linux-kernel - Re: [RFC 2/3] vmalloc: Support grouped page allocations

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Message-ID: <95a608e850a66af2add6ffcdeb5c6b5c057f1002.camel@intel.com>
Date:   Mon, 5 Apr 2021 21:38:22 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "luto@...nel.org" <luto@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
CC:     "daniel@...earbox.net" <daniel@...earbox.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "andrii@...nel.org" <andrii@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "hch@...radead.org" <hch@...radead.org>
Subject: Re: [RFC 2/3] vmalloc: Support grouped page allocations

On Mon, 2021-04-05 at 14:01 -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.

Yes sorry, that deserved more explanation. I tried putting it in struct
page actually. The problem was list_lru automatically determines the
node id from the list_head provided to it via
page_to_nid(virt_to_page(head)). I guess it assumes the list_head is on
the actual item. I started adding another list_lru function that let
the node id be passed in separately, but I remembered this trick from
the deferred free list in vmalloc.

If this ever expands to handle direct map unmapped pages (which would
probably be the next step), the list_head will have to be moved out of
the actual page anyway. But in the meantime it resulted in the smallest
change.

I can try the other way if it's still too weird.