, as there are no valid hugepages and that won't change at > > runtime. > > > > Signed-off-by: Nishanth Aravamudan <nacc@xxxxxxxxxxxxxxxxxx> > > > Looks good. Can you resubmit it as a proper patch ? Will Cc you on that. > You may also want to capture in commit message saying hugetlbfs file > system also will not be registered. I did that already: > > This does make hugetlbfs not supported in this environment. I > > believe this is fine, as there are no valid hugepages and that won't > > change at runtime. Thanks, Nish -- 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>