linux-kernel - Re: [PATCH] IB/ehca: Make sure user pages are from hugetlb before using MR large pages

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Date:	Wed, 12 Sep 2007 21:33:45 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Joachim Fenkes <fenkes@...ibm.com>
Cc:	"LinuxPPC-Dev" <linuxppc-dev@...abs.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"OF-General" <general@...ts.openfabrics.org>,
	Roland Dreier <rolandd@...co.com>,
	"OF-EWG" <ewg@...ts.openfabrics.org>,
	"Hoang-Nam Nguyen" <hnguyen@...ibm.com>,
	Christoph Raisch <raisch@...ibm.com>,
	Stefan Roscher <stefan.roscher@...ibm.com>
Subject: Re: [PATCH] IB/ehca: Make sure user pages are from hugetlb before using MR large pages

 > -#define HCA_CAP_MR_PGSIZE_4K  1
 > -#define HCA_CAP_MR_PGSIZE_64K 2
 > -#define HCA_CAP_MR_PGSIZE_1M  4
 > -#define HCA_CAP_MR_PGSIZE_16M 8
 > +#define HCA_CAP_MR_PGSIZE_4K  0x80000000
 > +#define HCA_CAP_MR_PGSIZE_64K 0x40000000
 > +#define HCA_CAP_MR_PGSIZE_1M  0x20000000
 > +#define HCA_CAP_MR_PGSIZE_16M 0x10000000

Not sure I understand what this has to do with things... is this an
unrelated fix?

 > +static int ehca_is_mem_hugetlb(unsigned long addr, unsigned long size)

This is rather awful -- another call to get_user_pages() to iterate
over all the vmas...

I would suggest extending ib_umem_get() to check the vmas and adding a
member to struct ib_umem to say whether the memory is entirely covered
by hugetlb pages or not.

 > +		ret = ehca_is_mem_hugetlb(virt, length);
 > +		switch (ret) {
 > +		case 0: /* mem is not from hugetlb */
 > +			hwpage_size = PAGE_SIZE;
 > +			break;
 > +		case 1:
 > +			if (length <= EHCA_MR_PGSIZE4K
 > +			    && PAGE_SIZE == EHCA_MR_PGSIZE4K)
 > +				hwpage_size = EHCA_MR_PGSIZE4K;
 > +			else if (length <= EHCA_MR_PGSIZE64K)
 > +				hwpage_size = EHCA_MR_PGSIZE64K;
 > +			else if (length <= EHCA_MR_PGSIZE1M)
 > +				hwpage_size = EHCA_MR_PGSIZE1M;
 > +			else
 > +				hwpage_size = EHCA_MR_PGSIZE16M;
 > +			break;
 > +		default: /* out of mem */
 > +			ib_mr = ERR_PTR(-ENOMEM);
 > +			goto reg_user_mr_exit1;

It seems like it would be better to just assume the memory is not from
a hugetlb is ehca_is_mem_hugetlb() fails its memory allocation and
fall back to the PAGE_SIZE case rather than failing entirely.

Also if someone runs a kernel with 64K pages on a machine where they
end up being simulated from 4K pages, do you have the same issue with
the hypervisor ganging together non-contiguous pages?

 - R.
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