linux-kernel - Re: [PATCH v33 1/4] mm: add a function to get free page blocks

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Date:   Mon, 18 Jun 2018 05:16:02 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Wei Wang <wei.w.wang@...el.com>, virtio-dev@...ts.oasis-open.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...nel.org, akpm@...ux-foundation.org,
        torvalds@...ux-foundation.org, pbonzini@...hat.com,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu0@...il.com, nilal@...hat.com, riel@...hat.com,
        peterx@...hat.com
Subject: Re: [PATCH v33 1/4] mm: add a function to get free page blocks

On Fri, Jun 15, 2018 at 09:50:05PM -0700, Matthew Wilcox wrote:
> I wonder if (to address Michael's concern), you shouldn't instead use
> the first free chunk of pages to return the addresses of all the pages.
> ie something like this:
> 
> 	__le64 *ret = NULL;
> 	unsigned int max = (PAGE_SIZE << order) / sizeof(__le64);
> 
> 	for_each_populated_zone(zone) {
> 		spin_lock_irq(&zone->lock);
> 		for (mt = 0; mt < MIGRATE_TYPES; mt++) {
> 			list = &zone->free_area[order].free_list[mt];
> 			list_for_each_entry_safe(page, list, lru, ...) {
> 				if (index == size)
> 					break;
> 				addr = page_to_pfn(page) << PAGE_SHIFT;
> 				if (!ret) {
> 					list_del(...);
> 					ret = addr;
> 				}
> 				ret[index++] = cpu_to_le64(addr);
> 			}
> 		}
> 		spin_unlock_irq(&zone->lock);
> 	}
> 
> 	return ret;
> }
> 
> You'll need to return the page to the freelist afterwards, but free_pages()
> should take care of that.

Yes Wei already came up with the idea to stick this data into a
MAX_ORDER allocation. Are you sure just taking an entry off
the list like that has no bad side effects?
I have a vague memory someone complained that everyone
most go through get free pages/kmalloc, but I can't
find that anymore.


-- 
MST