On Wed, 29 May 2013 14:09:02 -0700 (PDT) Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote: > > memory_failure() is merely an example of a general problem: code which > > reads from the memmap[] array and expects its elements to be of type > > `struct page'. Other examples might be memory hotplugging, memory leak > > checkers etc. I have vague memories of out-of-tree patches > > (bigphysarea?) doing this as well. > > > > It's a general problem to which we need a general solution. > > <Obi-tmem Kenobe slowly materializes... "use the force, Luke!"> > > One could reasonably argue that any code that makes incorrect > assumptions about the contents of a struct page structure is buggy > and should be fixed. Well it has type "struct page" and all code has a right to expect the contents to match that type. > Isn't the "general solution" already described > in the following comment, excerpted from include/linux/mm.h, which > implies that "scribbling on existing pageframes" [carefully], is fine? > (And, if not, shouldn't that comment be fixed, or am I misreading > it?) > > <start excerpt> > * For the non-reserved pages, page_count(page) denotes a reference count. > * page_count() == 0 means the page is free. page->lru is then used for > * freelist management in the buddy allocator. > * page_count() > 0 means the page has been allocated. Well kinda maybe. How all the random memmap-peekers handle this I do not know. Setting PageReserved is a big hammer which should keep other little paws out of there, although I guess it's abusive of whatever PageReserved is supposed to mean. It's what we used to call a variant record. The tag is page.flags and the protocol is, umm, PageReserved: doesn't refer to a page at all - don't touch PageSlab: belongs to slab or slub !PageSlab: regular kernel/user/pagecache page Are there any more? So what to do here? How about - Position the zbud fields within struct page via the preferred means: editing its definition. - Decide upon and document the means by which the zbud variant is tagged - Demonstrate how this is safe against existing memmap-peekers - Do all this without consuming another page flag :) -- 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>