On 07/30/2012 11:23 PM, Christoph Lameter wrote:

On Mon, 30 Jul 2012, Glauber Costa wrote:

quoted

On 07/27/2012 07:55 PM, Christoph Lameter wrote:

quoted

On Fri, 27 Jul 2012, Glauber Costa wrote:

quoted

But I am still wondering if there is anything I am overlooking.

put_page() is necessary because other subsystems may still be holding a
refcount on the page (if f.e. there is DMA still pending to that page).

Humm, this seems to be extremely unsafe in my read.

I do not like it either. Hopefully these usecases have been removed in the
meantime but that used to be an issue.

quoted

If you do kmalloc, the API - AFAIK - does not provide us with any
guarantee that the object (it's not even a page, in the strict sense!)
allocated is reference counted internally. So relying on kfree to do it
doesn't bode well. For one thing, slab doesn't go to the page allocator
for high order allocations, and this code would crash miserably if
running with the slab.

Or am I missing something ?

Yes the refcounting is done at the page level by the page allocator. It is
safe. The slab allocator can free a page removing all references from its
internal structure while the subsystem page reference will hold off the
page allocator from actually freeing the page until the subsystem itself
drops the page count.

pages, yes. But when you do kfree, you don't free a page. You free an
object. The allocator is totally free to keep the page around and pass
it on to someone else.

The use case that put_page protect against, would be totally and
absolutely broken with every other allocator. They could give an object
in the same address to another user in the very next moment.


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