On 07/30/2012 11:23 PM, Christoph Lameter wrote: > On Mon, 30 Jul 2012, Glauber Costa wrote: > >> On 07/27/2012 07:55 PM, Christoph Lameter wrote: >>> On Fri, 27 Jul 2012, Glauber Costa wrote: >>> >>>> 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. > >> 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>