Re: help on lock around vm_page

Some further reading give me a different message:
The pages returned by vm_page_grab should have VPO_BUSY flag which will
prevent anyone else to move this page to cache. So current approach should
be ok. Am I right? Maybe there is some edge cases since I only meet this bug
in a low memory situation.


--
-Howard

On 6/7/07, Howard Su <howard0su@xxxxxxxxx> wrote:

I want some helps from VM guru. I try to fix a panic in tmpfs. In
order to push tmpfs into -Current, I really want some help to solve
this.

1. we allocate an object from vm_pager_alloc(OBJT_SWAP, ...) when create a
file.
2. the panic is during handling write op:
a) find the first page we want to write
b) call vm_page_grab to get the page from object.
c) call use sf_buf_alloc to map it into kernel_map
d) use uiomove to move the data
e) mark page as dirty
f) loop to a until all pages are handled.

there is a race condition. while doing b-c & e, we hold the
OBJ_LOCK/page_queue_lock. when doing d, we have to drop the locks to
call uiomove. When calling uio move, the page may moved to cache queue
since in that time it is not dirty.

There is a solution that we allocate a page buffer. Before a), we
uiomove it to the buffer and replace uiomove with a bcopy in d). Then,
we can hold lock in b - e. I feel this will cause performance problem.

For the detailed code, please check:

http://perforce.freebsd.org/fileViewer.cgi?FSPC=//depot/user/howardsu/truss/sys/fs/tmpfs/tmpfs%5fvnops.c&REV=30

function: tmpfs_uio_xfer()

Any idea to close this race condition?

PS: If you can review my code about usage of vm, it will be appreciated.

--
-Howard

_______________________________________________
freebsd-arch@xxxxxxxxxxx mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-arch
To unsubscribe, send any mail to "freebsd-arch-unsubscribe@xxxxxxxxxxx"

Relevant Pages

  • Re: help on lock around vm_page
    ... use uiomove to move the data ... There is a solution that we allocate a page buffer. ... If you can review my code about usage of vm, ...
    (freebsd-arch)
  • help on lock around vm_page
    ... order to push tmpfs into -Current, I really want some help to solve ... use uiomove to move the data ... There is a solution that we allocate a page buffer. ... For the detailed code, please check: ...
    (freebsd-arch)
  • Re: pcm(4) related panic
    ... > This enables the panic. ... > Then the panic happens when another thread tries to grab the mutex. ... > to uiomove(), which can sleep if the userland buffer that it is trying ... Either the buffer has to be pre-wired before ...
    (freebsd-current)