Re: [PATCH v2 1/3] mm/memory-failure: userspace controls soft-offlining pages

[Jiaqi Yan <jiaqiyan@google.com>]

On Thu, Jun 13, 2024 at 8:28 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> On 2024/6/12 5:55, Jiaqi Yan wrote:
> > Correctable memory errors are very common on servers with large
> > amount of memory, and are corrected by ECC. Soft offline is kernel's
> > additional recovery handling for memory pages having (excessive)
> > corrected memory errors. Impacted page is migrated to a healthy page
> > if inuse; the original page is discarded for any future use.
> >
>
> Thanks for your update.
>
> > The actual policy on whether (and when) to soft offline should be
> > maintained by userspace, especially in case of an 1G HugeTLB page.
> > Soft-offline dissolves the HugeTLB page, either in-use or free, into
> > chunks of 4K pages, reducing HugeTLB pool capacity by 1 hugepage.
> > If userspace has not acknowledged such behavior, it may be surprised
> > when later mmap hugepages MAP_FAILED due to lack of hugepages.
>
> s/mmap hugepages MAP_FAILED/fails to mmap hugepages/ ?
>
> > In case of a transparent hugepage, it will be split into 4K pages
> > as well; userspace will stop enjoying the transparent performance.
> >
> > In addition, discarding the entire 1G HugeTLB page only because of
> > corrected memory errors sounds very costly and kernel better not
> > doing under the hood. But today there are at least 2 such cases:
>
> s/doing/doing so/ ?
>
> > 1. GHES driver sees both GHES_SEV_CORRECTED and
> >    CPER_SEC_ERROR_THRESHOLD_EXCEEDED after parsing CPER.
> > 2. RAS Correctable Errors Collector counts correctable errors per
> >    PFN and when the counter for a PFN reaches threshold
> > In both cases, userspace has no control of the soft offline performed
> > by kernel's memory failure recovery.
> >
> > This commit gives userspace the control of softofflining any page:
> > kernel only soft offlines raw page / transparent hugepage / HugeTLB
> > hugepage if userspace has agreed to. The interface to userspace is a
> > new sysctl called enable_soft_offline under /proc/sys/vm. By default
> > enable_soft_line is 1 to preserve existing behavior in kernel.
>
> s/enable_soft_line/enable_soft_offline/

Will fix these 3 typos in v3.

>
> >
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
> > ---
> >  mm/memory-failure.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index d3c830e817e3..23415fe03318 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -68,6 +68,8 @@ static int sysctl_memory_failure_early_kill __read_mostly;
> >
> >  static int sysctl_memory_failure_recovery __read_mostly = 1;
> >
> > +static int sysctl_enable_soft_offline __read_mostly = 1;
> > +
> >  atomic_long_t num_poisoned_pages __read_mostly = ATOMIC_LONG_INIT(0);
> >
> >  static bool hw_memory_failure __read_mostly = false;
> > @@ -141,6 +143,15 @@ static struct ctl_table memory_failure_table[] = {
> >               .extra1         = SYSCTL_ZERO,
> >               .extra2         = SYSCTL_ONE,
> >       },
> > +     {
> > +             .procname       = "enable_soft_offline",
> > +             .data           = &sysctl_enable_soft_offline,
> > +             .maxlen         = sizeof(sysctl_enable_soft_offline),
> > +             .mode           = 0644,
> > +             .proc_handler   = proc_dointvec_minmax,
> > +             .extra1         = SYSCTL_ZERO,
> > +             .extra2         = SYSCTL_ONE,
> > +     }
> >  };
> >
> >  /*
> > @@ -2771,6 +2782,11 @@ int soft_offline_page(unsigned long pfn, int flags)
> >       bool try_again = true;
> >       struct page *page;
> >
> > +     if (!sysctl_enable_soft_offline) {
> > +             pr_info("soft offline: %#lx: OS-wide disabled\n", pfn);
> > +             return -EINVAL;
> > +     }
> > +
>
> IMHO, callers might reach here with page refcnt increased. So we have to take care of releasing it first?

Ah, I think you are right when MF_COUNT_INCREASED.

I will move this after the pfn_to_online_page check, and release if disabled.

> Also will it be better to return -EOPNOTSUPP or some other better errno?
>
> Thanks.
> .