Re: [PATCH] um: Fix WRITE_ZEROES in the UBD Driver

["Frédéric Danis" <frederic.danis@collabora.com>]

Hi Anton,

On 25/01/2022 18:56, Anton Ivanov wrote:
> On 25/01/2022 17:14, Frédéric Danis wrote:
>> Call to fallocate with FALLOC_FL_PUNCH_HOLE on a device backed by a 
>> sparse
>> file can end up by missing data, zeroes data range, if the underlying 
>> file
>> is used with a tool like bmaptool which will referenced only used spaces.
>
> Can you elaborate on when do you miss data.
>
> A file with a hole created using FALLOC_FL_PUNCH_HOLE when reading 
> from the hole should return zeroes.
>
> If it returns anything else or if the zeroed range exceeds the one 
> specified in falloc - that is an underlying fs bug.

I use bmaptool to create a block map of the used part of a raw system 
image file, then compress the file and at the end use again bmaptool to 
flash the file to a SDCard.

iiuc fallocate() call with FALLOC_FL_PUNCH_HOLE doesn't "register" the 
space, as it is zeroed, so when bmaptool creates the block map it 
doesn't reference the "used holes", and when I flash the SDCard those 
parts are missing, so the filesystem on the SDCard is corrupted.

>
>>
>> Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
>> ---
>>   arch/um/drivers/ubd_kern.c  | 8 +++++++-
>>   arch/um/include/shared/os.h | 1 +
>>   arch/um/os-Linux/file.c     | 9 +++++++++
>>   3 files changed, 17 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
>> index 69d2d0049a61..b03269faef71 100644
>> --- a/arch/um/drivers/ubd_kern.c
>> +++ b/arch/um/drivers/ubd_kern.c
>> @@ -1526,13 +1526,19 @@ static void do_io(struct io_thread_req *req, 
>> struct io_desc *desc)
>>               }
>>               break;
>>           case REQ_OP_DISCARD:
>> -        case REQ_OP_WRITE_ZEROES:
>>               n = os_falloc_punch(req->fds[bit], off, len);
>>               if (n) {
>>                   req->error = map_error(-n);
>>                   return;
>>               }
>>               break;
>> +        case REQ_OP_WRITE_ZEROES:
>> +            n = os_falloc_zeroes(req->fds[bit], off, len);
>> +            if (n) {
>> +                req->error = map_error(-n);
>> +                return;
>> +            }
>> +            break;
>>           default:
>>               WARN_ON_ONCE(1);
>>               req->error = BLK_STS_NOTSUPP;
>> diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
>> index 00214059d9ec..fafde1d5416e 100644
>> --- a/arch/um/include/shared/os.h
>> +++ b/arch/um/include/shared/os.h
>> @@ -168,6 +168,7 @@ extern unsigned os_major(unsigned long long dev);
>>   extern unsigned os_minor(unsigned long long dev);
>>   extern unsigned long long os_makedev(unsigned major, unsigned minor);
>>   extern int os_falloc_punch(int fd, unsigned long long offset, int 
>> count);
>> +extern int os_falloc_zeroes(int fd, unsigned long long offset, int 
>> count);
>>   extern int os_eventfd(unsigned int initval, int flags);
>>   extern int os_sendmsg_fds(int fd, const void *buf, unsigned int len,
>>                 const int *fds, unsigned int fds_num);
>> diff --git a/arch/um/os-Linux/file.c b/arch/um/os-Linux/file.c
>> index e4421dbc4c36..fc4450db59bd 100644
>> --- a/arch/um/os-Linux/file.c
>> +++ b/arch/um/os-Linux/file.c
>> @@ -625,6 +625,15 @@ int os_falloc_punch(int fd, unsigned long long 
>> offset, int len)
>>       return n;
>>   }
>>   +int os_falloc_zeroes(int fd, unsigned long long offset, int len)
>> +{
>> +    int n = fallocate(fd, FALLOC_FL_ZERO_RANGE|FALLOC_FL_KEEP_SIZE, 
>> offset, len);
>> +
>> +    if (n < 0)
>> +        return -errno;
>> +    return n;
>> +}
>> +
>>   int os_eventfd(unsigned int initval, int flags)
>>   {
>>       int fd = eventfd(initval, flags);
>
>

-- 
Frédéric Danis
Senior Software Engineer

Collabora Ltd.
Platinum Building, St John's Innovation Park, Cambridge CB4 0DS, United Kingdom
Registered in England & Wales, no. 5513718


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