On Thu, Jun 16, 2016 at 09:47:46AM -0700, Mike Kravetz wrote:
> On 06/16/2016 09:31 AM, Michal Hocko wrote:
> > On Thu 16-06-16 09:05:23, Mike Kravetz wrote:
> >> On 06/16/2016 08:43 AM, Michal Hocko wrote:
> >>> [It seems that this patch has been sent several times and this
> >>> particular copy didn't add Kirill who has added this code CC him now]
> >>>
> >>> On Thu 16-06-16 17:42:14, Michal Hocko wrote:
> >>>> On Thu 16-06-16 19:36:11, zhongjiang wrote:
> >>>>> From: zhong jiang <[email protected]>
> >>>>>
> >>>>> when a process acquire a pmd table shared by other process, we
> >>>>> increase the account to current process. otherwise, a race result
> >>>>> in other tasks have set the pud entry. so it no need to increase it.
> >>>>>
> >>>>> Signed-off-by: zhong jiang <[email protected]>
> >>>>> ---
> >>>>> mm/hugetlb.c | 5 ++---
> >>>>> 1 file changed, 2 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >>>>> index 19d0d08..3b025c5 100644
> >>>>> --- a/mm/hugetlb.c
> >>>>> +++ b/mm/hugetlb.c
> >>>>> @@ -4189,10 +4189,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm,
> >>>>> unsigned long addr, pud_t *pud)
> >>>>> if (pud_none(*pud)) {
> >>>>> pud_populate(mm, pud,
> >>>>> (pmd_t *)((unsigned long)spte &
> >>>>> PAGE_MASK));
> >>>>> - } else {
> >>>>> + } else
> >>>>> put_page(virt_to_page(spte));
> >>>>> - mm_inc_nr_pmds(mm);
> >>>>> - }
> >>>>
> >>>> The code is quite puzzling but is this correct? Shouldn't we rather do
> >>>> mm_dec_nr_pmds(mm) in that path to undo the previous inc?
> >>
> >> I agree that the code is quite puzzling. :(
> >>
> >> However, if this were an issue I would have expected to see some reports.
> >> Oracle DB makes use of this feature (shared page tables) and if the pmd
> >> count is wrong we would catch it in check_mm() at exit time.
> >>
> >> Upon closer examination, I believe the code in question is never executed.
> >> Note the callers of huge_pmd_share. The calling code looks like:
> >>
> >> if (want_pmd_share() && pud_none(*pud))
> >> pte = huge_pmd_share(mm, addr, pud);
> >> else
> >> pte = (pte_t *)pmd_alloc(mm, pud, addr);
> >>
> >> Therefore, we do not call huge_pmd_share unless pud_none(*pud). The
> >> code in question is only executed when !pud_none(*pud).
> >
> > My understanding is that the check is needed after we retake page lock
> > because we might have raced with other thread. But it's been quite some
> > time since I've looked at hugetlb locking and page table sharing code.
>
> That is correct, we could have raced. Duh!
>
> In the case of a race, the other thread would have incremented the
> PMD count already. Your suggestion of decrementing pmd count in
> this case seems to be the correct approach. But, I need to think
> about this some more.
Yes, I made mistake by increasing nr_pmds, not descreasing here.
Testcase:
#include <errno.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/time.h>
#define HPGSZ 2097152UL
int main(int argc, char **argv) {
char *addr;
system("echo 1024 > /proc/sys/vm/nr_hugepages");
addr = mmap(NULL, 1024*HPGSZ, PROT_WRITE | PROT_READ,
MAP_SHARED | MAP_ANONYMOUS | MAP_HUGETLB |
MAP_POPULATE, -1, 0);
if (addr == MAP_FAILED) {
fprintf(stderr, "Failed to alloc hugepage\n");
return -1;
}
addr[0] = 1;
fork();
printf("addr[0]: %d\n", addr[0]);
sleep(1);
return 0;
}
You can simulate race by replacing 'if (pud_none(*pud))' with "if (0)". It
would produce "BUG: non-zero nr_pmds on freeing mm: 2" on the test-case.
Fix:
>From fd22922e7b4664e83653a84331f0a95b985bff0c Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <[email protected]>
Date: Fri, 17 Jun 2016 15:07:03 +0300
Subject: [PATCH] hugetlb: fix nr_pmds accounting with shared page tables
We account HugeTLB's shared page table to all processes who share it.
The accounting happens during huge_pmd_share().
If somebody populates pud entry under us, we should decrease pagetable's
refcount and decrease nr_pmds of the process.
By mistake, I increase nr_pmds again in this case. :-/
It will lead to "BUG: non-zero nr_pmds on freeing mm: 2" on process'
exit.
Let's fix this by increasing nr_pmds only when we're sure that the page
table will be used.
Signed-off-by: Kirill A. Shutemov <[email protected]>
Reported-by: zhongjiang <[email protected]>
Fixes: dc6c9a35b66b ("mm: account pmd page tables to the process")
Cc: <[email protected]> [4.0+]
---
mm/hugetlb.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e197cd7080e6..ed6a537f0878 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4216,7 +4216,6 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long
addr, pud_t *pud)
if (saddr) {
spte = huge_pte_offset(svma->vm_mm, saddr);
if (spte) {
- mm_inc_nr_pmds(mm);
get_page(virt_to_page(spte));
break;
}
@@ -4231,9 +4230,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long
addr, pud_t *pud)
if (pud_none(*pud)) {
pud_populate(mm, pud,
(pmd_t *)((unsigned long)spte & PAGE_MASK));
+ mm_inc_nr_pmds(mm);
} else {
put_page(virt_to_page(spte));
- mm_inc_nr_pmds(mm);
}
spin_unlock(ptl);
out:
--
Kirill A. Shutemov
