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    SubjectRe: [PATCH v1 7/7] fs/proc/kcore: use page_offline_(freeze|unfreeze)
    Date
    On 03.05.21 11:28, Mike Rapoport wrote:
    > On Mon, May 03, 2021 at 10:28:36AM +0200, David Hildenbrand wrote:
    >> On 02.05.21 08:34, Mike Rapoport wrote:
    >>> On Thu, Apr 29, 2021 at 02:25:19PM +0200, David Hildenbrand wrote:
    >>>> Let's properly synchronize with drivers that set PageOffline(). Unfreeze
    >>>> every now and then, so drivers that want to set PageOffline() can make
    >>>> progress.
    >>>>
    >>>> Signed-off-by: David Hildenbrand <david@redhat.com>
    >>>> ---
    >>>> fs/proc/kcore.c | 15 +++++++++++++++
    >>>> 1 file changed, 15 insertions(+)
    >>>>
    >>>> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
    >>>> index 92ff1e4436cb..3d7531f47389 100644
    >>>> --- a/fs/proc/kcore.c
    >>>> +++ b/fs/proc/kcore.c
    >>>> @@ -311,6 +311,7 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
    >>>> static ssize_t
    >>>> read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
    >>>> {
    >>>> + size_t page_offline_frozen = 0;
    >>>> char *buf = file->private_data;
    >>>> size_t phdrs_offset, notes_offset, data_offset;
    >>>> size_t phdrs_len, notes_len;
    >>>> @@ -509,6 +510,18 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
    >>>> pfn = __pa(start) >> PAGE_SHIFT;
    >>>> page = pfn_to_online_page(pfn);
    >>>
    >>> Can't this race with page offlining for the first time we get here?
    >>
    >>
    >> To clarify, we have three types of offline pages in the kernel ...
    >>
    >> a) Pages part of an offline memory section; the memap is stale and not
    >> trustworthy. pfn_to_online_page() checks that. We *can* protect against
    >> memory offlining using get_online_mems()/put_online_mems(), but usually
    >> avoid doing so as the race window is very small (and a problem all over the
    >> kernel we basically never hit) and locking is rather expensive. In the
    >> future, we might switch to rcu to handle that more efficiently and avoiding
    >> these possible races.
    >>
    >> b) PageOffline(): logically offline pages contained in an online memory
    >> section with a sane memmap. virtio-mem calls these pages "fake offline";
    >> something like a "temporary" memory hole. The new mechanism I propose will
    >> be used to handle synchronization as races can be more severe, e.g., when
    >> reading actual page content here.
    >>
    >> c) Soft offline pages: hwpoisoned pages that are not actually harmful yet,
    >> but could become harmful in the future. So we better try to remove the page
    >> from the page allcoator and try to migrate away existing users.
    >>
    >>
    >> So page_offline_* handle "b) PageOffline()" only. There is a tiny race
    >> between pfn_to_online_page(pfn) and looking at the memmap as we have in many
    >> cases already throughout the kernel, to be tackled in the future.
    >
    > Right, but here you anyway add locking, so why exclude the first iteration?

    What we're protecting is PageOffline() below. If I didn't mess up, we
    should always be calling page_offline_freeze() before calling
    PageOffline(). Or am I missing something?

    >
    > BTW, did you consider something like

    Yes, I played with something like that. We'd have to handle the first
    page_offline_freeze() freeze differently, though, and that's where
    things got a bit ugly in my attempts.

    >
    > if (page_offline_frozen++ % MAX_ORDER_NR_PAGES == 0) {
    > page_offline_unfreeze();
    > cond_resched();
    > page_offline_freeze();
    > }
    >
    > We don't seem to care about page_offline_frozen overflows here, do we?

    No, the buffer size is also size_t and gets incremented on a per-byte
    basis. The variant I have right now looked the cleanest to me. Happy to
    hear simpler alternatives.


    --
    Thanks,

    David / dhildenb

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