linux-kernel - Re: [RFC PATCH 39/39] KVM: guest_memfd: Dynamically split/reconstruct HugeTLB page

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Message-ID: <aAm9OHGt6Ag7ztqs@yzhao56-desk.sh.intel.com>
Date: Thu, 24 Apr 2025 12:25:28 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Ackerley Tng <ackerleytng@...gle.com>, <tabba@...gle.com>,
	<quic_eberman@...cinc.com>, <roypat@...zon.co.uk>, <jgg@...dia.com>,
	<peterx@...hat.com>, <david@...hat.com>, <rientjes@...gle.com>,
	<fvdl@...gle.com>, <jthoughton@...gle.com>, <seanjc@...gle.com>,
	<pbonzini@...hat.com>, <zhiquan1.li@...el.com>, <fan.du@...el.com>,
	<jun.miao@...el.com>, <isaku.yamahata@...el.com>, <muchun.song@...ux.dev>,
	<erdemaktas@...gle.com>, <vannapurve@...gle.com>, <qperret@...gle.com>,
	<jhubbard@...dia.com>, <willy@...radead.org>, <shuah@...nel.org>,
	<brauner@...nel.org>, <bfoster@...hat.com>, <kent.overstreet@...ux.dev>,
	<pvorel@...e.cz>, <rppt@...nel.org>, <richard.weiyang@...il.com>,
	<anup@...infault.org>, <haibo1.xu@...el.com>, <ajones@...tanamicro.com>,
	<vkuznets@...hat.com>, <maciej.wieczor-retman@...el.com>,
	<pgonda@...gle.com>, <oliver.upton@...ux.dev>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>, <kvm@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH 39/39] KVM: guest_memfd: Dynamically
 split/reconstruct HugeTLB page

On Thu, Apr 24, 2025 at 09:09:22AM +0800, Yan Zhao wrote:
> On Wed, Apr 23, 2025 at 03:02:02PM -0700, Ackerley Tng wrote:
> > Yan Zhao <yan.y.zhao@...el.com> writes:
> > 
> > > On Tue, Sep 10, 2024 at 11:44:10PM +0000, Ackerley Tng wrote:
> > >> +/*
> > >> + * Allocates and then caches a folio in the filemap. Returns a folio with
> > >> + * refcount of 2: 1 after allocation, and 1 taken by the filemap.
> > >> + */
> > >> +static struct folio *kvm_gmem_hugetlb_alloc_and_cache_folio(struct inode *inode,
> > >> +							    pgoff_t index)
> > >> +{
> > >> +	struct kvm_gmem_hugetlb *hgmem;
> > >> +	pgoff_t aligned_index;
> > >> +	struct folio *folio;
> > >> +	int nr_pages;
> > >> +	int ret;
> > >> +
> > >> +	hgmem = kvm_gmem_hgmem(inode);
> > >> +	folio = kvm_gmem_hugetlb_alloc_folio(hgmem->h, hgmem->spool);
> > >> +	if (IS_ERR(folio))
> > >> +		return folio;
> > >> +
> > >> +	nr_pages = 1UL << huge_page_order(hgmem->h);
> > >> +	aligned_index = round_down(index, nr_pages);
> > > Maybe a gap here.
> > >
> > > When a guest_memfd is bound to a slot where slot->base_gfn is not aligned to
> > > 2M/1G and slot->gmem.pgoff is 0, even if an index is 2M/1G aligned, the
> > > corresponding GFN is not 2M/1G aligned.
> > 
> > Thanks for looking into this.
> > 
> > In 1G page support for guest_memfd, the offset and size are always
> > hugepage aligned to the hugepage size requested at guest_memfd creation
> > time, and it is true that when binding to a memslot, slot->base_gfn and
> > slot->npages may not be hugepage aligned.
> > 
> > >
> > > However, TDX requires that private huge pages be 2M aligned in GFN.
> > >
> > 
> > IIUC other factors also contribute to determining the mapping level in
> > the guest page tables, like lpage_info and .private_max_mapping_level()
> > in kvm_x86_ops.
> >
> > If slot->base_gfn and slot->npages are not hugepage aligned, lpage_info
> > will track that and not allow faulting into guest page tables at higher
> > granularity.
>  
> lpage_info only checks the alignments of slot->base_gfn and
> slot->base_gfn + npages. e.g.,
> 
> if slot->base_gfn is 8K, npages is 8M, then for this slot,
> lpage_info[2M][0].disallow_lpage = 1, which is for GFN [4K, 2M+8K);
> lpage_info[2M][1].disallow_lpage = 0, which is for GFN [2M+8K, 4M+8K);
> lpage_info[2M][2].disallow_lpage = 0, which is for GFN [4M+8K, 6M+8K);
> lpage_info[2M][3].disallow_lpage = 1, which is for GFN [6M+8K, 8M+8K);
> 
>   ---------------------------------------------------------
>   |          |  |          |  |          |  |          |  |
>   8K        2M 2M+8K      4M  4M+8K     6M  6M+8K     8M  8M+8K
> 
> For GFN 6M and GFN 6M+4K, as they both belong to lpage_info[2M][2], huge
> page is allowed. Also, they have the same aligned_index 2 in guest_memfd.
> So, guest_memfd allocates the same huge folio of 2M order for them.
Sorry, sent too fast this morning. The example is not right. The correct
one is:

For GFN 4M and GFN 4M+16K, lpage_info indicates that 2M is allowed. So,
KVM will create a 2M mapping for them.

However, in guest_memfd, GFN 4M and GFN 4M+16K do not correspond to the
same 2M folio and physical addresses may not be contiguous.


> However, for TDX, GFN 6M and GFN 6M+4K should not belong to the same folio.
> It's also weird for a 2M mapping in KVM to stride across 2 huge folios.
> 
> > Hence I think it is okay to leave it to KVM to fault pages into the
> > guest correctly. For guest_memfd will just maintain the invariant that
> > offset and size are hugepage aligned, but not require that
> > slot->base_gfn and slot->npages are hugepage aligned. This behavior will
> > be consistent with other backing memory for guests like regular shmem or
> > HugeTLB.
> > 
> > >> +	ret = kvm_gmem_hugetlb_filemap_add_folio(inode->i_mapping, folio,
> > >> +						 aligned_index,
> > >> +						 htlb_alloc_mask(hgmem->h));
> > >> +	WARN_ON(ret);
> > >> +
> > >>  	spin_lock(&inode->i_lock);
> > >>  	inode->i_blocks += blocks_per_huge_page(hgmem->h);
> > >>  	spin_unlock(&inode->i_lock);
> > >>  
> > >> -	return page_folio(requested_page);
> > >> +	return folio;
> > >> +}