Re: kiobuf interface / PG_locked flag

["Joseph A. Knapka" <jknapka@earthlink.net>]

Martin Maletinsky wrote:
> 
> Hello,
> 
> I just read about the kiobuf interface in the Linux Device Driver book from Rubini/Corbet, and there is one point, which I don't understand:
> - map_user_kiobuf() forces the pages within a user space address range into physical memory, and increments their usage count, which subsequently prevents the pages from
> being swapped out.

While it's true that having a non-zero reference count will prevent
a page from being swapped out, such a page is still subject to
all normal VM operations. In particular, the VM might unmap
the page from your process, *decrement its reference count*, and
then swap it out.

> - lock_kiovec() sets the PG_locked flag for the pages in the kiobufs of a kiovec. The PG_locked flag prevents the pages from being swapped out, which is however already
> ensured by map_user_kiobuf().

I believe PG_locked will prevent the VM from unmapping the
page, which does, in fact, gaurantee that it won't be
swapped out.

Cheers,

-- Joe
  "I'd rather chew my leg off than maintain Java code, which
   sucks, 'cause I have a lot of Java code to maintain and
   the leg surgery is starting to get expensive." - Me
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