kernel-2.4.22-1.2174.nptlsmp + kiobuftest fails???

[Davide Rossetti]

hi all,
I'm experimenting a bit with kiobuffers in 2.4.22-1.2174.nptlsmp [plain 
rpm kernel]
 
- I took arch/cris/drivers/examples/kiobuftest.c
- compiled and insmod'd it
- mknod /dev/kiobuftest c <major> 0
- run the user-mode test ktest.c (attached)
 [note: O_DIRECT in open is just a trie, it doesn't modify the result. 
It is there to test whether the O_DIRECT code path does something better...]

problem is that map_user_kiobuf() often puts 0's in maplist[i] for 
some/all i.

[root at xeone driver]# ./ktest
buf=0xbf403008 addr=0xbf404000 len=8192
[root at xeone driver]# ./ktest
buf=0xbf4c6008 addr=0xbf4c7000 len=8192
[root at xeone driver]# ./ktest
buf=0xbf48f008 addr=0xbf490000 len=8192
[root at xeone driver]# dmesg
nr_pages == 2
offset == 0
length == 8192
buf == 0xbf404000
page_add(maplist[0]) == 0xff8cd000
page_add(maplist[1]) == 0x00000000
nr_pages == 2
offset == 0
length == 8192
buf == 0xbf4c7000
page_add(maplist[0]) == 0x00000000
page_add(maplist[1]) == 0xff9cb000
nr_pages == 2
offset == 0
length == 8192
buf == 0xbf490000
page_add(maplist[0]) == 0xffa90000
page_add(maplist[1]) == 0x00000000

[root at xeone driver]# dd if=/dev/kiobuftest of=miao bs=8192 count=1
1+0 records in
1+0 records out
[root at xeone driver]# dmesg |tail
nr_pages == 2
offset == 0
length == 8192
buf == 0x09650000
page_add(maplist[0]) == 0xffa91000
page_add(maplist[1]) == 0x00000000

[root at xeone driver]# dd if=/dev/kiobuftest of=miao bs=$[2*8192] count=1
1+0 records in
1+0 records out
[root at xeone driver]# dmesg |tail
nr_pages == 4
offset == 0
length == 16384
buf == 0x08ee8000
page_add(maplist[0]) == 0xffaa9000
page_add(maplist[1]) == 0xff8d9000
page_add(maplist[2]) == 0x00000000
page_add(maplist[3]) == 0x00000000

so it seems that sometimes I get a kernel virtual address = 0...

it is strange as the code in memory.c:get_user_pages(), called by 
map_user_kiobuf(), has guards against it happening:
            if (pages) {
                pages[i] = get_page_map(map);
                /* FIXME: call the correct function,
                 * depending on the type of the found page
                 */
                if (!pages[i])
                    goto bad_page;
                page_cache_get(pages[i]);
            }

seems like there is an error which is not propagated up to the caller of 
map_user_kiobuf()

I'm compiling a plain 2.4.25 + dprintk active to double check....

regards
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