Re: [Qemu-devel] [PATCH v2 3/3] hostmem-file: make option 'size' optional

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 10/27/16 12:55 -0200, Eduardo Habkost wrote:
>On Thu, Oct 27, 2016 at 12:23:00PM +0800, Haozhong Zhang wrote:
>> If 'size' option is not given, Qemu will use the file size of 'mem-path'
>> instead. If an empty file, a non-existing file or a directory is specified
>> by option 'mem-path', a non-zero option 'size' is still needed.
>>
>> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
>> ---
>>  backends/hostmem-file.c | 28 ++++++++++++++++++++--------
>>  exec.c                  | 33 ++++++++++++++++++++-------------
>>  2 files changed, 40 insertions(+), 21 deletions(-)
>>
>> diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
>> index 42efb2f..6ee4352 100644
>> --- a/backends/hostmem-file.c
>> +++ b/backends/hostmem-file.c
>> @@ -39,17 +39,14 @@ static void
>>  file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>>  {
>>      HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(backend);
>> +    Error *local_err = NULL;
>>
>> -    if (!backend->size) {
>> -        error_setg(errp, "can't create backend with size 0");
>> -        return;
>> -    }
>>      if (!fb->mem_path) {
>> -        error_setg(errp, "mem-path property not set");
>> -        return;
>> +        error_setg(&local_err, "mem-path property not set");
>> +        goto out;
>>      }
>>  #ifndef CONFIG_LINUX
>> -    error_setg(errp, "-mem-path not supported on this host");
>> +    error_setg(&local_err, "-mem-path not supported on this host");
>>  #else
>>      if (!memory_region_size(&backend->mr)) {
>>          gchar *path;
>> @@ -58,10 +55,25 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>>          memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
>>                                   path,
>>                                   backend->size, fb->share,
>> -                                 fb->mem_path, errp);
>> +                                 fb->mem_path, &local_err);
>>          g_free(path);
>> +
>> +        if (local_err) {
>> +            goto out;
>> +        }
>> +
>> +        if (!backend->size) {
>> +            backend->size = memory_region_size(&backend->mr);
>> +        }
>>      }
>>  #endif
>> +
>> +    if (!backend->size) {
>> +        error_setg(&local_err, "can't create backend with size 0");
>> +    }
>
>You need to move this check before the #endif line, as an error
>is already unconditionally set in local_err in the !CONFIG_LINUX
>path, and a second error_setg() call would trigger an assert()
>inside error_setv().
>

will change

>> +
>> + out:
>> +    error_propagate(errp, local_err);
>>  }
>>
>>  static char *get_mem_path(Object *o, Error **errp)
>> diff --git a/exec.c b/exec.c
>> index 264a25f..89065bd 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -1234,7 +1234,7 @@ static int64_t get_file_size(int fd)
>>  }
>>
>>  static void *file_ram_alloc(RAMBlock *block,
>> -                            ram_addr_t memory,
>> +                            ram_addr_t *memory,
>>                              const char *path,
>>                              Error **errp)
>>  {
>> @@ -1245,6 +1245,7 @@ static void *file_ram_alloc(RAMBlock *block,
>>      void *area = MAP_FAILED;
>>      int fd = -1;
>>      int64_t file_size;
>> +    ram_addr_t mem_size = *memory;
>>
>>      if (kvm_enabled() && !kvm_has_sync_mmu()) {
>>          error_setg(errp,
>> @@ -1309,21 +1310,27 @@ static void *file_ram_alloc(RAMBlock *block,
>>
>>      file_size = get_file_size(fd);
>>
>> -    if (memory < block->page_size) {
>> +    if (!mem_size && file_size > 0) {
>> +        mem_size = file_size;
>
>Maybe we should set *memory here and not below?
>

Qemu currently sets the memory region size to the file size, and block
length to the aligned file size, so the code here can be changed as below:

            memory_region_set_size(block->mr, mem_size);
            mem_size = HOST_PAGE_ALIGN(mem_size);
            *memory = mem_size;

The second line is necessary because Qemu currently passes the aligned
file size to file_ram_alloc().

>> +        memory_region_set_size(block->mr, mem_size);
>
>This could be simplified (and made safer) if the memory region
>got initialized by memory_region_init_ram_from_file() after we
>already mapped/allocated the file (so we avoid surprises in case
>other code does something weird because of the temporarily
>zero-sized MemoryRegion). But it would probably be an intrusive
>change, so I guess changing the memory size here is OK. Paolo,
>what do you think?
>

I will add a check to ensure that the size of block->mr is either 0 or
mem_size, and only set the region size in the former case. The former
corresponds to the case that 'size' option is not given or 0. The
latter corresponds to the case that 'size' option is given.

>> +    }
>> +
>> +    if (mem_size < block->page_size) {
>>          error_setg(errp, "memory size 0x" RAM_ADDR_FMT " must be equal to "
>>                     "or larger than page size 0x%zx",
>> -                   memory, block->page_size);
>> +                   mem_size, block->page_size);
>>          goto error;
>>      }
>>
>> -    if (file_size > 0 && file_size < memory) {
>> +    if (file_size > 0 && file_size < mem_size) {
>>          error_setg(errp, "backing store %s size %"PRId64
>>                     " does not match 'size' option %"PRIu64,
>> -                   path, file_size, memory);
>> +                   path, file_size, mem_size);
>>          goto error;
>>      }
>>
>> -    memory = ROUND_UP(memory, block->page_size);
>> +    mem_size = ROUND_UP(mem_size, block->page_size);
>> +    *memory = mem_size;
>
>Why exactly did you choose to set *memory to the rounded-up size
>and not file_size? Changing *memory to the rounded-up value would
>be additional behavior that is not described in the commit
>message. I believe we should change *memory only if *memory == 0,
>and avoid touching it otherwise.
>

Setting *memory here is buggy. I'll move it as above.

Thanks,
Haozhong

>>
>>      /*
>>       * ftruncate is not supported by hugetlbfs in older
>> @@ -1339,11 +1346,11 @@ static void *file_ram_alloc(RAMBlock *block,
>>       * those labels. Therefore, extending the non-empty backend file
>>       * is disabled as well.
>>       */
>> -    if (!file_size && ftruncate(fd, memory)) {
>> +    if (!file_size && ftruncate(fd, mem_size)) {
>>          perror("ftruncate");
>>      }
>>
>> -    area = qemu_ram_mmap(fd, memory, block->mr->align,
>> +    area = qemu_ram_mmap(fd, mem_size, block->mr->align,
>>                           block->flags & RAM_SHARED);
>>      if (area == MAP_FAILED) {
>>          error_setg_errno(errp, errno,
>> @@ -1352,7 +1359,7 @@ static void *file_ram_alloc(RAMBlock *block,
>>      }
>>
>>      if (mem_prealloc) {
>> -        os_mem_prealloc(fd, area, memory, errp);
>> +        os_mem_prealloc(fd, area, mem_size, errp);
>>          if (errp && *errp) {
>>              goto error;
>>          }
>> @@ -1363,7 +1370,7 @@ static void *file_ram_alloc(RAMBlock *block,
>>
>>  error:
>>      if (area != MAP_FAILED) {
>> -        qemu_ram_munmap(area, memory);
>> +        qemu_ram_munmap(area, mem_size);
>>      }
>>      if (unlink_on_error) {
>>          unlink(path);
>> @@ -1690,15 +1697,15 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>>      size = HOST_PAGE_ALIGN(size);
>>      new_block = g_malloc0(sizeof(*new_block));
>>      new_block->mr = mr;
>> -    new_block->used_length = size;
>> -    new_block->max_length = size;
>>      new_block->flags = share ? RAM_SHARED : 0;
>> -    new_block->host = file_ram_alloc(new_block, size,
>> +    new_block->host = file_ram_alloc(new_block, &size,
>>                                       mem_path, errp);
>>      if (!new_block->host) {
>>          g_free(new_block);
>>          return NULL;
>>      }
>> +    new_block->used_length = size;
>> +    new_block->max_length = size;
>>
>>      ram_block_add(new_block, &local_err);
>>      if (local_err) {
>> --
>> 2.10.1
>>
>
>-- 
>Eduardo