linux-kernel - RE: [PATCH] mm: fix hugetlb page unmap count balance issue

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Date:   Mon, 15 May 2023 00:08:30 +0000
From:   "Chang, Junxiao" <junxiao.chang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Hocko, Michal" <mhocko@...e.com>,
        "jmarchan@...hat.com" <jmarchan@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "muchun.song@...ux.dev" <muchun.song@...ux.dev>
Subject: RE: [PATCH] mm: fix hugetlb page unmap count balance issue

Thank you for review it! We only reproduced this hugetlb mapping issue with QEMU command which opens udmabuf driver(drivers/dma-buf/udmabuf.c).
I agree with you, it is better to map huge page with a pmd instead of a pte. 😊

-----Original Message-----
From: Andrew Morton <akpm@...ux-foundation.org> 
Sent: Saturday, May 13, 2023 5:04 AM
To: Chang, Junxiao <junxiao.chang@...el.com>
Cc: kirill.shutemov@...ux.intel.com; Hocko, Michal <mhocko@...e.com>; jmarchan@...hat.com; linux-mm@...ck.org; linux-kernel@...r.kernel.org; mike.kravetz@...cle.com; muchun.song@...ux.dev
Subject: Re: [PATCH] mm: fix hugetlb page unmap count balance issue

On Fri, 12 May 2023 15:20:36 +0800 Junxiao Chang <junxiao.chang@...el.com> wrote:

> hugetlb page usually is mapped with pmd, but occasionally it might be 
> mapped with pte. QEMU can use udma-buf to create host dmabufs for 
> guest framebuffers. When QEMU is launched with parameter "hugetlb=on", 
> udmabuffer driver maps hugetlb page with pte in page fault handler.

Are there any other situations in which a hugetlb page is mapped in this fashion?

If not, can QEMU be changed to map with a pmd?

So we get one less weird special case in MM.