Re: [PATCH v4 3/6] support UFFD write fault processing in ram_save_iterate()

[Peter Xu <peterx@redhat.com>]

On Thu, Nov 26, 2020 at 06:17:31PM +0300, Andrey Gruzdev wrote:
> In this particular implementation the same single migration
> thread is responsible for both normal linear dirty page
> migration and procesing UFFD page fault events.
> 
> Processing write faults includes reading UFFD file descriptor,
> finding respective RAM block and saving faulting page to
> the migration stream. After page has been saved, write protection
> can be removed. Since asynchronous version of qemu_put_buffer()
> is expected to be used to save pages, we also have to flush
> migraion stream prior to un-protecting saved memory range.
> 
> Write protection is being removed for any previously protected
> memory chunk that has hit the migration stream. That's valid
> for pages from linear page scan along with write fault pages.

Thanks for working on this version, it looks much cleaner.

> 
> Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
> ---
>  migration/ram.c | 155 +++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 147 insertions(+), 8 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 3adfd1948d..bcdccdaef7 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1441,6 +1441,76 @@ static RAMBlock *unqueue_page(RAMState *rs, ram_addr_t *offset)
>      return block;
>  }
>  
> +#ifdef CONFIG_LINUX
> +/**
> + * ram_find_block_by_host_address: find RAM block containing host page
> + *
> + * Returns pointer to RAMBlock if found, NULL otherwise
> + *
> + * @rs: current RAM state
> + * @page_address: host page address
> + */
> +static RAMBlock *ram_find_block_by_host_address(RAMState *rs, hwaddr page_address)

Reuse qemu_ram_block_from_host() somehow?

> +{
> +    RAMBlock *bs = rs->last_seen_block;
> +
> +    do {
> +        if (page_address >= (hwaddr) bs->host && (page_address + TARGET_PAGE_SIZE) <=
> +                ((hwaddr) bs->host + bs->max_length)) {
> +            return bs;
> +        }
> +
> +        bs = QLIST_NEXT_RCU(bs, next);
> +        if (!bs) {
> +            /* Hit the end of the list */
> +            bs = QLIST_FIRST_RCU(&ram_list.blocks);
> +        }
> +    } while (bs != rs->last_seen_block);
> +
> +    return NULL;
> +}
> +
> +/**
> + * poll_fault_page: try to get next UFFD write fault page and, if pending fault
> + *   is found, return RAM block pointer and page offset
> + *
> + * Returns pointer to the RAMBlock containing faulting page,
> + *   NULL if no write faults are pending
> + *
> + * @rs: current RAM state
> + * @offset: page offset from the beginning of the block
> + */
> +static RAMBlock *poll_fault_page(RAMState *rs, ram_addr_t *offset)
> +{
> +    struct uffd_msg uffd_msg;
> +    hwaddr page_address;
> +    RAMBlock *bs;
> +    int res;
> +
> +    if (!migrate_background_snapshot()) {
> +        return NULL;
> +    }
> +
> +    res = uffd_read_events(rs->uffdio_fd, &uffd_msg, 1);
> +    if (res <= 0) {
> +        return NULL;
> +    }
> +
> +    page_address = uffd_msg.arg.pagefault.address;
> +    bs = ram_find_block_by_host_address(rs, page_address);
> +    if (!bs) {
> +        /* In case we couldn't find respective block, just unprotect faulting page. */
> +        uffd_protect_memory(rs->uffdio_fd, page_address, TARGET_PAGE_SIZE, false);
> +        error_report("ram_find_block_by_host_address() failed: address=0x%0lx",
> +                page_address);

Looks ok to error_report() instead of assert(), but I'll suggest drop the call
to uffd_protect_memory() at least.  The only reason to not use assert() is
because we try our best to avoid crashing the vm, however I really doubt
whether uffd_protect_memory() is the right thing to do even if it happens - we
may at last try to unprotect some strange pages that we don't even know where
it is...

> +        return NULL;
> +    }
> +
> +    *offset = (ram_addr_t) (page_address - (hwaddr) bs->host);
> +    return bs;
> +}
> +#endif /* CONFIG_LINUX */
> +
>  /**
>   * get_queued_page: unqueue a page from the postcopy requests
>   *
> @@ -1480,6 +1550,16 @@ static bool get_queued_page(RAMState *rs, PageSearchStatus *pss)
>  
>      } while (block && !dirty);
>  
> +#ifdef CONFIG_LINUX
> +    if (!block) {
> +        /*
> +         * Poll write faults too if background snapshot is enabled; that's
> +         * when we have vcpus got blocked by the write protected pages.
> +         */
> +        block = poll_fault_page(rs, &offset);
> +    }
> +#endif /* CONFIG_LINUX */
> +
>      if (block) {
>          /*
>           * As soon as we start servicing pages out of order, then we have
> @@ -1753,6 +1833,55 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>      return pages;
>  }
>  
> +/**
> + * ram_save_host_page_pre: ram_save_host_page() pre-notifier
> + *
> + * @rs: current RAM state
> + * @pss: page-search-status structure
> + * @opaque: pointer to receive opaque context value
> + */
> +static inline
> +void ram_save_host_page_pre(RAMState *rs, PageSearchStatus *pss, void **opaque)
> +{
> +    *(ram_addr_t *) opaque = pss->page;
> +}
> +
> +/**
> + * ram_save_host_page_post: ram_save_host_page() post-notifier
> + *
> + * @rs: current RAM state
> + * @pss: page-search-status structure
> + * @opaque: opaque context value
> + * @res_override: pointer to the return value of ram_save_host_page(),
> + *   overwritten in case of an error
> + */
> +static void ram_save_host_page_post(RAMState *rs, PageSearchStatus *pss,
> +        void *opaque, int *res_override)
> +{
> +    /* Check if page is from UFFD-managed region. */
> +    if (pss->block->flags & RAM_UF_WRITEPROTECT) {
> +#ifdef CONFIG_LINUX
> +        ram_addr_t page_from = (ram_addr_t) opaque;
> +        hwaddr page_address = (hwaddr) pss->block->host +
> +                              ((hwaddr) page_from << TARGET_PAGE_BITS);

I feel like most new uses of hwaddr is not correct...  As I also commented in
the other patch.  We should save a lot of castings if switched.

> +        hwaddr run_length = (hwaddr) (pss->page - page_from + 1) << TARGET_PAGE_BITS;
> +        int res;
> +
> +        /* Flush async buffers before un-protect. */
> +        qemu_fflush(rs->f);
> +        /* Un-protect memory range. */
> +        res = uffd_protect_memory(rs->uffdio_fd, page_address, run_length, false);
> +        /* We don't want to override existing error from ram_save_host_page(). */
> +        if (res < 0 && *res_override >= 0) {
> +            *res_override = res;

What is this used for?  If res<0, I thought we should just fail the snapshot.

Meanwhile, res_override points to "pages", and then it'll be rewrite to the
errno returned by uffd_protect_memory().  Smells strange.

Can this ever be triggered anyway?

> +        }
> +#else
> +        /* Should never happen */
> +        qemu_file_set_error(rs->f, -ENOSYS);
> +#endif /* CONFIG_LINUX */
> +    }
> +}
> +
>  /**
>   * ram_find_and_save_block: finds a dirty page and sends it to f
>   *
> @@ -1779,14 +1908,14 @@ static int ram_find_and_save_block(RAMState *rs, bool last_stage)
>          return pages;
>      }
>  
> +    if (!rs->last_seen_block) {
> +        rs->last_seen_block = QLIST_FIRST_RCU(&ram_list.blocks);

Why setup the last seen block to be the first if null?

> +    }
> +
>      pss.block = rs->last_seen_block;
>      pss.page = rs->last_page;
>      pss.complete_round = false;
>  
> -    if (!pss.block) {
> -        pss.block = QLIST_FIRST_RCU(&ram_list.blocks);
> -    }
> -
>      do {
>          again = true;
>          found = get_queued_page(rs, &pss);
> @@ -1797,7 +1926,11 @@ static int ram_find_and_save_block(RAMState *rs, bool last_stage)
>          }
>  
>          if (found) {
> +            void *opaque;
> +
> +            ram_save_host_page_pre(rs, &pss, &opaque);
>              pages = ram_save_host_page(rs, &pss, last_stage);
> +            ram_save_host_page_post(rs, &pss, opaque, &pages);

So the pre/post idea is kind of an overkill to me...

How about we do the unprotect in ram_save_host_page() in the simple way, like:

  ram_save_host_page()
    start_addr = pss->page;
    do {
      ...
      (migrate pages)
      ...
    } while (...);
    if (background_snapshot_enabled()) {
      unprotect pages within start_addr to pss->page;
    }
    ...

>          }
>      } while (!pages && again);
>  
> @@ -3864,9 +3997,12 @@ fail:
>      rs->uffdio_fd = -1;
>      return -1;
>  #else
> +    /*
> +     * Should never happen since we prohibit 'background-snapshot'
> +     * capability on non-Linux hosts.

Yeah, yeah. So let's drop these irrelevant changes? :)

> +     */
>      rs->uffdio_fd = -1;
> -    error_setg(&migrate_get_current()->error,
> -            "Background-snapshot not supported on non-Linux hosts");
> +    error_setg(&migrate_get_current()->error, QERR_UNDEFINED_ERROR);
>      return -1;
>  #endif /* CONFIG_LINUX */
>  }
> @@ -3903,8 +4039,11 @@ void ram_write_tracking_stop(void)
>      uffd_close_fd(rs->uffdio_fd);
>      rs->uffdio_fd = -1;
>  #else
> -    error_setg(&migrate_get_current()->error,
> -            "Background-snapshot not supported on non-Linux hosts");
> +    /*
> +     * Should never happen since we prohibit 'background-snapshot'
> +     * capability on non-Linux hosts.
> +     */
> +    error_setg(&migrate_get_current()->error, QERR_UNDEFINED_ERROR);

Same here.

Thanks,

>  #endif /* CONFIG_LINUX */
>  }
>  
> -- 
> 2.25.1
> 

-- 
Peter Xu