Questions about slab and mmap

[Le Tan]

Hello, I am writing a driver module and doing something with slab and
mmap. Now I have some problems and need help.
1. I use kmem_cache_create() to create my own slab cache. It returns
success. But I can't find my slab item in /proc/slabinfo. Why?
2.If I want the object in the slab cache to be page-aligned, what
should I do? Because I want to alloc a memory of size 4096B, and want
the memory to be in one page.
3. My driver module maps pages, allocating by kmem_cache_alloc(), to
userspace through mmap(). In the userspace program, I first mmap() my
driver device, then read something from the address returned, then
munmap() it. Sometimes after the program calls munmap(), some error
messages will be printed to the log:
[42522.596729] BUG: Bad page map in process logger_pro
pte:8000000612a30025 pmd:314175067
What will be possible reasons?


Now I will explain my problems in detail. My module is called
"logger". Logger maintains a list of pages (each of them is 4096B).
Kernel will put some data into logger. Logger put them in the list of
pages in order. When a page is full, logger will alloc another page,
and then fill it with data. For efficiency, I use slab.

data_cache = kmem_cache_create("logger_data", 4096, 0, 0, NULL);  //I
use this to create my own cache. This is called in the init function
of my module
kmem_cache_alloc(data_cache, GFP_ATOMIC);      // I use this to alloc
a new page, use GFP_ATOMIC to avoid sleep. It is called in another
place

Then I insmod logger. There is no error message. And my driver seems
to work. However, I can't find the corresponding item in
/proc/slabinfo. Why?

I then write a userspace program called "logger_pro" to read data from
logger into a file. For efficiency, I ues mmap() to map one page at a
time, then read the data to the file.It is as follows:

while(1) {
      str = mmap(vma);   //always map the same address and offset
(they are set to zero), logger will handle this

      fwrite(str, 4096, fd);

      munmap(str);
}

In logger module, the logger_vma_fault() function will always returns
the first page in the list. When logger_pro munmap(), logger's
function logger_vma_close() will be called. In logger_vma_close(),
logger delete the page that has been mapped just now using
kmem_cache_free(). So, though logger_pro always mmap() the same
address and logger always return the fisrt page, logger_pro will
eventually read all the data out of logger and logger will delete all
the pages that have been read. Everything seems to be OK. But
sometimes there are errors in the log:

[42522.596689] logger_mmap():start:7f8ff57be000, end:7f8ff57bf000
                               //this is the mmap() function of my
device module
[42522.596694] logger_vma_fault():vmf->pgoff:0d,start:7f8ff57be000,pgoff:0,offset:0
                 //this is the fault function of struct
vm_operations_struct
[42522.596729] BUG: Bad page map in process logger_pro
pte:8000000612a30025 pmd:314175067                             //this
is the error
[42522.596740] page:ffffea00184a8c00 count:2 mapcount:-2146959356
mapping:          (null) index:0xffff880612a36000
[42522.596747] page flags: 0x200000000004080(slab|head)
[42522.596811] addr:00007f8ff57be000 vm_flags:04040071 anon_vma:
   (null) mapping:ffff880613b25f08 index:0
[42522.596824] vma->vm_ops->fault: logger_vma_fault+0x0/0x140 [logger]
[42522.596834] vma->vm_file->f_op->mmap: logger_mmap+0x0/0xd50 [logger]
[42522.596842] CPU: 1 PID: 21571 Comm: logger_pro Tainted: G    B
IO 3.11.0+ #1
[42522.596844] Hardware name: Dell Inc. PowerEdge M610/000HYJ, BIOS
2.0.13 04/06/2010
[42522.596846]  00007f8ff57be000 ffff880314199c98 ffffffff816ad166
0000000000006959
[42522.596851]  ffff880314539a98 ffff880314199ce8 ffffffff8114e270
ffffea00184a8c00
[42522.596854]  0000000000000000 ffff880314199cc8 00007f8ff57be000
ffff880314199e18
[42522.596858] Call Trace:
[42522.596867]  [<ffffffff816ad166>] dump_stack+0x46/0x58
[42522.596872]  [<ffffffff8114e270>] print_bad_pte+0x190/0x250
[42522.596877]  [<ffffffff8115027b>] unmap_single_vma+0x6cb/0x7a0
[42522.596880]  [<ffffffff81150bd4>] unmap_vmas+0x54/0xa0
[42522.596885]  [<ffffffff81155aa7>] unmap_region+0xa7/0x110
[42522.596888]  [<ffffffff81157f97>] do_munmap+0x1f7/0x3e0
[42522.596891]  [<ffffffff811581ce>] vm_munmap+0x4e/0x70
[42522.596904]  [<ffffffff811591bb>] SyS_munmap+0x2b/0x40
[42522.596915]  [<ffffffff816bc9c2>] system_call_fastpath+0x16/0x1b
[42522.596920] logger_vma_close():start:7f8ff57be000,
end:7f8ff57bf000, vmas:0                 //this is the close function
of struct vm_operations_struct

So what is wrong here? Any suggestions?
Thanks! I am sorry that this email is a little messy.