linux-kernel - Re: [PATCH] udmabuf: revert 'Add support for mapping hugepages (v4)'

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Message-ID: <015062b6-2d4a-7b91-8f64-1695f526f794@redhat.com>
Date:   Thu, 15 Jun 2023 11:48:34 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Kasireddy, Vivek" <vivek.kasireddy@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        Hugh Dickins <hughd@...gle.com>
Cc:     "Hocko, Michal" <mhocko@...e.com>,
        "jmarchan@...hat.com" <jmarchan@...hat.com>,
        "Kim, Dongwon" <dongwon.kim@...el.com>,
        "Chang, Junxiao" <junxiao.chang@...el.com>,
        "muchun.song@...ux.dev" <muchun.song@...ux.dev>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        James Houghton <jthoughton@...gle.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] udmabuf: revert 'Add support for mapping hugepages (v4)'


>> Skimming over at shmem_read_mapping_page() users, I assume most of
>> them
>> use a VM_PFNMAP mapping (or don't mmap them at all), where we won't be
>> messing with the struct page at all.
>>
>> (That might even allow you to mmap hugetlb sub-pages, because the struct
>> page -- and mapcount -- will be ignored completely and not touched.)
> Oh, are you suggesting that if we do vma->vm_flags |= VM_PFNMAP
> in the mmap handler (mmap_udmabuf) and also do
> vmf_insert_pfn(vma, vmf->address, page_to_pfn(page))
> instead of
> vmf->page = ubuf->pages[pgoff];
> get_page(vmf->page);
> 
> in the vma fault handler (udmabuf_vm_fault), we can avoid most of the
> pitfalls you have identified -- including with the usage of hugetlb subpages?

Yes, that's my thinking, but I have to do my homework first to see if 
that would really work for hugetlb.

The thing is, I kind-of consider what udmabuf does a layer violation: we 
have a filesystem (shmem/hugetlb) that should handle mappings to user 
space. Yet, a driver decides to bypass that and simply map the pages 
ordinarily to user space. (revealed by the fact that hugetlb does never 
map sub-pages but udmabuf decides to do so)

In an ideal world everybody would simply mmap() the original memfd, but 
thinking about offset+size configuration within the memfd that might not 
always be desirable. As a workaround, we could mmap() only the PFNs, 
leaving the struct page unaffected.

I'll have to look closer into that.

-- 
Cheers,

David / dhildenb