Re: read failed EINVAL with O_DIRECT flag

[Yves Crespin <crespin.quartz@wanadoo.fr>]

>| How can I obtains an buffer alignement from a "user program" ?
>
>I actually left that as an exercise (after I did it at home
>last night).  Did you read the hint (below)?

Well ... either with malloc() and alignement or posix_memalign(),
read() still failed!
My read buffer is in user space, so it's copy to kernel space.
Inside the read() call, it's the kernel buffer which must be aligned?

>
>| >In fs/buffer.c, it wants the buffer & the length (size) to be aligned:
>| >
>| >function: brw_kiovec()
>| >
>| >    /*
>| >     * First, do some alignment and validity checks
>| >     */
>| >    for (i = 0; i < nr; i++) {
>| >        iobuf = iovec[i];
>| >        if ((iobuf->offset & (size-1)) ||
>| >            (iobuf->length & (size-1)))
>| >            return -EINVAL;
>| >        if (!iobuf->nr_pages)
>| >            panic("brw_kiovec: iobuf not initialised");
>| >    }
>| >
>| >so in your program, malloc() the buf [pointer] (larger than needed)
>| >and then align it to a page boundary and pass that aligned pointer
>| >to read().
>| >
/* --- start code --- */
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

#define O_BINARY    0

int main(int argc,char *argv[])
{
   struct stat    sbuf;
   char *        buf;
   int        openFlags;
   int        fd;
   int        nb;
   off_t        size;
   blksize_t    blksize;
   off_t        offset;

   if (argc!=2){
       printf("Missing file name\n");
       exit(2);
   }
   printf("[%s]\n",argv[1]);
   openFlags = O_RDWR|O_BINARY|O_NOCTTY;
   openFlags |= O_DIRECT;    /* Not POSIX */
   fd = open(argv[1],openFlags,0666);
   if (fd==-1){
       printf("open failed [%s] %#o %#o errno 
%d\n",argv[1],openFlags,0666,errno);
       exit(1);
   }
   if (fstat(fd,&sbuf)<0){
       printf("fstat failed\n");
       exit(1);
   }
   blksize = sbuf.st_blksize;
   size = sbuf.st_size;
   nb = posix_memalign((void **)&buf,blksize,size);
   if (nb!=0){
       printf("posix_memalign blksize %lu size %lu failed 
%d\n",blksize,size,nb);
       exit(3);
   }
#if 0
   free(buf);
   buf = malloc(2*blksize);
#endif
   printf("direct: buf %p buf & (blksize-1) %lu\n",buf,(unsigned 
long)buf & (unsigned long)(blksize-1));
   for (offset=0;offset<blksize;offset++){
       if (!((unsigned long)(buf+offset) & (unsigned long)(blksize-1))){
           printf("offset: buf %p buf & (blksize-1) %lu offset 
%lu\n",buf+offset,(unsigned long)(buf+offset) & (unsigned 
long)(blksize-1),offset);
           break;
       }
   }
   printf("align:  buf %p buf & (blksize-1) %lu\n",buf+offset,(unsigned 
long)(buf+offset) & (unsigned long)(blksize-1));
   nb = read(fd,buf+offset,blksize);
   if (nb != blksize){
       printf("read failed fd %d buf %p buf & (blksize-1) %lu blksize 
%lu size %lu nb %d errno %d\n",
               fd,buf+offset,(unsigned long)(buf+offset) & (unsigned 
long)(blksize-1),
               (unsigned long)blksize,size,nb,errno);
       exit(1);
   }
   if (close(fd)){
       printf("close failed\n");
       exit(1);
   }
   free(buf);
   return 0;
}
/* --- end code --- */


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