Re: [PATCH 5/5] migration/ram: Merge save_zero_page functions

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Tue, Aug 15, 2023 at 11:38:28AM -0300, Fabiano Rosas wrote:
>> We don't need to do this in two pieces. One single function makes it
>> easier to grasp, specially since it removes the indirection on the
>> return value handling.
>> 
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> ---
>>  migration/ram.c | 41 +++++++++++------------------------------
>>  1 file changed, 11 insertions(+), 30 deletions(-)
>> 
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 8ec38f69e8..13935ead1c 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -1128,32 +1128,6 @@ void ram_release_page(const char *rbname, uint64_t offset)
>>      ram_discard_range(rbname, offset, TARGET_PAGE_SIZE);
>>  }
>>  
>> -/**
>> - * save_zero_page_to_file: send the zero page to the file
>> - *
>> - * Returns the size of data written to the file, 0 means the page is not
>> - * a zero page
>> - *
>> - * @pss: current PSS channel
>> - * @block: block that contains the page we want to send
>> - * @offset: offset inside the block for the page
>> - */
>> -static int save_zero_page_to_file(PageSearchStatus *pss, RAMBlock *block,
>> -                                  ram_addr_t offset)
>> -{
>> -    uint8_t *p = block->host + offset;
>> -    QEMUFile *file = pss->pss_channel;
>> -    int len = 0;
>> -
>> -    if (buffer_is_zero(p, TARGET_PAGE_SIZE)) {
>> -        len += save_page_header(pss, file, block, offset | RAM_SAVE_FLAG_ZERO);
>> -        qemu_put_byte(file, 0);
>> -        len += 1;
>> -        ram_release_page(block->idstr, offset);
>> -    }
>> -    return len;
>> -}
>> -
>>  /**
>>   * save_zero_page: send the zero page to the stream
>>   *
>> @@ -1167,12 +1141,19 @@ static int save_zero_page_to_file(PageSearchStatus *pss, RAMBlock *block,
>>  static int save_zero_page(RAMState *rs, PageSearchStatus *pss, RAMBlock *block,
>>                            ram_addr_t offset)
>>  {
>> -    int len = save_zero_page_to_file(pss, block, offset);
>> +    uint8_t *p = block->host + offset;
>> +    QEMUFile *file = pss->pss_channel;
>> +    int len = 0;
>>  
>> -    if (!len) {
>> -        return -1;
>> +    if (!buffer_is_zero(p, TARGET_PAGE_SIZE)) {
>> +        return 0;
>>      }
>>  
>> +    len += save_page_header(pss, file, block, offset | RAM_SAVE_FLAG_ZERO);
>> +    qemu_put_byte(file, 0);
>> +    len += 1;
>> +    ram_release_page(block->idstr, offset);
>> +
>>      stat64_add(&mig_stats.zero_pages, 1);
>>      ram_transferred_add(len);
>>  
>> @@ -1186,7 +1167,7 @@ static int save_zero_page(RAMState *rs, PageSearchStatus *pss, RAMBlock *block,
>>          XBZRLE_cache_unlock();
>>      }
>>  
>> -    return 1;
>> +    return len;
>
> I don't think it's correct.. We need to keep the retval definition (how
> many pages were sent) rather than returning num of bytes, I think.
>
> I'm curious how did this pass any form of test.. because I think we did
> assert that:
>
>             /* Be strict to return code; it must be 1, or what else? */
>             if (migration_ops->ram_save_target_page(rs, pss) != 1) {
>                 error_report_once("%s: ram_save_target_page failed", __func__);
>                 ret = -1;
>                 goto out;
>             }
>
> Did I miss something?

Kind of, this code is correct. It's just that I made save_zero_page()
return bytes like save_zero_page_to_file() used to do and made
ram_save_target_page() return 1 instead of passing the value from
save_zero_page() along.

But there's a bug in patch 3 because what I described above should only
happen in this patch 5.