linux-kernel - Re: [PATCH v3 1/6] mm/thp: add prep_transhuge_device_private

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Date:   Fri, 6 Nov 2020 12:56:47 -0800
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Christoph Hellwig <hch@....de>
CC:     <linux-mm@...ck.org>, <nouveau@...ts.freedesktop.org>,
        <linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Jerome Glisse" <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        "Alistair Popple" <apopple@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Zi Yan <ziy@...dia.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Ben Skeggs <bskeggs@...hat.com>, Shuah Khan <shuah@...nel.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/6] mm/thp: add prep_transhuge_device_private_page()

On 11/5/20 11:55 PM, Christoph Hellwig wrote:
> On Thu, Nov 05, 2020 at 04:51:42PM -0800, Ralph Campbell wrote:
>> +extern void prep_transhuge_device_private_page(struct page *page);
> 
> No need for the extern.

Right, I was just copying the style.
Would you like to see a preparatory patch that removes extern for the other
declarations in huge_mm.h?

>> +static inline void prep_transhuge_device_private_page(struct page *page)
>> +{
>> +}
> 
> Is the code to call this even reachable if THP support is configured
> out?  If not just declaring it unconditionally and letting dead code
> elimination do its job might be a tad cleaner.

The HMM test driver depends on TRANSPARENT_HUGEPAGE but the nouveau SVM
option doesn't and SVM is still useful if TRANSPARENT_HUGEPAGE is not configured.

The problem with defining prep_transhuge_device_private_page() in huge_mm.h
as a static inline function is that prep_compound_page() and prep_transhuge_page()
would have to be EXPORT_SYMBOL_GPL which are currently mm internal only.
The intent is to make this helper callable by separate device driver modules
using struct pages created with memremap_pages().

>> +void prep_transhuge_device_private_page(struct page *page)
> 
> I think a kerneldoc comment explaining what this function is useful for
> would be helpful.

That is a good idea. I'll add it to v4.