linux-kernel - Re: Struct page proposal

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Message-ID: <20210927183350.obd756wnsctukf63@box.shutemov.name>
Date:   Mon, 27 Sep 2021 21:33:50 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Struct page proposal

On Mon, Sep 27, 2021 at 07:05:26PM +0100, Matthew Wilcox wrote:
> On Mon, Sep 27, 2021 at 07:48:15PM +0200, Vlastimil Babka wrote:
> > On 9/23/21 03:21, Kent Overstreet wrote:
> > > So if we have this:
> > > 
> > > struct page {
> > > 	unsigned long	allocator;
> > > 	unsigned long	allocatee;
> > > };
> > > 
> > > The allocator field would be used for either a pointer to slab/slub's state, if
> > > it's a slab page, or if it's a buddy allocator page it'd encode the order of the
> > > allocation - like compound order today, and probably whether or not the
> > > (compound group of) pages is free.
> > 
> > The "free page in buddy allocator" case will be interesting to implement.
> > What the buddy allocator uses today is:
> > 
> > - PageBuddy - determine if page is free; a page_type (part of mapcount
> > field) today, could be a bit in "allocator" field that would have to be 0 in
> > all other "page is allocated" contexts.
> > - nid/zid - to prevent merging accross node/zone boundaries, now part of
> > page flags
> > - buddy order
> > - a list_head (reusing the "lru") to hold the struct page on the appropriate
> > free list, which has to be double-linked so page can be taken from the
> > middle of the list instantly
> > 
> > Won't be easy to cram all that into two unsigned long's, or even a single
> > one. We should avoid storing anything in the free page itself. Allocating
> > some external structures to track free pages is going to have funny
> > bootstrap problems. Probably a major redesign would be needed...
> 
> Wait, why do we want to avoid using the memory that we're allocating?

Intel TDX and AMD-SEV have concept of unaccpeted memory. You cannot use
the memory until it got "accepted". The acceptance is costly and I made a
patchset[1] to pospone the accaptance until the first allocation. So pages
are on free list, but page type indicate that it has to go though
additional step on allocation.

[1] https://lore.kernel.org/all/20210810062626.1012-1-kirill.shutemov@linux.intel.com/

-- 
 Kirill A. Shutemov