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SubjectRe: [PATCH v2] mm: hwpoison: disable memory error handling on 1GB hugepage
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Hi Punit,

On Mon, Feb 05, 2018 at 03:05:43PM +0000, Punit Agrawal wrote:
> Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> writes:
>
> > Recently the following BUG was reported:
> >
> > Injecting memory failure for pfn 0x3c0000 at process virtual address 0x7fe300000000
> > Memory failure: 0x3c0000: recovery action for huge page: Recovered
> > BUG: unable to handle kernel paging request at ffff8dfcc0003000
> > IP: gup_pgd_range+0x1f0/0xc20
> > PGD 17ae72067 P4D 17ae72067 PUD 0
> > Oops: 0000 [#1] SMP PTI
> > ...
> > CPU: 3 PID: 5467 Comm: hugetlb_1gb Not tainted 4.15.0-rc8-mm1-abc+ #3
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-1.fc25 04/01/2014
> >
> > You can easily reproduce this by calling madvise(MADV_HWPOISON) twice on
> > a 1GB hugepage. This happens because get_user_pages_fast() is not aware
> > of a migration entry on pud that was created in the 1st madvise() event.
>
> Maybe I'm doing something wrong but I wasn't able to reproduce the issue
> using the test at the end. I get -
>
> $ sudo ./hugepage
>
> Poisoning page...once
> [ 121.295771] Injecting memory failure for pfn 0x8300000 at process virtual address 0x400000000000
> [ 121.386450] Memory failure: 0x8300000: recovery action for huge page: Recovered
>
> Poisoning page...once again
> madvise: Bad address
>
> What am I missing?

The test program below is exactly what I intended, so you did right testing.
I try to guess what could happen. The related code is like below:

static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
int write, struct page **pages, int *nr)
{
...
do {
pud_t pud = READ_ONCE(*pudp);

next = pud_addr_end(addr, end);
if (pud_none(pud))
return 0;
if (unlikely(pud_huge(pud))) {
if (!gup_huge_pud(pud, pudp, addr, next, write,
pages, nr))
return 0;

pud_none() always returns false for hwpoison entry in any arch.
I guess that pud_huge() could behave in undefined manner for hwpoison entry
because pud_huge() assumes that a given pud has the present bit set, which
is not true for hwpoison entry. As a result, pud_huge() checks an irrelevant
bit used for other purpose depending on non-present page table format of
each arch.
If pud_huge() returns false for hwpoison entry, we try to go to the lower
level and the kernel highly likely to crash. So I guess your kernel fell back
the slow path and somehow ended up with returning EFAULT.

So I don't think that the above test result means that errors are properly
handled, and the proposed patch should help for arm64.

Thanks,
Naoya Horiguchi

>
>
> --------- >8 ---------
> #include <stdio.h>
> #include <string.h>
> #include <sys/mman.h>
>
> int main(int argc, char *argv[])
> {
> int flags = MAP_HUGETLB | MAP_ANONYMOUS | MAP_PRIVATE;
> int prot = PROT_READ | PROT_WRITE;
> size_t hugepage_sz;
> void *hugepage;
> int ret;
>
> hugepage_sz = 1024 * 1024 * 1024; /* 1GB */
> hugepage = mmap(NULL, hugepage_sz, prot, flags, -1, 0);
> if (hugepage == MAP_FAILED) {
> perror("mmap");
> return 1;
> }
>
> memset(hugepage, 'b', hugepage_sz);
> getchar();
>
> printf("Poisoning page...once\n");
> ret = madvise(hugepage, hugepage_sz, MADV_HWPOISON);
> if (ret) {
> perror("madvise");
> return 1;
> }
> getchar();
>
> printf("Poisoning page...once again\n");
> ret = madvise(hugepage, hugepage_sz, MADV_HWPOISON);
> if (ret) {
> perror("madvise");
> return 1;
> }
> getchar();
>
> memset(hugepage, 'c', hugepage_sz);
> ret = munmap(hugepage, hugepage_sz);
> if (ret) {
> perror("munmap");
> return 1;
> }
>
> return 0;
> }
>

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