On Mon, 2021-04-05 at 22:32 +0100, Matthew Wilcox wrote: > On Mon, Apr 05, 2021 at 02:01:58PM -0700, Dave Hansen wrote: > > On 4/5/21 1:37 PM, Rick Edgecombe wrote: > > > +static void __dispose_pages(struct list_head *head) > > > +{ > > > + struct list_head *cur, *next; > > > + > > > + list_for_each_safe(cur, next, head) { > > > + list_del(cur); > > > + > > > + /* The list head is stored at the start of the > > > page */ > > > + free_page((unsigned long)cur); > > > + } > > > +} > > > > This is interesting. > > > > While the page is in the allocator, you're using the page contents > > themselves to store the list_head. It took me a minute to figure > > out > > what you were doing here because: "start of the page" is a bit > > ambiguous. It could mean: > > > > * the first 16 bytes in 'struct page' > > or > > * the first 16 bytes in the page itself, aka *page_address(page) > > > > The fact that this doesn't work on higmem systems makes this an OK > > thing > > to do, but it is a bit weird. It's also doubly susceptible to bugs > > where there's a page_to_virt() or virt_to_page() screwup. > > > > I was *hoping* there was still sufficient space in 'struct page' > > for > > this second list_head in addition to page->lru. I think there > > *should* > > be. That would at least make this allocator a bit more "normal" in > > not > > caring about page contents while the page is free in the > > allocator. If > > you were able to do that you could do things like kmemcheck or page > > alloc debugging while the page is in the allocator. > > > > Anyway, I think I'd prefer that you *try* to use 'struct page' > > alone. > > But, if that doesn't work out, please comment the snot out of this > > thing > > because it _is_ weird. > > Hi! Current closest-thing-we-have-to-an-expert-on-struct-page here! > > I haven't read over these patches yet. If these pages are in use by > vmalloc, they can't use mapping+index because get_user_pages() will > call > page_mapping() and the list_head will confuse it. I think it could > use > index+private for a list_head. > > If the pages are in the buddy, I _think_ mapping+index are free. > private > is in use for buddy order. But I haven't read through the buddy code > in a while. > > Does it need to be a doubly linked list? Can it be an hlist? It does need to be a doubly linked list. I think they should never be mapped to userspace. As far as the page allocator is concerned these pages are not free. And they are not compound. Originally I was just using the lru member. Would it be ok in that case?