Re: [PATCH V5 02/23] physmem: qemu_ram_alloc_from_fd extensions

[Steven Sistare <steven.sistare@oracle.com>]

On 12/24/2024 12:18 PM, Peter Xu wrote:
> On Tue, Dec 24, 2024 at 08:16:47AM -0800, Steve Sistare wrote:
>> Extend qemu_ram_alloc_from_fd to support resizable ram, and define
>> qemu_ram_resize_cb to clean up the API.
>>
>> Add a grow parameter to extend the file if necessary.  However, if
>> grow is false, a zero-sized file is always extended.
>>
>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>> ---
>>   include/exec/ram_addr.h | 13 +++++++++----
>>   system/memory.c         |  4 ++--
>>   system/physmem.c        | 35 ++++++++++++++++++++---------------
>>   3 files changed, 31 insertions(+), 21 deletions(-)
>>
>> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
>> index ff157c1..94bb3cc 100644
>> --- a/include/exec/ram_addr.h
>> +++ b/include/exec/ram_addr.h
>> @@ -111,23 +111,30 @@ long qemu_maxrampagesize(void);
>>    *
>>    * Parameters:
>>    *  @size: the size in bytes of the ram block
>> + *  @max_size: the maximum size of the block after resizing
>>    *  @mr: the memory region where the ram block is
>> + *  @resized: callback after calls to qemu_ram_resize
>>    *  @ram_flags: RamBlock flags. Supported flags: RAM_SHARED, RAM_PMEM,
>>    *              RAM_NORESERVE, RAM_PROTECTED, RAM_NAMED_FILE, RAM_READONLY,
>>    *              RAM_READONLY_FD, RAM_GUEST_MEMFD
>>    *  @mem_path or @fd: specify the backing file or device
>>    *  @offset: Offset into target file
>> + *  @grow: extend file if necessary (but an empty file is always extended).
>>    *  @errp: pointer to Error*, to store an error if it happens
>>    *
>>    * Return:
>>    *  On success, return a pointer to the ram block.
>>    *  On failure, return NULL.
>>    */
>> +typedef void (*qemu_ram_resize_cb)(const char *, uint64_t length, void *host);
>> +
>>   RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>>                                      uint32_t ram_flags, const char *mem_path,
>>                                      off_t offset, Error **errp);
>> -RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>> +RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, ram_addr_t max_size,
>> +                                 qemu_ram_resize_cb resized, MemoryRegion *mr,
>>                                    uint32_t ram_flags, int fd, off_t offset,
>> +                                 bool grow,
>>                                    Error **errp);
>>   
>>   RAMBlock *qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
>> @@ -135,9 +142,7 @@ RAMBlock *qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
>>   RAMBlock *qemu_ram_alloc(ram_addr_t size, uint32_t ram_flags, MemoryRegion *mr,
>>                            Error **errp);
>>   RAMBlock *qemu_ram_alloc_resizeable(ram_addr_t size, ram_addr_t max_size,
>> -                                    void (*resized)(const char*,
>> -                                                    uint64_t length,
>> -                                                    void *host),
>> +                                    qemu_ram_resize_cb resized,
>>                                       MemoryRegion *mr, Error **errp);
>>   void qemu_ram_free(RAMBlock *block);
>>   
>> diff --git a/system/memory.c b/system/memory.c
>> index 78e17e0..290c522 100644
>> --- a/system/memory.c
>> +++ b/system/memory.c
>> @@ -1680,8 +1680,8 @@ bool memory_region_init_ram_from_fd(MemoryRegion *mr,
>>       mr->readonly = !!(ram_flags & RAM_READONLY);
>>       mr->terminates = true;
>>       mr->destructor = memory_region_destructor_ram;
>> -    mr->ram_block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, offset,
>> -                                           &err);
>> +    mr->ram_block = qemu_ram_alloc_from_fd(size, size, NULL, mr, ram_flags, fd,
>> +                                           offset, false, &err);
>>       if (err) {
>>           mr->size = int128_zero();
>>           object_unparent(OBJECT(mr));
>> diff --git a/system/physmem.c b/system/physmem.c
>> index c76503a..48c544f 100644
>> --- a/system/physmem.c
>> +++ b/system/physmem.c
>> @@ -1942,8 +1942,10 @@ out_free:
>>   }
>>   
>>   #ifdef CONFIG_POSIX
>> -RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>> +RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, ram_addr_t max_size,
>> +                                 qemu_ram_resize_cb resized, MemoryRegion *mr,
>>                                    uint32_t ram_flags, int fd, off_t offset,
>> +                                 bool grow,
>>                                    Error **errp)
>>   {
>>       RAMBlock *new_block;
>> @@ -1953,7 +1955,9 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>>       /* Just support these ram flags by now. */
>>       assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_NORESERVE |
>>                             RAM_PROTECTED | RAM_NAMED_FILE | RAM_READONLY |
>> -                          RAM_READONLY_FD | RAM_GUEST_MEMFD)) == 0);
>> +                          RAM_READONLY_FD | RAM_GUEST_MEMFD |
>> +                          RAM_RESIZEABLE)) == 0);
>> +    assert(max_size >= size);
>>   
>>       if (xen_enabled()) {
>>           error_setg(errp, "-mem-path not supported with Xen");
>> @@ -1968,12 +1972,14 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>>   
>>       size = TARGET_PAGE_ALIGN(size);
>>       size = REAL_HOST_PAGE_ALIGN(size);
>> +    max_size = TARGET_PAGE_ALIGN(max_size);
>> +    max_size = REAL_HOST_PAGE_ALIGN(max_size);
>>   
>>       file_size = get_file_size(fd);
>> -    if (file_size > offset && file_size < (offset + size)) {
>> +    if (file_size && file_size < offset + max_size && !grow) {
> 
> Is this a bugfix for the case offset < fsize?  If so, better make it a
> small patch and copy stable..
> 
> $ touch ramfile
> $ truncate -s 64M ramfile
> $ ./qemu-system-x86_64 -object memory-backend-file,mem-path=./ramfile,offset=128M,size=128M,id=mem1,prealloc=on
> qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address
> 
> So yes, it's a bug..

Yes, it's a bug I noticed by inspection.
I will split and submit to stable.

>>           error_setg(errp, "backing store size 0x%" PRIx64
>>                      " does not match 'size' option 0x" RAM_ADDR_FMT,
>> -                   file_size, size);
>> +                   file_size, max_size);
>>           return NULL;
>>       }
>>   
>> @@ -1988,11 +1994,13 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>>       new_block = g_malloc0(sizeof(*new_block));
>>       new_block->mr = mr;
>>       new_block->used_length = size;
>> -    new_block->max_length = size;
>> +    new_block->max_length = max_size;
>> +    new_block->resized = resized;
>>       new_block->flags = ram_flags;
>>       new_block->guest_memfd = -1;
>> -    new_block->host = file_ram_alloc(new_block, size, fd, !file_size, offset,
>> -                                     errp);
>> +    new_block->host = file_ram_alloc(new_block, max_size, fd,
>> +                                     file_size < offset + max_size,
> 
> Same here, looks like relevant to above.

This line would not be part of the fix for stable.  The pre-cpr code should only
truncate (allocate) if !file_size.  If file_size > 0, the fixed conditional above
verifies that file_size is large enough.

The fix will be a 1-liner:
  -    if (file_size > offset && file_size < (offset + size)) {
  +    if (file_size && file_size < offset + size) {

- Steve

> In general, we want to either fail properly detecting wrong-sized files, or
> making sure qemu can boot correctly with the fd.  I confess pretty corner
> cases.. but still.
> 
> So the whole patch looks correct, but maybe need a split.
> 
>> +                                     offset, errp);
>>       if (!new_block->host) {
>>           g_free(new_block);
>>           return NULL;
>> @@ -2044,7 +2052,8 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>>           return NULL;
>>       }
>>   
>> -    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, offset, errp);
>> +    block = qemu_ram_alloc_from_fd(size, size, NULL, mr, ram_flags, fd, offset,
>> +                                   false, errp);
>>       if (!block) {
>>           if (created) {
>>               unlink(mem_path);
>> @@ -2059,9 +2068,7 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>>   
>>   static
>>   RAMBlock *qemu_ram_alloc_internal(ram_addr_t size, ram_addr_t max_size,
>> -                                  void (*resized)(const char*,
>> -                                                  uint64_t length,
>> -                                                  void *host),
>> +                                  qemu_ram_resize_cb resized,
>>                                     void *host, uint32_t ram_flags,
>>                                     MemoryRegion *mr, Error **errp)
>>   {
>> @@ -2113,10 +2120,8 @@ RAMBlock *qemu_ram_alloc(ram_addr_t size, uint32_t ram_flags,
>>   }
>>   
>>   RAMBlock *qemu_ram_alloc_resizeable(ram_addr_t size, ram_addr_t maxsz,
>> -                                     void (*resized)(const char*,
>> -                                                     uint64_t length,
>> -                                                     void *host),
>> -                                     MemoryRegion *mr, Error **errp)
>> +                                    qemu_ram_resize_cb resized,
>> +                                    MemoryRegion *mr, Error **errp)
>>   {
>>       return qemu_ram_alloc_internal(size, maxsz, resized, NULL,
>>                                      RAM_RESIZEABLE, mr, errp);
>> -- 
>> 1.8.3.1
>>
>