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

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Date:   Wed, 27 Jun 2018 22:07:02 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     wei.w.wang@...el.com, virtio-dev@...ts.oasis-open.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        KVM list <kvm@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu0@...il.com, nilal@...hat.com,
        Rik van Riel <riel@...hat.com>, peterx@...hat.com
Subject: Re: [PATCH v33 1/4] mm: add a function to get free page blocks

On Wed, Jun 27, 2018 at 09:05:39AM -0700, Linus Torvalds wrote:
> [ Sorry for slow reply, my travels have made a mess of my inbox ]
> 
> On Mon, Jun 25, 2018 at 6:55 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > Linus, do you think it would be ok to have get_from_free_page_list
> > actually pop entries from the free list and use them as the buffer
> > to store PAs?
> 
> Honestly, what I think the best option would be is to get rid of this
> interface *entirely*, and just have the balloon code do
> 
>     #define GFP_MINFLAGS (__GFP_NORETRY | __GFP_NOWARN |
> __GFP_THISNODE | __GFP_NOMEMALLOC)
> 
>     struct page *page =  alloc_pages(GFP_MINFLAGS, MAX_ORDER-1);
> 
>  which is not a new interface, and simply removes the max-order page
> from the list if at all possible.
> 
> The above has the advantage of "just working", and not having any races.
> 
> Now, because you don't want to necessarily *entirely* deplete the max
> order, I'd suggest that the *one* new interface you add is just a "how
> many max-order pages are there" interface. So then you can query
> (either before or after getting the max-order page) just how many of
> them there were and whether you want to give that page back.
> 
> Notice? No need for any page lists or physical addresses. No races. No
> complex new functions.
> 
> The physical address you can just get from the "struct page" you got.
> 
> And if you run out of memory because of getting a page, you get all
> the usual "hey, we ran out of memory" responses..
> 
> Wouldn't the above be sufficient?
> 
>             Linus

I think so, thanks!

Wei, to put it in balloon terms, I think there's one thing we missed: if
you do manage to allocate a page, and you don't have a use for it, then
hey, you can just give it to the host because you know it's free - you
are going to return it to the free list.

-- 
MST